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How to Install an Aprilaire Whole-House Humidifier, and More

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Dan has been in the HVAC industry for 23 years, with experience ranging from installation and service to sales and distribution.

Introduction to Home Humidification

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

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

In this article:

  • 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
Aprilaire humidifier installed.

Aprilaire humidifier installed.

Thinking About Buying A Whole-House Humidifier?

The cost of having a heating and cooling professional install a humidifier can be unreasonable. If you're doing it yourself, you can expect to spend around $250 for the unit and the necessary installation materials. Even if you have to purchase a tool or two, it's worth it: if you have an HVAC contractor perform the installation for you, the cost will in the neighborhood of $400-$500, if not more.

The whole-house humidifier pays for itself. There are countless benefits, listed below.

Benefits of Humidifiers

The benefits far outweigh the cost!

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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.

Materials

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)
This humidifier kit gets you started but you'll still need a couple items to go with it.

This humidifier kit gets you started but you'll still need a couple items to go with it.

  • 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.

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 setup. 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!

Location is important. This is a very easy set-up ,and clearly, I will mount the unit to the return air on the right and bypass to the back of the plenum.

Location is important. This is a very easy set-up ,and clearly, I will mount the unit to the return air on the right and bypass to the back of the plenum.

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.

humidifier-installing-an-aprilaire-whole-house-humidifier

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

humidifier-installing-an-aprilaire-whole-house-humidifier
  • 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.

humidifier-installing-an-aprilaire-whole-house-humidifier
  • 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.

Top left: the control for this system's zoning. Top right: control body. Bottom: humidifier casing.

Top left: the control for this system's zoning. Top right: control body. Bottom: humidifier casing.

  • 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.

The take off is installed at the top left in this photo. Notice how it's above the coil and facing the humidifier casing.

The take off is installed at the top left in this photo. Notice how it's above the coil and facing the humidifier casing.

  • 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.

Install the 6" elbow to the casing. Be sure the bypass damper can swing.

Install the 6" elbow to the casing. Be sure the bypass damper can swing.

  • 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.

Measure to cut your 6" warm air pipe to fit and finish up your bypass. Remember, edge to edge plus 3".

Measure to cut your 6" warm air pipe to fit and finish up your bypass. Remember, edge to edge plus 3".

  • 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.
Finish the bypass by taping the joints and installing two screws to each joint.

Finish the bypass by taping the joints and installing two screws to each joint.

Tap into existing drains.

Tap into existing drains.

Tap into the drain by cutting in a "tee".

Tap into the drain by cutting in a "tee".

Install PVC drain between "tee" and humidifier with the path of least resistance.

Install PVC drain between "tee" and humidifier with the path of least resistance.

Be sure to glue the fittings and secure the pipe.

Be sure to glue the fittings and secure the pipe.

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

humidifier-installing-an-aprilaire-whole-house-humidifier
  • 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

The 24V humidifier wiring is done in a very simple series.

The 24V humidifier wiring is done in a very simple series.

Put your spade connectors on to your transformer wires.

Put your spade connectors on to your transformer wires.

Carefully push the spade connectors onto the circuit board. Black to HUM and white to the 110V Neutral.

Carefully push the spade connectors onto the circuit board. Black to HUM and white to the 110V Neutral.

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

Connect your red and white low-voltage wires to the transformer. It doesn't matter which color goes to which side; just remember where they're going.

Connect your red and white low-voltage wires to the transformer. It doesn't matter which color goes to which side; just remember where they're going.

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.
Here are the connections to the humidifier control. Red is from the transformer and white goes to the humidifier.

Here are the connections to the humidifier control. Red is from the transformer and white goes to the humidifier.

I poked a hole in the duct and inserted the wires from the humidifier to hide my connections. White is from the humidistat to humidifier unit and from unit back to the transformer. Red is just a continuation from the transformer to humidistat.

I poked a hole in the duct and inserted the wires from the humidifier to hide my connections. White is from the humidistat to humidifier unit and from unit back to the transformer. Red is just a continuation from the transformer to humidistat.

Here, I protected my wiring from the duct to the furnace.

Here, I protected my wiring from the duct to the furnace.

10. Install the water line and valve.

Notice how the needle is recessed into the rubber gasket and that the gasket is seated on the valve so that the curvature of the two match up.

Notice how the needle is recessed into the rubber gasket and that the gasket is seated on the valve so that the curvature of the two match up.

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.

Install the saddle valve on the hot water line with the discharge pointing toward the humidifier. Be sure everything is securely tightened but not overtightened.

Install the saddle valve on the hot water line with the discharge pointing toward the humidifier. Be sure everything is securely tightened but not overtightened.

This is how you place the nut and ferule onto the 1/4" water line you are about to connect.

This is how you place the nut and ferule onto the 1/4" water line you are about to connect.