DIY whole house humidifier with Ecobee smart thermostat

We live in the Midwest, which means that in summer it’s devastatingly hot, while in winter it will regularly hit 0 F and the heat will run constantly, drying out your sinuses and turning your carpet into a giant fuzzy capacitor. We’ve been making do with little portable humidifiers for awhile, but I finally got around to installing a whole-home humidifier directly into our HVAC system for less than $250 and a few hours of work. It’s definitely made a difference, and if you do it yourself you can probably save around $1,000.

All in all, if you’re reasonably handy around the house, the work itself isn’t too hard. But it is a bit confusing to figure out what parts you need, and the wiring especially can be a bit of a head-scratcher because it’ll vary depending on your setup. Here I’ll go through what worked for me, and from what I can tell I have a fairly common setup, so I hope this helps some others.

Choosing a humidifier

This could be a whole post in itself, and I’m not an expert so if you want the full story I’ll just recommend you Google around. But suffice to say that it seems the most common kind are evaporative humidifiers — ones that drip water onto a manifold/water panel which has air forced through it and, thus, humidifies your home. There are also steam humidifiers, but those are much more expensive and more complicated. I chose an evaporative style because they’re simple and, from what I could tell, perfectly effective.

Most evaporative humidifiers are pretty similar in design — there’s a water connection, an electronically controlled valve, and a water panel through which the air flows. How the water valve gets controlled — when it is turned on and off to regulate the humidity — is really the thing that’s going to vary most depending on your setup.

This is where having a smart thermostat comes in handy. Since my Ecobee 3 has humidifier control built into it, I could skip all the “automatic” versions of humidifiers that involved hooking up outdoor sensors, wiring in an additional control panel, etc. (Those versions are also, predictably, more expensive.) Instead, I was able to get the AprilAire 600M (M for “manual”), which comes with no automatic controllers of its own and thus is very inexpensive at around $150. It’s good enough for 4,000 sq ft, and AprilAire is a well-known brand with plenty of history and seemingly a good reputation. Don’t let the “manual” designation fool you — this is exactly what you need if your smart thermostat can handle humidifiers.

Parts and Tools

Other than the humidifier itself, there are some other parts you’ll need to get (this list may vary depending on your setup).


  1. Relay – Functional Devices RIB24C-FA. You need this if you want a call for humidity (from the thermostat) to automatically spin up the blower motor, even when heat isn’t being called for at the same time. Trust me, yes, you want this. That Functional Devices one is the exact same as the “official” AprilAire relay, just without the AprilAire sticker and with $20 back in your pocket. You’re welcome.
  2. Hose clamps, sufficient to clamp the 1/2″ drain line onto the drain plug at the bottom of the humidifier.
  3. Pex valve. Exactly which one you need will depend on how you’re going to supply water to the humidifier. I had a 1/2″ hot water line for the bathroom sink less than 6 feet from the spot I was mounting the humidifier, so I went with the SharkBite 1/2″ to 1/2″ tee stop valve. More on that later in the article.
  4. Sheet metal screws. Matt got the kind without self-tapping (self-cutting) tips. Don’t be like Matt. Get the kind that have self-tapping tips.
  5. Sheet metal tape aka aluminum duct tape. The shiny stuff that’s used all over ductwork for sealing up joints.
  6. 1/4″ water supply line with brass fittings. You can sometimes find these marketed as an ice maker installation kit.
  7. 1/2″ drain tube. Flexible tubing is fine as long as you have a way to secure it/mount it so it stays pointed at the drain. You can also use PVC if you’re willing to deal with the cutting and sealing the joints.
  8. Appropriate 6″ ductwork and fittings (not pictured). I got a flexible/expandable 6″ HVAC duct and a 6″ mounting collar like this one. But you need to get whatever ductwork is right for your installation — the point is that you’re going to mount the humidifier unit on the air supply and you have to install a duct from the unit to the air return (or vice versa if you mount the unit on the air return).
  9. Some way to mount the relay and transformer inside your furnace housing (not pictured). I wound up using a 2-gang metal electrical box.
  10. Some small wire nuts.
  11. A couple feet of standard 14-gauge electrical wire and a few feet of 18-gauge wire. How much and if you need this depends on the exact layout of your wiring.


  1. Sheet metal cutters. The angled kind are easier to use on HVAC work.
  2. GLOVES. Do not attempt this without thick, heavy duty leather or rubber work gloves. You will slice your hands without them.
  3. Power screwdriver or variable-speed drill with screw driver head.
  4. Some pliers or a wrench to snug up the water connections.
  5. You might need some pipe cutters, depending on how you’re doing the water connection.

Plan your layout

I had a nice spot right between my return and supply that was just large enough to mount the unit.

The instruction card that comes with the AprilAire 600M is pretty straightforward so I won’t rehash it here. From my own research, I learned that while the instructions say you can install the unit either on the air return or on the air supply, people generally recommend putting it on the supply side (the air going out of your furnace to supply the house with air) if possible. This is because it’s considered better not to have the freshly humid air flowing directly into your furnace filter and the inner workings of the furnace itself. But either way works.

The other consideration is the ability to get the water supply and drain lines to the right places. Your furnace is probably near a drain, so hopefully that won’t be an issue. Mine was less than 10 feet away. I also had a hot water line (to my basement bathroom sink) about 6 feet away, so that was a great break too.


Once you have the layout sorted, physical installation is pretty easy. Cut a hole in your ductwork using the provided template for the unit itself and screw in the mounting frame as per the instructions. Cut another hole for the air line, using the 6″ mounting collar as a template, then mount that collar with the sheet metal screws and connect the air duct. Tape around all the seams.

Fully mounted with duct and water supply/drain lines installed. The position necessitated having the water connection on the far side of the unit, which isn’t ideal but I can still get to it if necessary.

My house has a pex water distribution manifold, with point-to-point water connections and valves for every individual tap in the house. So I could have installed a dedicated branch off the main manifold with its own valve, and I would have done this except my pex manifold is full (after finishing the basement I had used up all the open spots on it). I didn’t want to deal with expanding it, plus the humidifier uses such a small line, I decided it was more convenient to just tap an existing sink line running near the furnace using the tee valve. Use a hot water line if you can, it will work better.

To install the valve, obviously shut off the water supply first. Then just cut the pex with some appropriate pipe cutters and install the valve as shown. The valve uses compression fittings, so there’s no adhesive or soldering to worry about. Super easy.

Tapping a sink hot water line for the humidifier supply.


This is where things get a bit more complicated, but I eventually got it figured out. The main source of confusion is that there’s so many possible ways to wire this thing up, so sorting through the options (and comparing what you see on the diagrams to what you see inside your furnace and behind your thermostat) takes a little time.

The below diagram (one of the several included with the 600M) wound up being right for me. You can see that it’s labeled “manual control”, but really that’s what allows the system to be controlled by your smart thermostat.

The wiring diagram I used. For both the Ecobee and 24V transformer connections, it doesn’t matter which wire goes to which terminal.

One key question is whether you have the accessory wires already running up to where your thermostat is mounted. Fortunately, I did. I found three unused wires in the bundle coming out of the wall behind my thermostat, so I was able to allocate two of them to the ACC+ and ACC- terminals behind the Ecobee.

Congratulations orange and black, you get to control the humidifier now.

The humidifier needs a 24V power supply to control the water valve. The 600M comes with a 24V transformer. Sometimes, depending on your furnace model, you can get a 24V tap for accessories right from your furnace’s control panel. But when I popped off the panel for my furnace (be sure to shut the power off first — mine had a handy cutoff switch mounted right next to the furnace), I found that all the taps on my control board were line voltage (120V). It was pretty easy to tell; they were labeled as such. There was a 24V transformer already in there for the blower motor, mounted right next to the control board. I looked at the specs, and it probably would have worked to tap into that, but just to be safe (from overloads) I decided to use the transformer the 600M came with.

After deciding that, the main question was how to physically mount the new transformer and relay inside the furnace. As you can see in the picture below, I grabbed a metal 2-gang electrical box with flanges and just screwed one of them into the mounting plate that the furnace control board was on (which was how the existing transformer was mounted, so I figured that was cool). Then I was able to install the transformer and relay using the knockout holes in the box, along with the appropriate ring nuts. Note that this would probably not be “code” if all this was to be installed in a wall, but since we’re already inside the “line hot” environment of a furnace’s electrical box, I’m purely using this 2-gang as a mounting convenience and not for any other purpose.

New transformer and relay physically installed, but not yet wired up.

Once you sort out how to get the stuff mounted in the furnace, it’s just a matter of wiring it all up according to the diagram above. This is where you may need some extra 14 gauge (for the line side of the transformer) and 18 gauge (everything else) wire, depending on how physically close everything is.

Triple-check that your wiring is all correct, and then power everything up!

Configuring the Ecobee to manage humidity

After all the above, I figured I was home free and just had to configure some options in the Ecobee app. But after spending more than 5 minutes digging through the app, I couldn’t find any way to tell it I had a humidifier! I’m embarrassed to say I had to call Ecobee support. Turns out…you have to do this from the thermostat itself, not the app. I’m so used to just completely controlling the thing with the app that it didn’t even occur to me that the thermostat might have slightly different options.

Anyway, what you have to do is go through the equipment setup process again. To get to that process (this is for an Ecobee 3):

  1. Click on the Menu icon
  2. Tap Settings
  3. Tap Installation Settings
  4. Tap Reconfigure Equipment

Then follow the prompts. On the “Verify Wire Connection” screen, don’t worry if it doesn’t show your ACC+ and ACC- connections yet — that’s ok at this point. Keep going and answer the remaining questions. Make sure to select “2 wire” when you get to that prompt.

Once you complete the setup, you can test your humidifier to ensure that it works. Go to the test equipment screen, then turn on your humidifier. If you wired everything correctly, you should hear the blower fan spin up (thanks to the relay), and you should be able to feel/hear water running through the supply line. Turn off test mode, and you’re done! Then you can configure the humidifier level through your Ecobee’s comfort settings.


After a week, we’ve definitely noticed a difference. More than I expected, actually, considering that prior to this project our humidity level was around 30% according to the thermostat reading (Ecobee recommends a 36% setting). I wasn’t entirely sure how big of a difference 6-8% would make, but as it turns out…quite a lot. We wake up now without having our noses clogged and throats dried out. My skin isn’t as dry. The kids’ hair isn’t quite as effective for building up electrostatic charges on balloons. So suffice to say, for a few hours of work and less than $250, this was well worth it. Now we can finally ditch those annoying portable humidifiers.

19 Responses

  1. Paul says:

    I don’t see any mention of an outdoor temperature sensor. How does the thermostat balance indoor humidity with outside temperature?

    • Matt says:

      The Ecobee is controlling it, and from what I can tell it uses weather data for our location from the internet to get the outside temperature.

  2. JR says:

    The winter/summer switch on the duct side of the humidifier. What do you place that to in the summer or winter and do you switch it based on the seasons? I’ve been doing that for years but that was prior to my new ecobee that is now managing it

    • Matt says:

      I still close it off for summer. I just have a reminder on my phone that fires off every 6 months to remind me to flip it.

  3. Catalin says:

    Any reason you needed the 24V transformer? wouldn’t it be OK for the Ecobee & furnace to provide the 24V to the solenoid valve instead of wiring another transformer?

    • Matt says:

      I addressed that in the article…the humidifier came with the transformer, and to be safe I decided to just use it rather than tap into the one for the blower motor and risk messing up the motor.

  4. Catalin says:

    I guess I missed that. Thank you very much.

  5. Jey says:

    Your article helped me setup my ecobee4 and my Aprilaire 600m with the relay switch to kick on without call for heat. I tested and i see that the blower turns on and water flows as well. I had setup my humidity to recommended 36% and configured as evaporative humidifier, since i have hot water running to it. What’s puzzling is, so far i have not seen my ecobee call for humidity even when humidity fell to 30%. Not sure what i am missing or what i should check.

    • Chris Mizeur says:

      Hi Jey, Same problem. Did you get this sorted out? I turned the humidity up to 50% and it still won’t turn on. When I test equipment it fires right up.

      Thanks Chris

      • jey says:

        I have the ecobee 2-wire setup and i did make it work by changing the Humidifier type to steam. In evaporative mode it won’t fire with just the fan.

  6. Catalin says:

    I finally got around to installing mine. I do a test run using the Ecobee, and it seems it is running ok. One thing I noticed while it is running is that the water keeps flowing continuously down the drain. the flow rate seems to be quite high. I was thinking it will trickle down and not run. Is that normal, or is there something wrong with mine?

    • Evan says:

      I’m not an HVAC expert, but I believe it’s quite common for bypass or flow-through humidifiers to do this. Google around to verify.

  7. Jonathan says:

    Great article, thank you. Question on the blower relay – do you actually need it if you install the manual controller? I would think that if you configure it to keep the blower switch on you would accomplish the same thing as having the relay installed. Or is this not possible with the Ecobee controlling the system?

    I’m also considering the 700M instead if that makes a difference. I like that I don’t have to install a bypass with the 700M and that there isn’t a seasonal switch.

  8. Chris Mizeur says:

    Hi Matt

    Thank you for the article. I am removing an older humidifier and installing the Aprilaire 600m. My current Honeywell trusteam humidifier (discontinued junk) calls for the blower fan to turn on with no heat. I can’t figure if it is built into the unit or there is a relay somewhere. I don’t see it. Is it possible that I don’t need the additional relay Functional Devices RIB24C-FA mentioned in the article? Your two cents?

    • Chris Mizeur says:

      Also using ecobee4 thermostat.


    • Ed says:

      I’m using the same setup at my home. You don’t need any relay. Just use one wire (C wire for my case as I don’t have any other available wire) to connect the ACC+ of your ecobee to one wire of your aprilaire. Then the other wire of the aprilaire to the C on your furnace.

      (Ecobee) Acc+ to Humidifer – to C (furnace board)

      Then go to setting on the Ecobee to setup your humidifer as evaporative and you’re done.

      This video may give you a better picture at the wiring. It’s for the Nest but the wiring is the same for ecobee.

      • Chris Mizeur says:

        Hi thank you. I had it working the way you describe. I want the humidity to turn on without the heat. I need the blower to turn on when there is a call for humidity. I have a wood stove near the return. Sucks in a lot of heat. Matt’s description isn’t quite clear on actually physically installing the manual humidistat. I think I need to but I have been unable to locate any clear directions.

  9. Cristiano says:

    Hi! I read your wiring section two times but wasn’t clear for me, you connected both ACC+ and ACC- between ecobee and humidifier. What happens if I just connect the ACC+ to humidifier, and the other wire I leave connected to the power on the furnace panel? Will it be less efficient or will have less functionality with ecobee?

  10. Ben says:

    “This is because it’s considered better not to have the freshly humid air flowing directly into your furnace filter and the inner workings of the furnace itself. But either way works.”

    Regardless of how you mount the humidifier on the return or supply ducts, the air is going to travel from the supply to the return because when the fan is on (which is always when the humidifier is on) there is negative pressure in the return and positive pressure in the supply. The humidifier isn’t pushing any air, it is relying on this pressure to pass the air through the filter and humidify it. This is why bypass humidifiers are generally mounted on the return duct. Also if you happen to have a leak there is typically less risk on the return side since the furnace controls aren’t right below it (depends on installation obviously).

Leave a Reply

Your email address will not be published. Required fields are marked *