How to Install an Aprilaire Whole-House Humidifier and More

How to Install an Aprilaire Whole-House Humidifier and More

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Introduction to Home Humidification

Adding a whole-house humidifier to your heating and air conditioning system is something you can do yourself to improve your air quality and comfort.

These easy-to-follow, step-by-step instructions and photos will show you all you need to know to install, operate, maintain and understand your furnace humidifier using an Aprilaire 600M unit as an example.

Table of Contents

  • Thinking About Buying a Whole-House Humidifier?
  • Benefits of Humidifiers
  • What You Need for Installation (Materials/Tools With Photos)
  • Where to Install a Humidifier
  • 12-Step Humidifier Installation Guide (With Photos)
  • Setting and Controlling Your Humidity Level
  • Basic Humidifier Maintenance and Parts
  • Conclusion

Thinking About Buying A Whole-House Humidifier?

The cost of installing or having a heating and cooling professional install a humidifier for you is really rather reasonable. If you're doing it yourself, you can expect to spend around $250 for the unit and necessary materials to install it. Even if you have to purchase a tool or two, you'd likely get out of the HVAC system upgrade for around $300 since you likely have most of the tools you need. If you have an HVAC contractor perform the installation for you, the cost will still be "earthly" and in the neighborhood of $400-$500.

Besides cutting down on your use of moisturizers and boiling pots of water on the stove, here is a list of benefits a whole house humidifier will provide to help pay for itself.

Benefits of Humidifiers

The benefits far outweigh the cost!


Significantly reduces the amount of airborne dust in the home, thus reducing the suffering of those with allergies.

Lowers or even eliminates static electricity.

Save on energy bills—your furnace works harder to heat dry air.

Deters the spread of viruses that thrive in drier environments.

Reduces snoring

Wood floors and cabinets will maintain their appearance and last longer.

Prevents skin from drying and itching and lips from chapping

Helps keep house plants healthy—looking at dying plants doesn't conduce a feeling of comfort.

Increases home value—humidifiers are rather inexpensive but add value to your HVAC system, thus your home. In this way, they will more than pay for themselves.

What You Need for Installation

Now that we've decided adding a humidifier is a good and affordable idea, let's get started with installing a unit ourselves using a rather basic Aprilaire humidifier as an example. If you're going to hire a heating and cooling service provider for your install, perhaps you'll want to skip down to the maintenance and humidity control sections of this page.


  • Aprilaire Humidifier Kit (recommended) – Confirm the model is sized properly for your home.

Each kit comes with:

  • Unit with filter/panel and built-in damper
  • Transformer (110v/24v)
  • Saddle valve (to tap into existing hot water piping)
  • Humidistat (humidity control)
  • Installation template (for fitting the unit and humidistat)
  • 3/4" PVC pipe – 10' should be sufficient if you have an A/C or furnace drain you can tap into.)
  • 3/4" PVC fittings – 1-tee, 4-90o elbows, 2-45o elbows, and 1-coupling should do, in most cases.
  • PVC cement – Very little is required and can be substituted with a waterproof silicone.
  • Small wire nuts – Small and usually blue. Six would be the most you'll use
  • 2 spade terminal connectors – These are usually small and blue.
  • 1" sheet metal screws – 6-10 should be fine.
  • 1/2" sheet metal screws – 10 or so should do.
  • 18/2 thermostat wire – This is low voltage wire (24V).
  • Foil or duct tape – Again, very little is needed.
  • 1/4" copper tubing – The required length will be the distance from the humidifier to the nearest hot water line.
  • 18/2 thermostat wire –This is low voltage wire (24V).
  • 6" round warm air pipe – You'll need a 5' pipe.
  • 1–6" take off – A 6" start collar and elbow could be used instead.
  • 6" adjustable elbows – Depending on your set up, you'll need no more than three.

PVC pipe and fittings, wire nuts, spade terminals, screws, wire ties, and thermostat wire.

Note: You may only need some of these, depending on the humidifier you're installing. You may find that you already have tools that can be used as substitutes for some of these HVAC specialty tools. Before you buy any of these items, perhaps you should assess your installation compared to this example.

  • Tape measure
  • Marker
  • Hammer
  • Flathead and Philip's screwdriver
  • Cordless (battery-powered) drill and 1/4" hex driver
  • Tin snips
  • Pliers – You will want two pairs of standard pliers or channel locks.
  • Torpedo level
  • Tubing cutter
  • Awl
  • Crimping tool

Where to Install a Humidifier

Before we start cutting holes into our ductwork, we need to decide where we're going to install our humidifier. Your options may be limited by your existing heating and cooling system's set up. Again, I suggest you follow this example installation to the end to have a good understanding of what you will be doing and then make your decision.

Here are a few things to keep in mind—I've applied them to this home humidifier install.

  • Mount the unit on the return air duct whenever possible. You can mount to the supply side, but the system works much better on the return side and poses less of a threat to your furnace, should there ever be a malfunction involving water and electrical components.
  • Make sure there is enough room above the unit to mount your humidistat. It's suggested to be at least 6" above the unit.
  • Try to make the bypass connection as short and straight as possible.
  • Aprilaire units are able to be flipped around so that the bypass can be on either side.
  • Be sure you have room to remove the unit's cover, water pad/panel, and other parts for easy maintenance and repair in the future.
  • Visualize your drain path to be sure you'll be able to maintain a downward slope towards its destination. This ensures proper drainage and prevents stagnant water from building up in the line. Again, the shorter, the better.

Without further delay, let's start installing our humidifier!

12-Step Humidifier Installation Guide (With Photos)

A picture is worth a thousand words, so why don't I keep this as simple as possible? Below, I've listed the steps for my Aprilaire 600M installation example, but you don't lose anything to your imagination, I've provided pictures to go along with these steps. The two combined should give you a very clear sense of how to install your own humidifier.

1. Level and trace your humidifier template onto your return air duct.

2. Make a slit in the duct just inside the pattern about 3" long.

  • Place your flat head screwdriver against the duct at a slight angle and hammer it into the metal.
  • Alternatively, you may want to drill a screw in the duct and remove it for an easier start.
  • Twist the screwdriver to pry the slit open a little so that you can start your snips for the next step.

3. Cut out the humidifier and humidistat patterns.

  • Place the humidistat 6" above where the humidifier will be. It does not have to be located on the same side of the return air duct.

4. Mount the humidifier casing and control body.

  • Remove the cover and water pad from the humidifier and place the casing into the cutout on the duct.
  • Using 1" long sheet metal screws, mount the casing to the duct by running screws through the provided holes in the casing.
  • This Aprilaire unit requires 6 screws. Pull off the control knob so that you can pull the facing off of the main control body.
  • Using four 1/2" long sheet metal screws, fasten the control body to the duct in the hole you cut. Don't forget the foam gasket that goes between the humidity control body and the duct.

5. Locate, trace, cut out, and install the 6" take off on the supply duct for your bypass.

  • Remember, the shorter and straighter the bypass, the better.
  • Warning: If you have air conditioning, avoid mounting the take off right on the coil case, and if that’s the only place you can place it, definitely don't pierce a hole in the coil when cutting into the duct.
  • Now, slide the teeth of the take off or collar into the hole and fold them over to lock it in place.

6. Install the bypass piping.

  • Your path may vary from mine—just keep in mind that you want to take the path of least resistance when possible.
  • Connect your 6" elbow to the humidifier casing with two 1" screws. The casing has holes at the top and bottom of the damper connection for this. Make sure you adjust the elbow first and check that the damper can swing freely.
  • When cutting straight pipe sections, measure from the edge of point A to the edge of point B and add an extra 3" for your connections.
  • As you complete each connection, wrap tape around them, and install two 1/2" screws in each, across from each other.
  • If it's summer, close the damper. If it's winter, leave it open.

7. Install the drain piping.

  • Assuming you have air conditioning or a high-efficiency furnace, you can easily tap into one of those 3/4" lines by cutting in a "tee" and running your pipe from there to the humidifier.
  • You can measure edge to edge plus 1" to get your cuts for straight pipe.
  • Be sure to glue each joint and secure the pipe to the furnace or duct with 1/2" screws.
  • You can use pieces of your scrap metal for strapping.
  • If you don't have a drain already, you can run the pipe along the floor to a laundry drain, or you'll have to add a condensate pump to pump the water to its destination.

8. Connect your transformer. *Make sure the unit is powered off.*

  • Modern furnaces have terminals on the circuit board marked HUM and NEUTRAL for easier humidifier hook up.
  • Be sure you don't tie into the 24V common by mistake. The NEUTRAL is for 110V use.
  • Use pliers to squeeze your spade connectors onto the transformer's black and white wires.
  • Then, just securely press the black connector onto the HUM terminal and the white connector onto the NEUTRAL.
  • If you don't have these terminals, you'll have to tap into another 110V source using wire nuts.

Humidifier Wiring Diagram

9. Run the thermostat wiring (low-voltage wiring).

I have chosen to run most of my wiring through my ductwork, in this example, because it keeps it protected and looks much neater. However, with so many options and situations, I've provided photos and a wiring diagram that I believe will give you a better idea of how to tackle your specific needs. Just keep this in mind:

  • Any wiring outside of the duct from the ceiling down should be protected by some sort of flexible conduit.
  • Don't let the wire rub on sharp edges.
  • Make sure you make good connections. Don't over tighten, but be sure the wire is secure in whatever terminal type you're using.

10. Install the water line and valve.

Again, I think the photos will help you the most, so as you look at those, keep in mind:

  • Make sure the rubber gasket is seated properly, the "tee" handle is tightened to the mounting bracket, and the valve needle is retracted when mounting the valve to the hot water line.
  • Don't over tighten anything, and use two pairs of pliers to make sure you don't twist things as you tighten.
  • Once you've cut the line and completed the install of it, twist the valve "tee" handle all the way down to pierce the main and then loosen it to open the valve and allow water to flow to the humidifier.
  • Make sure you know where the nearest water shut off is.

In case there is a problem once you pierce the line, you'll have to shut off the water to that pipe to stop the leak and fix the issue. It is probably just a matter of tightening the connections a bit more, but worst case scenario—you can install an inline valve and make the repair to the main at the same time using push fittings. Simple as pie.

11. Re-install the water feed tube, panel and humidistat cover.

  • Place the tube in the valve.
  • Slide the nut and ferrule up to the valve
  • Tighten the two by using one plier to hold the valve and the other to tighten the nut.
  • With the tube connected, tighten the valve down to pierce the main.
  • Then, loosen it to allow water to flow.
  • Insert the feed tube into the water panel housing where it was
  • Snap the panel back into the humidifier by putting the bottom in first and snapping the top back into the humidifier housing.
  • The humidistat cover will also just snap into place.
  • Put the knob back on the control post. The knob only goes on one way.

12. Run a test cycle.

  • Make sure the power to the furnace is turned back on and that the main thermostat is set high enough to make the furnace run.
  • Place the doors back onto the furnace and turn the humidistat all the way up or to the "test" marking.
  • Within a minute or so, you should hear the "click" of the humidifier's solenoid valve opening and the water flowing into the water panel.
  • You can remove the hose to see if the water is coming, but when you do, make sure you point it at the panel near the bottom to catch the water.
  • If all is well, replace the cover on the humidifier and adjust your humidity control to your liking.

Setting and Controlling the Humidity Level

Once your humidifier installation is complete—and you've checked that all of your plumbing connections are leak free—you can run a test to make sure it's operating properly and set your desired humidity level.

Turn the humidistat all the way up or to the "test" level. This will force the unit to operate. You can confirm this by listening for the "click" of the solenoid valve open and for the flow of water and drainage coming from the unit.

The recommended starting setting will vary a bit based on the region you're in. That being said, 35-45% is the suggested range for most of us, but we want to ease into that. Start at 30% and let the humidity settle into the home before deciding to make adjustments.

Remember that things will begin to expand and soak up the initial humidity, so you won't get a true feel for what the proper level is until you let the unit work for a while and equilibrate. Consider waiting 4-5 days before fine-tuning the settings.

Tips for making adjustments:

  • Do a little at a time. You'd be surprised how much 1 or 2% can make a difference.
  • If windows and horizontal surfaces feel damp, the level is too high. This can cause wood to take in too much humidity and ice to form on windows.
  • If static is still keeping you from wanting to be near your loved ones, and you feel that the air is still dry, the level is likely too low. You should bump up your humidistat until you're comfortable (without making your home too damp).
  • Once you find a level that you're comfortable with, mark it on the humidistat so that when you shut down the humidifier for the summer, you'll know where to set it again when cold weather swings back in.

Improve Your Air Quality Even More

  • How to Clean Air Ventilation Ducts Yourself
    Cleaner duct work equals cleaner air and less dusting. Learn how you can improve the quality of the air in your home without any cost and specialized tools.

Basic Humidifier Maintenance and Parts

One of the best things about humidifiers is that they're so simple to maintain and there are only a couple of working parts.

Below are photos of these parts as well as a brief description of each one for easy reference, should you need them. As far as maintenance goes, just replace your water panel as needed.

A water panel doesn't have an exact lifespan. It largely depends on the water that it uses and the home that it's in. Be sure to check it for mineral build-up and/or deterioration. These are signs that your water panel needs replacing.

Other than that, remember: When we have plenty of summer humidity, and we want our humidifier shut down, turn your humidistat all the way down or off, and shut the bypass damper between the unit and the plenum (turn it to "summer").

This may be a good time to replace your water pad or panel since it's also a good time to replace your furnace filter with spring coming.

Tip: Change your filters after you do your spring cleaning since you'll likely be stirring up dust.

Humidifier Parts

The water panel casing holds the water panel.

Aprilaire 600M Humidifier Kit Review

This Concludes Our Session of Humidifier 101

Perhaps this is more than you ever wanted to know about humidifiers, but I wanted to provide a complete guide, not just on installing a whole-house humidifier but also all the things that go with it.

Whether it be the Aprilaire unit I used in this example or any other humidifier brand, all of this information is applicable, and the project should only take about four hours to complete for the average homeowner.

Questions & Answers

Question: Can the 600 be installed on a horizontal furnace in a home that’s 1,340 square feet?

Answer: Yes. Your manual should show that. If you don’t have one, you can probably find it on the internet.

© 2014 Dan Reed

B_Erie on January 19, 2020:

Hi Dan,

Great write up, very helpful!

I have everything hooked up as you described. But not getting power to the solenoid. I am wondering if it's the 24v converter. I have it hooked up to the HUM and NEUTRAL as you describe. I did notice however that on my furnace circuit board, next to HUM it says 24v. Wondering if it is already at 24v, and the converter isn't needed? Or do you think I just need to tap into a different power source?

Nuno Freire on December 11, 2019:

Will this setup run the humidifier if only the blower is on and not the heat. I would like the humidifier to run when the fan comes on independent of the heat.

Brian can on January 02, 2019:

I put a new powerd April air on and my hum numbers are real low. I have good water flow

Dan Reed (author) on October 16, 2018:

The drain must be at the bottom or all the water will run all over the place.

Dave on October 13, 2018:

Hey Dan - is it an issue to install a humidifier horizontally?

Dan Reed (author) on September 05, 2018:

Shiva - it is different because you have an outdoor sensor you have to wire in but your instructions should tell you how to do so in your specific application.

Shiva on September 04, 2018:

Hello Dan. Thank you for the instructions. They are very helpful. I have bought the same humidifier but with automatic humidistat control [600 model]. Is the wiring to the humidistat is any different or it is the same?

Dan Reed (author) on March 07, 2018:

Jayme - Yes. You can install a humidifier on ductboard and as long as it's installed proper you'll be fine but that goes for any install.

Jayme on March 07, 2018:

Can you install on installation foam board as I don’t have metal ductwork.

Dan Reed (author) on February 19, 2018:

I really can't say for sure on a specific board but I do know the neutrals are often sectioned off on their own sort of and if they are all occupied there spade terminal splitters you can get that will turn a single into a double.

Dan Reed (author) on February 19, 2018:

There should be a neutral section on the board that's common to all the 110 circuits, it just may not be located right there by the HUM terminal.

Eric on February 15, 2018:

Follow up to my last post, the circuit board is a Nordyne 624790-A for reference. In the corner of the board, it has two terminals labeled "LINE N" and a terminal labeled "XFMR N", are these the dedicated neutrals I can hook up to? (all three are currently in use) If so, does it matter which one I hook up to?

Eric on February 15, 2018:

I have the Aprilaire 500m unit. My circuit board has the HUM (110v) terminal but no neutral terminal. Where else can I hook the white wire from the transformer?

Dan Reed (author) on February 04, 2018:

It sounds like you have an upgraded model. Keep in mind the example in my article is using a 600M...a very simple unit. You should have wiring diagrams in with the unit you bought that will help with yours.

Aprilaire on February 03, 2018:

Thanks for putting so much effort into this article. Helps but it still didn’t work. I have on my April aire humidistat r,c,w,h,h,gf,a,b, g,odt

So the diagram doesn’t workout.

I bypassed it for now so it will run. Any suggestions will help

Dan Reed (author) on February 15, 2015:

Yes but not much and your installation guide should help you with the outdoor sensor wiring. I don't remember off the top of my head however I know it's quite simple. The rest of the wiring is basically a circle going from the furnace to the stat (red), from there to the humidifier solenoid (red still) and back to the furnace (white/common). The stat acts as a switch triggered by the humidity reading so that it won't come on unless the humidity is less than what you a setting it to call for so the 24v just goes to the stat when the heat comes on but only makes it to the solenoid if the stat then "closes the bridge" allowing it to pass to the valve so it will open. This is where the work is done so it then sends the negative voltage back to the common on the furnace and that's it.

Kenny on February 15, 2015:

Thank you soo much. Very helpful. But the wiring still confuses Me. I have an automatic 500 aprilaire with outside temperature. Will wiring be different for me.

Dan Reed (author) on March 04, 2014:

Hello Millionaire Tips. The only time you have to worry about mold with a humidifier is if it's not used properly. People who say their humidifier caused mold usually have it set too high and/or forget to shit it down in the winter. Like most products, it just takes proper care...I highly recommend having a humidifier. It's one of the most beneficial of hvac accessories and affordable. Thank you for reading!

Shasta Matova from USA on March 03, 2014:

Congratulations on being one of the top 10 hubs for Friday. I've seen advertisements for the Aprilaire humidifier, and have been seriously tempted. Do you have to worry about mold with a humidifier?

Watch the video: Presents Aprilaire Model 400 Drainless Bypass Humidifier (July 2022).


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