How a Furnace Works and How It Turns Itself On and Off
Often the best way to figure out what is wrong with something is to know how it is supposed to work. If you know the sequence of operations, you can pinpoint where the sequence is being disrupted. Your furnace is no exception to the rule.
When your furnace is called into action by the thermostat, there is a rhyme and reason to the procedure it follows in safely turning itself on. Next time your furnace doesn't respond to the call, you will be able to see where the problem lies, and you can either decide that it is within your skills and resources to repair it, or that you need a professional HVAC technician to handle the issue for you.
The following discussion can save you time and money, whether you fix the problem yourself, or end up showing the problem to your service technician.
Firing Order: Steps in the Operation of a Modern Forced-Air Furnace
When the thermostat asks for heat, the furnace jumps into action, going through the steps and components below. You can see many of these components in the video.
- The thermostat tells the furnace to come on.
- The inducer motor starts up.
- The pressure switch confirms proper venting of the chimney.
- The hot-surface ignition module (if you have one, and not a spark ignitor or a pilot) begins to glow.
- The gas valve opens and the gas is ignited by the ignition source.
- The flame sensor verifies that the gas has been lit.
- The high-limit switch reaches its set temperature.
- The blower motor comes on.
- The furnace runs until the thermostat is satisfied and tells it to shut down.
- The gas valve shuts.
- The high-limit switch reaches its low-temperature setting.
- The fan shuts down.
A newer furnace may have even more bells and whistles; an old furnace may have just the minimum, a gas valve and a thermocouple (like an old-fashioned flame sensor). The thermocouple or flame sensor tests whether the pilot is lit, and stops the main gas valve from opening if it is not. Of course, a broken thermocouple can also stop the gas valve from opening.
The thermostat is where it all begins. The thermostat is really just a set of switches that open and close depending on the temperature, allowing power to flow to certain circuits in the heating and cooling system. Think of them like drawbridges that swing up and down. They are normally drawn up, but can be lowered to close the bridge and allow power to pass.
When the temperature in your home drops, the thermostat drops its bridge, sending power to the furnace to let it know that heat is needed. As the room temperature rises again, the thermostat raises its bridge and shuts off the power. For air conditioning, the thermostat works the same way but in a mirror image, closing its bridge when the house gets too warm.
If the temperature in the house is lower than the temperature to which you set your thermostat, your thermostat may not be functioning as it should. You can test this by jumping (touching together) the red and white wires to your thermostat. If the furnace comes on, your thermostat is quite likely the problem.
What Do the Wires in the Thermostat Do?
|Wire Color||What it Controls|
Red is for power. This carries 24-volt power, supplied by the furnace. This power waits at the “bridge” until it is told where to go.
White is for heat. When the bridge (switch) between the red and white wires closes, the thermostat is calling for heat.
Yellow is for cooling. When the bridge (switch) between the red and yellow wires closes, the thermostat is calling for cooling.
Green is for the fan. When the bridge between red and green closes, only the fan runs. No heat or cooling is called for.
Blue is a rogue or wild card. It can be used to power a display or for advanced features. Usually, though, it is wrapped back into the wall and not used at all.
The Inducer Motor and Fan
The inducer motor and fan push exhaust up the chimney, getting rid of carbon monoxide (CO). Older systems relied solely on "natural draft" to vent these gases, but inducers have been a great addition to the furnace system. They extend the life of chimneys, and when used with pressure switches they help prevent CO poisoning.
For each question, choose the best answer. The answer key is below.
- How many people are injured each year from CO?
- How many people die each year from CO?
The Pressure Switches
The function of the pressure switch is to verify that the inducer motor is actually pushing air up the chimney. If the chimney is blocked, for example by a bird's nest, the pressure switch shuts down the system in order to eliminate the risk of CO entering the home. Of course, the pressure switch may also shut down if the motor is old and running too slow to satisfy the switch, or the hose from the fan to the switch is pinched or broken, or the switch itself is bad.
The Hot Surface Ignitor
Again, a rather simple device in a complex system. The hot surface ignitor uses electrical power to heat a very fragile ceramic element, charcoal-like in appearance, to such a high temperature that natural gas or propane bursts into flame as it flows by. The hot surface ignitor is located at the orifice at the first burner port, in order to light the gas immediately as it's introduced to the port. This prevents a buildup of unburned gas in the unit.
Some furnaces employ what is called spark ignition. Instead of a red-hot surface, a spark ignitor creates a series of sparks to ignite the gas.
It's not important whether you have hot-surface or electronic ignition in your furnace; the important thing is that you are avoiding using a pilot light. Pilot lights waste gas; they burn gas constantly during the heating season, and some people let them burn year round.
Want to save a few dollars? Make sure you shut off the pilot on your furnace when it's not being used for the season. Just be sure you know how to relight it next year before you do.
The Gas Valve
The gas valve is basically an electronically controlled gateway. When the unit calls for heat, and the circuit board confirms that the right conditions have been met, the unit passes energy to the valve, causing it to open and release gas to be ignited by the ignitor or pilot. When the heat cycle has run its course, the energy to the valve is cut, and the valve once again shuts, cutting off the gas supply to the burners.
The Flame Sensor
The flame sensor is very simple, yet causes a lot of problems for homeowners. Its only job is to verify that the gas has been lit, by sensing the heat. If there's no heat, it shuts off the gas, to avoid any dangerous buildup of gas in the unit. The flame sensor is simple to fix when it goes bad or just needs some TLC.
I wrote a simple step-by-step guide for people needing to clean their furnace flame sensor. Have a look—a lot of issues can be fixed just by a simple cleaning of a flame sensor.
Video: Cleaning a Flame Sensor
The High-Limit Switch
The high-limit switch serves several purposes. First, it keeps the furnace fan from turning on until it detects that a set temperature has been reached. Otherwise, the fan would come on prematurely, and blow cold air into the house each time the thermostat asks for heat.
Second, it also tells the blower how long it should continue to run before shutting off, once the thermostat has been satisfied. Otherwise, hot air remaining in the ductwork and heat exchanger, after the furnace has cut off, would just go to waste.
Lastly, the limit switch will turn the unit off if the heat exchanger reaches a certain high temperature, to prevent dangerous overheating.
The Furnace Fan and Blower Motor
Ah, finally. Heat for us all. Now that all the sensors have been satisfied, and the limit switch tells the blower that the air is warm enough to send into the living space, it will start the air flowing.
Our furnace's blower will typically use 120 volts and push anywhere from 800 to 2000 cubic feet per minute (cfm). Most blowers require the use of a capacitor, and have a half-inch drive that hooks directly to the fan, unless you have a belt-driven motor.
Tid-Bit: In the case of a malfunction, or a cut in power to the unit, it is common for the blower to turn on and run for a period of time. This may be because there is too much heat, and a cool down is called for, or there may be a build up of "bad gases" which need to be spead out or dispersed by the fan.
The Roll-Out Switch
Save Time and Money by Knowing Your Furnace
I hope this helped you to understand your heating system a bit better. Here are some troubleshooting ideas involving the parts you just read about, and here are a few more parts tips.
- In case of a malfunction, your furnace will usually try to restart three times. If on the third try it cannot reach successful ignition, it will go into a lockout mode for a specified amount of time, usually an hour. Resetting the power (turning it off and on) will usually reset the board and allow the unit to show you what was happening in case you missed it.
- The little blinking light in the unit isn't a Christmas decoration or little disco in your furnace. This light blinks a sort of Morse Code that can lead you to the problem, or say "Hey, all is well here." Look inside and outside your unit. There is likely a code chart to tell you what the dot, dot, dash, dash sequence you're seeing means.
- Some annoying noises your home furnace makes may be simple and cheap to quiet down.
- Take care of your furnace and it will likely take care of you. But if you don't clean the unit and change the filter like you're supposed to, your furnace may break down, or break your bank.
Questions & Answers
Question: What could be the problem when the heater shuts off and restarts a few hours later?
Answer: The problem can be a lot of things. This “lockout” that the furnace is going into is a default after 3 consecutive failures. Unfortunately that doesn’t tell me what the failure is.
Question: Had a technician come out last night change some things and now the heat exchanger is glowing?
Answer: Are you sure it's the heat exchanger? I rather doubt it's getting hot enough to glow without tripping a hi-limit switch.
Question: The furnace cycles and runs fine but the blower does not shut off and keeps blowing cold air. What do I do?
Answer: It seems as if the thermostat wire, usually the green one, is somehow touching another wire, likely red, and is keeping the fan running full time. Or your stat is set to "fan" instead of "auto". These are my initial guesses from afar. You could test the wire theory by removing the green stat wire from the control board at the furnace and then let the furnace run another cycle and see if it shuts off. That would at least tell you if the problem is on the wire/thermostat side or the furnace side. If it shuts off, it's on the stat/wire side. If not, the problem is in the furnace.
Question: My gas furnace comes on at 4° below the set temperature on the thermostat. Is this a thermostat issue?
Answer: If it's always been this way, then it may just be an old stat and not very accurate anymore so you may want to replace it. If not and your stat is newer, then it's faulty, and they should give you a new one.
Question: I have a Payne furnace and the inducer motor and fan constantly are running. My heat won't kick on and the led code is stating that the pressure switch did not open. I have checked the hose and there are no issues there. Should I try and replace my furnace's pressure switch?
Answer: It sounds like perhaps the switch is bad so yes. Because it’s cheap I would at least try it and if it doesn’t work then maybe call a tech but if it does it can save you a lot of money.
Question: My Goodman 96% 2-stage furnace starts normally in the early morning when the thermostat first calls for heat. After several minutes of heating, the furnace cycles off as if the thermostat call had been satisfied. However, the system immediately resumes the start cycle and continues to heat for several more minutes until the thermostat call is truly satisfied. Does this seem like a high-limit switch issue?
Answer: I don’t think so. I think that is your furnace switching from 1st to 2nd stage. This is perhaps it’s normal function and not a problem at all.
Question: Why does my furnace only run properly with the bottom draft door off?
Answer: More than likely it means the unit is not getting enough return air. Perhaps the ducts are too small or unit too big. (Meaning the fan is too powerful for the duct) Maybe the fan could be set to a slower speed but you need to be careful it’s not too slow or you can trade one problem for another and you’ll need to consult a tech to make sure. Unfortunately there is far to much to that decision for me to explain here.
Question: What would cause the blower to work in on mode but not in auto mode?
Answer: The blower will only come on if there is a call for heating/cooling in “auto” mode. Fan “on” mode turns on the fan by itself but still allows heat and cool to work as normal. If it’s not coming on in heat/cool “auto” mode then some must be wrong at the stat or board I believe.
Question: I have a Goodman furnace and it runs fine as long as I keep the thermostat at 80 and above but if I turn it to lower than 80, the gas valve will not open. Can you help me?
Answer: It sounds like the stat could be your problem. I can’t think of any other reason that would happen.
Question: Furnace kicks on. Home temp reaches desired temp. Then the burners turn off but the blower keeps blowing. Normally the blower turns off after approx 100 seconds but occasionally will continue to blow and the only way to turn off is the fuse box. Can it be the switch above the burners or the switch that is by the blower?
Answer: The blower usually only does that when there is a fault. If no fault and no changes have been made recently then perhaps the board is going bad and there is a short causing the blower to run continuously. Can't say that for sure but this is what I'd be looking at.
Question: How many years do limit switches and thermocouples on a furnace last?
Answer: The lifespan of mechanical parts of any kind will vary greatly on a lot of different things. I can only say they are intended to last the length of their warranty. This is how companies figure most of the terms of their warranties, based on the expected lifespan. If you know how long the warranty on your parts are then you have a pretty good idea of their expected life. That all said, if a part is continuously failing then either the manufacturer has a defect or there is an underlying reason the part keeps failing and you’ll need to figure that out. Usually, it’s the latter.
Question: I have a Goodman furnace that is about a year old. It works for some time blowing hot air and tries to reach the temperature set on the thermostat. After some time trying to reach the target it cuts off. When I look at the furnace, I see a red light flashing four times. Would replacing a filter and placing it incorrectly in the direction of flow cause this problem?
Answer: No, I don't see the filter being in backwards causing this issue. I'm not sure what is but that I don't think is.
Question: What is wrong with my furnace when it ignites and then shuts off?
Answer: If it's really quick then maybe it's the flame sensor.
Question: My furnace does not usually start by itself. I can turn the switch on the side of the furnace on and off, and then it will start and run, but it will seldom start otherwise. I replaced the thermostat, but the problem remains. The inducer motor is also fairly new. The light is not blinking. What could it be?
Answer: While I don't have enough info here to suggest a resolution, I wouldn't keep doing what you're doing without having a tech look at it. It could potentially be unsafe and cost you more money the longer you wait, not to mention the inconvenience.
Question: Is 80 degrees too hot to run your gas furnace?
Answer: In my opinion yes. It can do it but will likely cost a small fortune to do so.
Question: What would be the main cause of having to replace the ignitor on my furnace three times in the last four months?
Answer: I would guess that you either have the improper part or improper voltage coming from the board. Different ignitors require different voltages...usually 80 or 120V and if the part is right but the voltage is not, then that could cause it. Otherwise, if you touch the surface of the ignitor where it glows, that could be ruining your new ignitors too. The oils from our skin can cause the HSI to burn out.
Question: I'm getting 4 flashing lights from my furnace, meaning an open limit switch. It is -40c today so I'm wondering is it overheating and turning off just because it's having a hard time keeping up? Or is there an actual issue?
Answer: Check your filter for good airflow to keep cool. If possible, check you a/c coil to see if it's plugged or clean for good airflow. These things are free. If not, you may just have a bad limit switch.
Question: My light is blinking nine times, and my code suggests that's it's the ignitor. I've changed out the ignitor two times and still no heat! Any help would be greatly appreciated?
Answer: I'm sorry but these codes are not all the same per brand and I don't know them all. The code just means it's failed to ignite, not that the igniter is bad. (You can't touch the ignition part of the igniter while installing or it will short out the new however so if it was the problem and you did touch that part of it, it is possible but if it glows then it shouldn't be. That said there are a lot of reasons a furnace won't let itself ignite and I would need more info on what's happening up to that point or run electrical tests to pinpoint the issue.
Question: My furnace will light and run, but when it shuts off the pilot light always goes out, how can I fix this?
Answer: It sounds like you need a thermocouple.
Question: My outside furnace exhaust faintly smells of natural gas, but nothing inside. Is this normal?
Answer: I've never heard of it or ran into it and cannot think of how that would be. That said, if you really do, then I would have it tested. You never want to smell gas.
Question: What does it mean if white blower motor wire attached to a black wire that connects to fan limit wire touches the blower housing, and it starts running? That's a short in the motor which means replace right? The blower will not turn on any other way, and the breaker keeps tripping.
Answer: It sounds like a short, yes.
Question: Why does the pilot light go out when my furnace kicks off?
Answer: It could be a weak flame so any sort of air back flow might be putting it out. Maybe it's an intermittent pilot though. I don't know of too many set ups like that. Could be a bad thermocouple. Maybe bad heat exchanger. I really can't say for sure without being there and running some tests.
Question: What does "pressure switch open with combustion air inducer energized" mean?
Answer: It means the electrical connection the pressure switch is supposed to "close" didn't and it being "open" won't allow electricity to pass to the next step so either the switch is bad or there is something preventing enough suction to make it "close". Could be a bad inducer or a blocked chimney but typically it is the switch. This would be the first thing I would check.
Question: I have a ruud furnace that's about 4 yrs old. Recently it has been starting to short cycle. I have checked everything and even changed the filter but it still does the same thing. I have been able to take the door off and it works fine but when I put the door back, it shorts cycles again. What could be the problem?
Answer: It could be dirty and wants more air, but it also could just be drastically low temps making the unit run more to keep up. Mine does.
Question: I have a Goodman forced air furnace that is about 13yrs old. I replaced the pressure switch a few yrs back and have had the blower replaced about a yr ago as well. I’m currently having trouble with getting heat. The furnace will run, but sometimes it doesn’t blow hot air. When that’s happening, it sounds like the burners are coming on and off. I replaced the thermostat already, but that didn’t work. Any ideas?
Answer: If the burners are kicking off and on almost immediately my first thought would be the flame sensor but beyond that, I cannot advise based on the info provided.
Question: Why won't the outside fan turn on my furnace? It's stuck.
Answer: If your fan is “stuck,” then the motor has likely locked up and will need to be replaced.
Question: What temperature should air coming out of the heat exchange be? Is 113 degrees correct or should it be between 120-125 degrees?
Answer: The fan limit switch should tell you what range the temperatures should be running in. EX- 130-160 degrees may be stamped on it.
Question: While the furnace is in a long heat cycle (~30 minutes), I will hear the inducer motor cut off for 1 second and then it restarts and continues running. This 1 second shutdown of the inducer may happen 3 or 4 times during the 30 minute heat cycle. Only the inducer motor cuts off for this 1 second time period. The furnace blower continues running. Does the IDM need replacing or possibly the round manual Honeywell thermostat? 14 yo Tempstar Furnace.
Answer: The stat would have nothing to do with that. Do you have a variable speed inducer? Perhaps changing its speeds? If not then it should run the whole cycle because if it stops the pressure switch will open and shut the unit down and if that's the case for you be happy it's only a second or you wouldn't have heat. You may want to have it checked out before it's urgent.
Question: My gas furnace turns o, runs for a few minutes, and then shuts off. I have replaced the fuse, and have also replaced the motor. What else could it be?
Answer: Well, I wouldn't replace things unless I knew they were bad for the sake of saving money. I can't say based on the information here, but figuring out at what point in the cycle it's stopping would help. Perhaps testing the circuit would be best if you know what you're looking for. If not, you should call a tech and allow them to diagnose.
Question: My cold air intake pipe is blowing hot air after the inducer fan shut is off, is that normal?
Answer: No. The intake should never blow air out.
Question: If an inducer fan is working but maybe getting worn out, can it cause a pressure switch not to function properly?
Answer: It could, yes. It wouldn’t pull enough of a vacuum to trigger the switch.
Question: My 1979 Rheem furnace in my upstairs heater closet works well with 1/2 blue 1/2 yellowish pilot flame. Inside the door where it's housed is very hot to the touch and also the inside of the closet is heated up! I'm afraid to use my heater, could it be a fire hazard? What could cause the hot door and heat inside the closet? What should be checked?
Answer: I'm thinking it's hot like that because it's a natural draft unit most likely and is in a confined space. Seeing that it's almost 40 years old if you can, I'd consider budgeting for a replacement and definitely invest in a smoke/CO detector.
Question: I installed my new Patriot 80 oil furnace. Fired it up and everything worked! The burner started up, ran for its cycle, and shut down. But then I noticed that the blower for moving heat did not start. What should I check?
Answer: Given that the unit is new I would check first for a loose connection. From there, you really just have to test the circuits and see where the breakdown is and then determine why. There are a few too many possibilities to list based on the information provided.
Question: Hi. I have a two year old RUUD gas furnace. It runs for exactly 8 minutes then everything shuts off and restarts after 30 seconds. This cycle repeats until the desired temperature is reached. I bypassed the thermostat, and the same thing happens. Any suggestions?
Answer: Sometimes a 2 stage furnace will do that between stages, but if it's consistently happening throughout the cycle and you've jumped the stat, I would first suspect the hi limit. Check your filter and if you haven't had the unit cleaned...it could be too much air is not getting out of the plenum. This is just a guess from afar and what I would check next if I were there.
Question: My furnace is giving me an error that says “check igniter or ground.” Everything functions properly until the blower kicks on. At that point, the burners turn off, and the blower runs for about a minute. Then it turns off, and the process starts all over again. Is this something I can address?
Answer: Unfortunately, though it may be something simple that you may be able to fix, I can't help pinpoint the problem without running tests. Try resetting the breaker and check your filter. See if anything happens that seems in ordinary but otherwise electrical tests will probably need to be run. Sorry.
Question: WHy does the electric furnace trip the breaker but not the a/c?
Answer: Those two units are on separate circuits (or should be), so that's not abnormal.
Question: Does the outside HVAC unit fan need to be running when using the heater?
Answer: That would be a heat pump and not always. The fan cools the compressor...not a problem for most right now so it won't run if it's not needed.
Question: My furnace is 10 years old and is cleaned and checked by the same company twice a year. A month ago a fella came out and cleaned it like no one else ever has. He had pieces all over the place! Now, the filters are filthy every two weeks... any idea why? Blackish color.
Answer: Well, if the blower hadn't been taken out and cleaned before then it could be working better thus drawing in more dust.
Question: How do I fix a furnace when there's banging when turned on and when it's shut off?
Answer: If it's oil canning the duct work (which is usually that type of sound...pressurizing and depressurizing with the fan) you need to locate the weak point and reinforce it.
Question: why the blower is running all the time and never shut off?
Answer: If it's running "all the time," then either your fan switch is set to "fan" instead of "auto," or there is a short in either the low voltage wiring, stat, or at the board. You could disconnect the low voltage from the board to see if it stops. That would tell you if the problem is the board (if it keeps running) or if it's in the wiring and stat. Then, reconnect the wires there and disconnect the stat. If the fan stays running, then the stat is fine, and the problem is the wire. Green wire controls the fan. So red touching green would cause the fan to run continuously. Only try this if you're confident that you know what you're doing.
Question: I'm installing a new draft inducer (exhaust blower) that came with four limit switches that have four different colored dots (white, yellow, red & blue) on them. I have only two limit switches on my furnace, the upper limit switch is blue dotted and the lower is yellow dotted. Am I correct to use the new yellow dotted limit switch since I am only replacing my current yellow dotted lower limit switch?
Answer: It sounds like you got a "kit" that can be used on various units so I would say yes you should be fine in doing that though I'd probably do both limits but at least keep the ones that go w your unit as spares.
Question: My furnace is working fine. It is vented out the front of the house. Why does it have such a strange odor outside only when the furnace turns on?
Answer: The only foul smells (rotten egg-like) I'm aware of have been a result of a bad heat exchanger. I'm not saying for sure that's you because I don't know what you're smelling but usually, there is no bad odor from these vents.
Question: The inducer motor does not start or makes a half turn spin when a call for heat is initiated from the thermostat. The motor is spinning rather well when hand spun. Is my furnace's inducer motor broken or is something else, like the pressure switch, malfunctioning? The light gives me a code 31.
Answer: You may have a capacitor on your inducer that’s bad. Not all units have these but some do. If not then perhaps the board isn’t sending power to the inducer. You’d have to check for voltage to make sure that’s happening and if it is the perhaps the motor is still bad though spinning freely.
Question: The furnace runs and heats the house fine, but the house is dry. The furnace humidifier is working, but the house is still dry? What can I try?
Answer: Have you turned up the percentage of humidity created by the humidifier? Adding humidity is the only way to fix the issue. Perhaps your humidifier isn't sized properly. I wish I could add something more, but there's really no "fix" if everything is working.
Question: Almost every morning when we wake up, and our house is freezing. When I go to our furnace, I see error codes 10 and 61. Why do we consistently have a blower error? The furnace is brand new!
Answer: While I can't be sure this is your issue, often when someone has consistent blower issues it's because the air coming to the furnace is insufficient. It could be a dirty filter, undersized duct or not enough return air registers. This is especially common when installing a new unit on older home ducting, and the ducting hasn't been redone properly.
Question: My gas furnace model nugm100ehb1 takes combustion air from inside the home, is this correct or proper?
Answer: Yes as long as there is enough free air space. There is a chart in the install manual that will tell you how many sq. ft. You should have. Also, I'm not sure if it applies to your model, but some say it will shorten the warranty on the unit. You may want to look at your manual and warranty details to find out.
Question: Our furnace works only when we detach the air intake pipe from its connection at the furnace. We have ensured that the air intake pipe is clear. The furnace is a ninety percent efficient furnace. Any suggestions?
Answer: It’s possible the air intake is too small for the distance it’s traveling. You would need to consult the install manual to see what the proper size based on distance is recommended.
Question: Why does my furnace gas valve stick open all the time?
Answer: I would venture to say either it’s defective and needs replaced or is wired in wrong and receiving constant power rather than opening on command.
Question: My thermostat has a timer (leave/return, etc.) so my furnace will come up to temperature the first time it is asked to, but then will shut off until the next interval. What do you think it is?
Answer: There are hundreds of thermostats on the market so I cannot tell you much beyond reading your manual and double checking your programming. It sounds like something has perhaps been programmed improperly. Unfortunately they all program differently so I wouldn't want to guess.
© 2012 Dan Reed
Suzie N on March 13, 2020:
I have a 6 year old Goodman propane furnace and just recently the furnace inducer has had to blow air 4 to 5 times before heat kicks on. A service man came out and cleaned everything and of course it worked perfectly while he was here;however, it started the blowing sounds again later. He thinks its the inducer but i thought the task of the blower inducer was to blow air. Do you have any ideas
Merlin M on October 06, 2019:
Fantastic article, thank you.
We have a old Heil furnace that will need to be replaced soon. Until then we are trying to limp by (small town and contractors are very busy)
Our furnace get between step 7 & 8 but the short cycles. After the system warms up (flame burns for ~60 sec) the blower comes on but then the system starts to short cycle. Flame is on for about 2sec then off, one 2sec off and so on. Heat is delivered to the vents but clearly this is not right.
I have done the basic, check filter, change thermostat batteries, looks on roof to ensure eventing. At first I thought flame sensor but tow problems with that,
One, if that was the issue why would the furnace warm up. My thought is a bad sensor would prevent this. And two, for the life of me I cannot see a flame sensor on this unit.
Camster on March 13, 2019:
Hi Dan. So yesterday a furnace technician came to inspect my 23 year old Lennox High Efficiency Gas furnace. He said that the temperature in one of the ducts registered 68 degrees and if it reaches 70 the furnace will shut down. He said I probably need a new furnace. What is he talking about please?
[email protected] on March 08, 2019:
Every time we cut our furnace on into the old one it's one of the fan blowers fucking ancient and now it smells and it's smoking so when we cut it off and then we try to cut it back on the next day it just doesn't even turn on what could it be it was really dusty and stuff my life but another filter for it and it's just not working we cut the switches back on through the fuse box and it doesn't cut on and it's getting cold man what do I do what could it be
connie on January 01, 2019:
I have a new York furnace in my garage. I noticed that I feel warm air coming out of the furnace box in garage, while it is dispenseing warm air into the living space.
Dan Reed (author) on November 21, 2018:
Joe689 - So here's a theory. The old stat was perhaps inaccurate thus maybe the furnace wasn't heating up as much. No perhaps it's running longer thus increasing the temperature in the duct. As it heats up, the duct may expand and as the heat shuts off but the fan is still running and the temperature dropping the duct contracts and causes a "thump" or oil canning sound. I can't really see any other way a thermostat could cause this to start happening other than pure coincidence.
Joe689 on November 19, 2018:
Hi Dan, thank you for the detailed run down. Very easy to follow and intuitive. Thanks for making a complex subject a little easier.
I have a Luxaire gas furnace. I just installed a new thermostat, and started to observe some different behavior. When the thermostat reaches its temp, I hear the "clicks", and then right after the furnace make a thump, deep enough to hear through the floor, and then one more thump almost right after. The fans continues to run for a minute, and turns off. This did not happen with old thermostat. The "thumps" also does not happen all the time. I try to recreate it by toggle the temp up and down but can not get it to repeat. Only when it has been on for some time, does this thump upon signal to turn off occur. And it does not happen all the time. By the way, this is for heat. What could this be? I know it is not the fan because the fan does not turn off right away. It is right might the thermostat clicks.
It never happened with old thermostat. I am about 99% sure I wired it correctly. All my wires were labeled, and I also wrote down what lettered slots they were going in on the former thermostat. With that, it was pretty much 1:1 with the new thermostat.
Thanks in advance
Dan Reed (author) on November 15, 2018:
Ray - While I can't be sure what the issue you're having is I can say that typically a unit that runs until there is a failure will automatically kick on the blower as a default safety feature but it should shut off within 5 minutes or so after that. If you're having a limit issue, this could be why you're having problems. Perhaps, since the part is cheap and easy to change, try replacing it and see if that doesn't resolve the issue.
Ray Brewer on November 15, 2018:
I have a furnace that if it stays on for a long period of time will trip the limit switch and blow cold air and the only way to shut it off is switch the breaker off. I have a new filter and all my vents are open. Not sure what else to check?
Dan Reed (author) on October 29, 2018:
Ryan - There are a few things that could be going on and it would probably be best to get an HVAC technician out there to look at it. Gas isn't anything to mess with and hopefully not but you could have a heat exchanger issue and that would definitely require a tech to repair. Good news would be that you likely still have a warranty on the exchanger if it is your problem. (Not the labor to install it but the part itself.)
Casey Rohman on October 28, 2018:
Thank you very much for your reply, Mr. Robbins. My post here didn't appear when I checked back after posting, thus my late response. The breaker is the proper size, and no other electrical problems are appearing in the house.
However, your advice regarding an electrical problem was correct. I think that even though I was careful to reconnect the new control panel properly, it seems likely that at least one of the connections was a bit loose. After I went through and rechecked them all as well as other electrical connections in the furnace, the problem disappeared and the furnace has been functioning properly since. Your patience and willingness to help are much appreciated!
Ryan Jewell on October 26, 2018:
I have aTrane 2 stage furnace. It makes it to around item 7 in your start up list. The furnace will be lit for about 15-20 seconds and then go out... I can hear a waffling whistle noise and see fluttering in the flame at the exit of the gas tubes like it has too much air or not enough gas flowing. And then the middle to further away from ignitor gas lines blow out and it cuts gas to the unit to shut off. Any thoughts on what wrong?
Dan Reed (author) on October 26, 2018:
You're welcome Brian. Glad I could help some. Best wishes!
Dan Reed (author) on October 26, 2018:
Tom - I'm sorry no. There isn't enough here for me to offer sound advice. Perhaps the limit? Those are a common failure in older furnaces.
Brian S. Pollack on October 25, 2018:
Thanks for the input. It is the pressure switch. I can now return the control panel I ordered and had someone else install the pressure switch. Next time i'll know. thanks for your expertise!
Tom on October 25, 2018:
I have a old furnace it’s cal a Stewart Warner . It about 40 years old. I just had the thermal coupling replace and it was working fine then it warm up outside so I turn it off. Then when I needed heat again it wouldn’t come on. I can hear the click in the thermostat To on the furnace but nothing any idea?
Dan Reed (author) on October 23, 2018:
Don - This sounds more like a pressure switch problem to me. Perhaps check out my article on that and see if it helps.
Dan Reed (author) on October 23, 2018:
Brian - I would be more inclined to think it's the flame sensor or pressure switch. If the sensor is dirty or pressure switch is weak, it could be giving a false failure that can happen within split seconds causing the valve to click open and closed but so fast that the flames don't actually go out. This is my thought from a far. Perhaps check them and see.
Dan Reed (author) on October 23, 2018:
Casey - When there are multiple part failures in a row like this I always consider the incoming power and circuit. Is the breaker sized properly? Is the furnace on it's own dedicated breaker? Are there any other "occurrences" in the home with other appliances or lights? I wish I had an easy answer here but testing will definitely be required to definitively pinpoint the issue.
don murray on October 20, 2018:
Hi Dan, I have an older carrier furnace, light is on constantly, and no blinking noticed. Inducer spins, I hear the blower sound, igniter glows hot, then everything shuts down.
I just replaced the gas valve, and now the same thing is happening. Any ideas? could the gas valve I just put in be bad ? The gas is turned on to the furnace, lol...
Brian Pollack on October 20, 2018:
I am trying to figure out what going on with my Trane XE80. It can be either the limit switch, control panel, or gas valve that is clicking. The furnace runs, no problems with the start. It remains on and about ten minutes into blowing hot air it will click about 5-10x. The flames will flicker on and off with each click. The furnace is still running, the flames come back continually and the clicking goes away.
I have already checked the thermistor. Any thoughts?
Dan Reed (author) on October 18, 2018:
While your message isn't clear to me I can say that changing the filter will help keep temps down but has nothing to do with the pressure switch. The problem may be the pressure switch however the 2 are not related. Perhaps check out this article and see if it helps...https://dengarden.com/appliances/Everything-You-Ne...
Reed Wickstrom on October 17, 2018:
I have a Heil 2 stage filter and it does nothing when thermostat calls for heat; however, if I unplug the limit switch the inducer motor starts and then shuts off around 30 seconds later after I reconnect it. FYI, I checked the furnace filter and it was dirty. I am thinking that it overheated. Do you have any suggestions.
Dan Reed (author) on October 11, 2018:
Call the company who installed it and ask them to check it out.
Carol miller on October 11, 2018:
I have a 2 month old heater/air con. Installed and it gets very very hot to touch. What should i do?
Dan Reed (author) on May 16, 2018:
Craigh S - I can't be sure without testing. Maybe the board is bad? Are you sure it's the rollout switch and not the temperature sensor on the blower housing?
Craigh S on May 14, 2018:
Why would rollout switch error show when it a/c is used but not when heat is used?
Dan Reed (author) on May 01, 2018:
Derek - I really can't think off why these two would be running simultaneously because of your cutting concrete unless you got water on the board and shorted something out. I could maybe understand just the blower but not along with the inducer. And are you sure this is a result of your work or that you just happened to notice because you spent so much time down there?
dgrandstaff on May 01, 2018:
I was cutting concrete close to the furnace and there was a lot of dust, now the exhaust fan and the blower run at the same time nonstop.
Dee Kae on March 25, 2018:
Thank you for responding.
Dan Reed (author) on March 25, 2018:
Well Dee, Like other mechanical working things, they sometimes just do something weird and turning off and on the stat can trigger a new call and voila, it works. I wish I could offer a better explanation but it does happen. My furnace will sometimes shutdown on a real cold night because I believe the "heavy" cold air along with a right wind direction overcomes the pressure switch/inducer and can trigger a lock out. Wind stops, I reset the unit, voila...it works. It is very difficult when a cause can't be pinpointed. Next time the furnace shuts down see if you can catch the blinking red light code and maybe that would help.
Dee Kae on March 25, 2018:
I have a Tud-B series older Trane furnace. It is cleaned professionally every fall and last fall HVAC said it's still in great condition. On Feb. 22, March 3, March 6 and March 25, the furnace stopped working but started again when the thermostat was turned off and back on. I haven't called HVAC yet because it's happened late at night and on weekends so it has seemed to be better to try turning it off and on first. Once last summer the newer air conditioner stopped and they couldn't duplicate the problem because it started working when they got here and turned the thermostat back on. I have a newer electronic programmable thermostat. I've changed the batteries twice (display was still working), cleaned it (no visible dust) and tightened a loose anchor screw. What else could cause my problem besides a faulty thermostat? Turning the thermostat off and right back on has turned the furnace back on all four times. Thank you!
Dan Reed (author) on March 06, 2018:
I would say perhaps you have a slow close gas valve however if it keeps running or goes into lock out then I don't think that's it. It sounds like the valve itself maybe going bad. It's a "normally closed" valve and if the power is being lost in the valve then it will not hold it open and thus creating this intermittent issue. I can't swear to it without running tests but I would have it checked out because it will cause other components to wear out if it's constantly starting and stopping.
RandysPCs on March 06, 2018:
I have a Coleman furnace that acts like it's running out of gas sometimes. I have watched it and what happens is, the induction fan runs, ignitor glows, gas lights but then gradually the flame gets smaller and goes out. Then the gas valve clicks off and the cycle starts over, sometimes it will stay running sometimes it goes into lock out. Any ideas?
Dan Reed (author) on March 06, 2018:
Lowski - I'm nearly positive it's the flame sensor and is probably just a bit in need of a cleaning. You likely won't even be able to see dirt on it but it doesn't take much at all. I've written on how to do this so perhaps if you need some guidance you could check that out. Thanks for reading.
lowski on March 05, 2018:
I recently replaced the circuit board on my Ruud 80%. Apparently there was a bad relay, causing my inducer motor to constantly switch between hi and low, delaying the furnace from ever igniting. Since I switched out the board, it has worked fine, with the caveat that every once in awhile, when the inducer motor turns on and the pressure switch makes, I hear the gas ignite but then turn off right away; but the blower motor still comes on and blows cold air. Once the blower motor turns off, the inducer comes immediately on and the whole sequence starts again, but this time with the flame staying lit and working properly. Thoughts?
Dan Reed (author) on February 26, 2018:
Roger, There are a lot of possibilities based on the info you've provided. I would need more to offer any sound advice. Maybe igniter is bad? Maybe inducer/pressure switch is preventing it due to obstruction or part failure? Maybe board? I'm sorry but I can't help much at this time.
Roger on February 26, 2018:
furnace turns on but the igniters don't light up.
Dan Reed (author) on January 27, 2018:
I don't think it's either to be honest. Sounds more like an intermittent pressure switch issue. See my reply to John.
Dan Reed (author) on January 27, 2018:
Sounds similar to how a pressure switch will act sometimes. It's cheap and easy to fix so maybe give it a try if it's in the budget.
Jakobsdad on January 26, 2018:
My furnace works perfectly for one or two cycles then it goes through the start up process but the igniter doesn't come on. The furnace continues through the whole start up process then blows regular air through the system. if I turn the thermostat to the off position and then back to heat it works perfectly for one or two cycles then repeats. do you think this is a thermostat issue or an igniter issue? the igniter works as it should after the manual power cycle from the thermostat.
john on January 26, 2018:
My furnace works perfectly for one or two cycles then when the thermostat calls for heat the igniter doesn't light and it blows cold air only. I can turn the unit off then right back on and it works perfectly again until one or two more cycles then I have to repeat a powering down to get it to work
Kathleen8 on January 26, 2018:
You are probably right! hahahhaha I figure it there was a problem, it would have shown itself by now. Thanks!
Dan Reed (author) on January 25, 2018:
And though I hate to say it, that may be exactly why this is an issue for you. Furnaces aren't as quite when you have to sleep next to one as they are in a basement and is likely why the evenings seem worse...it's quieter, colder, and you're right there for it all.
Kathleen8 on January 25, 2018:
Honestly, I don't know about furnaces. It's a Carrier 59SC5A I believe.. I don't believe the manual says it's 2 stage. I thought the filter was dirty, so I changed it with a nice Filtrete filter and no sluggish noises upon the start of cycling. Then 3 weeks later, it started doing it again. I looked at the filter and it was dirty again. So, I jut put in a filter from ACE Hardware and the noise came back. It will only usually happen in the evening when it actually does happen.. You hear the switch and then it's this sluggish noise like it's having a bit of trouble getting going) from what I assume is the air draft inducer and then it starts blowing just fine. My brother thinks maybe it's just working harder? I can't believe I'm the only person with this issue..then again, I live in a tiny house and the furnace is right there next to my bedroom so I hear EVERYTHING.
Dan Reed (author) on January 25, 2018:
Kathleen8 - I can't really say much based on the information here but since it's condensing...is there a chance it's also a 2 stage unit??? If so, the gas output and blower speeds may vary by cycle and whether it's calling for the first or second stage.
Kathleen8 on January 25, 2018:
Hi, I have a Carrier Condensing gas furnace. Sometimes, when the furnace kicks on and starts it's cycle, it sounds sluggish before the blowing begins. Most times, you hear the click, the cycling starts and it starts blowing but it seems like since we are in the midst of winter and it's on all the time, it happens for a few cycles during the evening, but not all the time... Any ideas? Could there be dust stuck somewhere? Thanks!
Dan Reed (author) on January 16, 2018:
Unfortunately there are a too many possibilities for me to say anything certain. Maybe a fan limit? Intermittent pressure switch? Gas valve? Circuit board? Unless you just want to start throwing parts at it, you'd need a tech to troubleshoot the circuit.
Phyrfitr on January 16, 2018:
i have a york 90+ 100,000 btu single stage unit, the thermostat will call , the inducer motor will spin up the solid igniter will glow the gas valve will open and the furnace will cycle, for five minutes or so then the flame will go out but the inducer and blower motor will continue to run, the igniter will glow and the gas valve will start clicking constantly and the flame will ignite multiple times causing multiple fuel ignitions sometimes making a Poof sound, I sit and watch it thru the peep hole, this has been going on now for about A week, Any thoughts ?
Bill Fish on January 14, 2018:
Thanks for responding to my question. You said that my problem might be related to the flame sensor, with the limited info I gave ya. So I cleaned the flame sensor, which was very simple, and my furnace is working great again. I probably saved a couple hundred dollars by doing that myself. Thanks again.
Steve Stark on January 08, 2018:
I have a 14 yo Tempstar gas furnace. During a long heating cycle, the inducer motor will cut off for a second and then come back on. This happens maybe 3 or 4 times during the heating cycle. Does the IDM need replacement? Or, could the thermostat be faulty? (round manual Honeywell thermostat) Thanks
Dale on January 03, 2018:
Have a 10 year old furnace, have it cleaned and checked twice a year. A month ago the fella took the thing apart like never before, put it back together and left. Now the filter is filthy after two weeks. That never happened before. Have any ideas what happened? Thank you. 1/3/18. [email protected]
Earl on January 03, 2018:
My furnace will not stay running unless the door is off, why?
Dan Reed (author) on December 30, 2017:
Angelo - It seems maybe the thermostat is the issue since the furnace will only turn on when the stat says to so perhaps it's not reading properly???
Dan Reed (author) on December 30, 2017:
Jeff - I'm sorry but there isn't enough for me to offer much. Check your filter. Could be a high limit switch??? Not sure though.
Angelo on December 29, 2017:
I have Lennox Meritt series that's 13 years old. It started to act weird yesterday. It will only turn the furnace on and maintain the house temperature if the thermostat is set at 73 and above. If I set the thermostat below 73, it will will not turn on the furnace.
Jeff on December 28, 2017:
I have a Coleman I replaced the pressure switch and the thermostat it is running a cycle at two set to 70 but any higher it will run as well then shut off and not fire in less I turn it back down to 70 and cut power and reset it
Dan Reed (author) on December 26, 2017:
RK - It sounds like you either need a new motor and/or motor capacitor. I don't recommend using your hand. I suppose it may be getting bad voltage from the board as another possibility but you may want to call a tech.
RK on December 26, 2017:
Hello, I have Carrier weathermaker 8000 gas furnance. Very often furnace would not start and I have to rotate the motor by hand and press the cover switch to start again.. If i leave it, it may start randomly after few hours on its own Any suggestions
Dan Reed (author) on December 17, 2017:
Craig, Though I can't say for sure from here, I would first check the hi limit and is the coil for your a/c new too? If not, perhaps it is clogged and blocking air flow??? Is the new furnace fan perhaps more powerful? If so, your ducts may not be big enough on the return side...especially if you added an air cleaner. These are my suspects. Thanks for reading.
Craig on December 15, 2017:
Have a Goodman furnace installed on 2016. I set the thermostat to 72 the furnace starts up. Runs for a while tries to make temperature but then cuts off and there is red light blinking 4 times. After a while when the temperature has dropped it kicks back in. I replaced the filter. The thermostat was recently replaced. I have checked the airflow outlet and inlet they don't have any objects. Would a leaking window sill or older windows result in this kind of behavior where the furnace cuts off? This is again a brand new furnace.
Dan Reed (author) on December 14, 2017:
Greg, I'm not sure what advice to offer based on this info. You experienced some sort of failure that is apparently intermittent so when you reset the breaker it started over and apparently didn't have the same problem at that time. In short, something is happening to shut down the unit and you'd need to know at what point in the cycle the unit shut down and caused the unit to "lockout". Any failure that happens 3 times will put the furnace into "lockout" for a set period of time before it tries again. Sorry I can't be of much help based on this info.
Greg Staples on December 13, 2017:
Furnace stopped working the other day, wiggled wires, tripped breaker back and forth, after a bit it came on and worked through the night fine. Same thing happened again the next night. Replaced the breaker because there didn't seem to be any power going to the furnace. Furnace is working so far today but when it is running it seems to ossolate power now and then. Any ideas?
Dan Reed (author) on December 11, 2017:
Thomas - It sounds like you're not getting enough air flow when the filter is in. Perhaps it's restricting the airflow just enough to make the unit overheat and shut down prior to satisfying the thermostat. Often when adding a high end air cleaner, the ducts coming to it need increased in size to allow for more air to keep the unit temperature within it's confines and not overheat the motor or exchanger. Hope this helps at least to understand what the issue MIGHT be. Thanks for reading.
Thomas on December 10, 2017:
If I remove the air filter on my electric furnace then I heat until it reaches the Nest thermostat selected target temperature. If I place the air filter in, it doesn’t reach the selected temperature but is about 4 degrees colder. What is wrong and what is the fix?
Dan Reed (author) on December 08, 2017:
Dave - They can vary. The best way to know your range is to check the High Limit/Fan Switch. There is usually a temp range on there for your unit.
Dave on December 05, 2017:
What should the heated air coming out of heat exchange Be is there a minimum temp ??? Like 120 deg ?
Dan Reed (author) on November 12, 2017:
John - That's an odd one but likely an issue with communication between the stat and furnace. I'm sorry but can't offer much there other than rechecking your programming and running electrical tests to see what's making the unit shut down in that first cycle.
John Beattie on November 11, 2017:
My gas furnace comes on for the 1st program cycle of the morning but doesn't complete the cycle and will stop with out reaching the programed setting. If I turn the furnace on/off it then acts like it completes a heat cycle without the burners on. Then in a minute or so later the furnace comes on and then regularly runs the rest of the day. what might be wrong?
Dan Reed (author) on November 10, 2017:
Ozzie - A high efficient furnace has an intake and exhaust pipe and connection usually. Sometimes guys will let the unit pull air from in the home on the intake and not run it all the way outside. In order to do that, they are supposed to make sure there is enough free square footage to provide the needed intake. Taking that door off may be giving you just enough to do the job but you shouldn't have to leave any covers off your unit to make it run. Hope that helps.
OzzieSch on November 10, 2017:
What exactly brings in the outside air for an efficient furnace? I don't get enough outside air to combust the gas. When I take off a little side cover on the furnace to let in inside air, the furnace works properly. What is my problem?
Dan Reed (author) on November 08, 2017:
Lee - If that's the case then they may need a good cleaning. I'm a little stunned though since a flame sensor should detect that the burners aren't lit since one needs to fire to fire the next and the next. ???
Dan Reed (author) on November 06, 2017:
Marco - I'm sorry but for me to help I would have to perform electrical circuit testing to pinpoint your problem here. I would start with the thermostat. Sounds like it may be suspect.
Dan Reed (author) on November 02, 2017:
Thank you Jonas!
Jonas Porter on November 01, 2017:
This was just an awesome explanation. Thank you. Thank you.
Dan Reed (author) on October 21, 2017:
Kevin - There is obviously a short somewhere. Likely culprits are the transformer and/or entire board. There could be other causes but this is what I'd suspect. Thanks for reading.
Trina Bowers on October 19, 2017:
Ok i checked the fuse, i clean the heating sensor and the blower is working, glow ignitor comes on, slow flashing green light. I checked the thermostat ....the problem is i cant seem to get a flame ignited i dint know what else to do...i want to call for help to be my last resort.
Jw clink on October 19, 2017:
I replaced my ignition last year and furnace work fine.
This year it will not even get hot.
Fan works fine.
Kevin Zoch on October 17, 2017:
I have a Trane Furnace that I determined has a blown fuse on the control panel. When I replace the fuse and push the black switch that the metal panel touches it instantly blows again.
Aaron on October 13, 2017:
I have a Colman Evcon. It is flashing 4 flashes meaning bad limit switch. Tested all switches and checked out. Have a green flash when unit not heating. Bad board?
Dan Reed (author) on October 07, 2017:
Duane - It sounds like it. If the inducer fan/motor is shot then nothing will work as it's the first step in the heating cycle and if it is bad, the whole assembly is usually sold as a piece and is typically specific to it's brand so universal parts aren't likely going to be an option for repair. You'll need to find the distributor for your brand of furnace to find the part most likely. Thanks for reading.
Duane on October 06, 2017:
The induction fan does not start it just hums. It does not spin. Is the fan bad or is there something else I should look for?
Dan Reed (author) on September 11, 2017:
Amacblue - The coil sits on top of the furnace. It's not part of the furnace. It is not always possible for a homeowner to do this since sometimes there is not way to access the underneath of the coil where dirt and such collects in which case it would need to be removed and that DEFINITELY requires a service technician. If you can however access the underside, congrats and it should be cleaned but carefully and not to bend the fins over so that air can't pass through. Thanks for reading.
Amacblue on September 08, 2017:
I would like to clean my coil on my furnace however o can't seem to locate it. It is a carrier just like the one pictured thank you
Dan Reed (author) on July 05, 2017:
Luis - You may have a blown fuse on the board. It's usually 3 amp fuse that is like that in a car. Can't say for sure but that's where I would start. Thank you for reading.
Dan Reed (author) on March 26, 2017:
Robby =- There is a laundry list of things to check from here however it doesn't seem coincidental that this is happening after the stat was changed. Sounds like perhaps your white wire isn't making good contact or in the right spot. Double check your stat wiring. Beyond that, there are too many things to check to list here. Fuse on board? Gas valve? Ignitor bad? Limt? Pressure Switch? I'm sorry I can't be more helpful from here. Thanks for reading.
robby on March 25, 2017:
My heater stopped working yesterday nigh (it had stopped once few months ago, we changed the thermostat battery and it worked fine). After it stopped working, I put a new thermostat and still no luck. These are the few observations, please help in letting me know what is wrong
1) the AC works fine with both thermostat. The AC turns on (outside the house) and the blower blows air
2)On heat, the fan ON position gets the blower working (and you can hear the blower unit below the furnace making noise).
2)On heat, auto, nothing is working. The blower does not work, the flame is not lit (however the pilot flame is lit fine)
While changing to new thermostat, twice on Heat/AUTO, the blower came on but it kept pushing cold air for minutes. Now even that is not happening. Manually pushing the switch in the furnace gets the blower going, but there is no furnace flame(only pilot flame).
Please let me know what to do next
Dan Reed (author) on March 08, 2017:
Arash - It sounds like you had/have a short circuit somewhere. If you don't have a voltmeter you will have a very hard time finding it. You may want to call a tech or you could find yourself spending a lot of money on parts you don't need or buying a part that will short again because you didn't identify the source. Thank you for reading.
Arash on March 07, 2017:
Hi Daniel, I have a Lennox hvac with two zone. My heater stopped working last month an no power on thermostats. I notice the fuse on zone controller board is blown. I changed the fuse with a 5 amp. The power return to thermostats but heater didn't turned on again. I ordere a new zone controller but again the fuse blown in new board. So I thought the problem is from main controller board. I ordered and replaced the main controller plus adaptor. Still heater doesn't work. The egniter glows but the gas valve doesn't release gas to flame. I checked the voltage on gas valve and it's not 24v. It keeps trying ignite and doesn't go to watch guard mode. Would you please help me to figure it up? Thank you
Ccole456 on February 21, 2017:
Thank you. I looks up the difference and it appears it is an ignition board. Thanks for confirming. We have a number of things to try and hoping to keep the unit going for at least a few more months!
Dan Reed (author) on February 21, 2017:
If you have a spark ignition board then you perhaps will not have what I was thinking of. My apologies. There are so many boards out there and this was not what I was envisioning. It would be in plain site if you could change it and I don't believe you can open those without causing further damage.
Ccole456 on February 21, 2017:
Daniel- Curious if you can assist in locating the fuse on the board. The white Rogers control seems to be sealed. The fuse would be inside the case correct? The main fuse at the switch is fine we did check that one.
Dan Reed (author) on February 21, 2017:
Perhaps you've blown the fuse on the board. Typically a 3 or 5 amp automotive type fuse and/or perhaps the 120v/24v transformer took a hit. That could be the humming you hear. All of these are cheap places to start. I'm sorry but troubleshooting over the net is very tough so I try to recommend some of the simplest, cheap repairs to folks.
Ccole456 on February 20, 2017:
Daniel Robbins thanks. Nothing we have read or viewed yet has led us to the capacitor. Haven't been able to make any of the initial trouble shooting techniques give us a clear yes or no. We had a number of power failures from wind the day before it stopped working. If I understand you the the old White Rodgers control panel should illuminate to indicate at least something.
We will look into the capacitor then probably need to call technician if that doesn't work?
Dan Reed (author) on February 20, 2017:
Ccole456 - Sounds like the main blower isn't turning and that humming may be the motor trying to start. There is a capacitor usually mounted to the blower housing that may be bad and would be a cheap easy place to start and if that doesn't work, perhaps the blower is shot. That said, if the light on the board isn't lit, then you may have taken a hit electrically and then the potential possibilities get to be too many to cover here. That said, I would start at that capacitor. Thanks for reading.
Ccole456 on February 19, 2017:
York gas furnace clicks on and then you hear a low continuous buzzing noise (seems to come from near the control panel) and nothing else happens. Replaced inducer motor thinking that was the problem, same issues.
Stat working and fan doesn't turn on when switched "on" at stat. Getting power where stat wires connect to pressure switch. Tubing at pressure switch appears unblocked and in good shape.
Could it be the pressure switch? Could it be the control panel? There is one light on control panel but it doesn't illuminate, no blink code, nothing. Could it be something else? Thoughts?
Jill on February 08, 2017:
Just replaced an oversized furnace with the right sized one as i was having issues with the furnance constantly coming on. I lowered the thermostat several degrees during the day so it would not come on as much. With the new furnace I thought this would all be resolved but it has not. It comes on more frequently and even with the temp set lower it comes on every 10 mins. Any advice? I've been dealing with this for 2 years and I just want an answer!
Wayne Hogan on January 31, 2017:
Have a 2-stage furnace. furnace runs great on high, runs for about 2 minutes on low then gas shuts the gas valve and the cycle repeats many times till thermostat is met. I have replaced the roll out switch and flame rod and it still occurs. Any suggestions?
Chris taft on January 22, 2017:
Carrier weathermaker has been having flame out problems, had a tech come out and tell me that the intake/exhaust wad too small and needed to be 3", so all that is changed and now it seems like it won't cycle back to back without having an issue. Turn power off to furnace and back on will work for 1 maybe 2 times. Any advice?
Dan Reed (author) on January 22, 2017:
Michael Riemer - Not real sure. Could be too large of BTU. Could be the LP kit to make it LP wasn't adjusted properly. Could be a new stat is more accurate and the old one wasn't keeping the temp as consistent. Sorry but this is a tough one that would require further investigation onsite than I can provide here.