The instructions for the humidifier I'm installing says to connect the humidistat wiring to the two terminals "marked 24 VAC IN". My furnace doesn't have terminals marked that.

I have R,C,W,Y.G. Am I out of luck or is this just different terminology for the terminals I already have?

  • Please include the make and model of the furnace. – Tester101 Nov 18 '13 at 13:45

Your furnaces controller is typically powered from a 24VAC transformer. Trace the incoming line-voltage power source - it should connect to a transformer. You can verify the output voltage is 24VAC with a multi-meter. Even if there is not a dedicated terminal, you can connect to the transformer output in parallel in order to power your humidifier. Often there will be a wirenut already present, but if not you can cut the transformer output leads, strip both ends and as well your output to your humidifier and connect all the stripped ends with a wirenut or other sutiable connector.

Alternatively you can power it with a dedicated 24VAC transformer.

  • 3
    Wiring the humidistat directly to the 24V transformer, will cause the humidifier to become active whenever the humidistat says. This is not ideal, since the humidifier needs airflow to work. The humidifier should only be active when humidity is low and the blower is on. – Tester101 Nov 19 '13 at 12:30

This is going to completely depend on the make and model furnace you have. If you simply wire the humidistat to the 24V transformer, the humidifier will be active only based on the humidistat. This is not ideal, since the furnace has to be moving air for the humidifier to function properly. You want the humidifier to be active when the humidistat is closed (calling for humidity), and the furnace is on.

Locate the control terminals

If you check the schematic on the furnace (often found on the inside of the access panel cover), you may find there are separate terminals on the control board for a humidifier. In some models these are 24V terminals, in others they're line voltage.

24 volt terminals

If they're 24V terminals, you're in luck. All you have to do is connect the humidistat wiring to these terminals, and you're done.

Line voltage terminals

If they're line voltage terminals, you're going to need a second step down transformer. You'll connect the primary side of the transformer to the terminals on the control board, then connect the humidistat to the secondary side of the transformer.

No humidifier control terminals

If there are no humidifier control terminals present, you'll have to get creative. Check the furnaces user manual, as it may have instructions on how to connect a humidifier. Though a common solution to this problem, is to connect one wire to the C terminal, and the other to the W terminal. This causes the humidifier to be active only when the furnace is calling for heat.

- Always follow all manufacturer installation instructions.
- Do not forget to turn off the power to the furnace while working.

  • This is promising. I've got no humidifier controls. So from your answer, connect C,W. I know W is the relay for the heat. What's C? – kareem Nov 18 '13 at 3:36
  • Ah. Common/Ground. Got it. So now it can turn on when the heat is on. Does that mean it can't be used in the summer when the A/C is used? – kareem Nov 18 '13 at 3:51
  • @kareem If you want it to also be active when the A/C is on, you'll have to find a way to switch between the W and Y terminals. So you'll use W in the winter, an Y in the summer. However, humidifiers aren't typically used with air conditioning. – Tester101 Nov 18 '13 at 12:21

R + C = constant 24x7 24vac

Use these lines only if your humidifier has an electronics board that needs a constant supply of power separate from call for humidity.

In addition to constant power you will need some way to switch it on and off so that humidifier is humidifying only when humidity is called for. I recommend investing in a thermostat with humidifier control built in and let it and or your programming of it figure out when to turn on and off rather than sensing W/Y/G especially if you have a need for humidity + AC during summer Honeywell and others have intelligence to safely support this without rusting out your duct work.


Get yourself a desktop or standup humidifier unit for your summer humidity needs. If you're trying to moisten your house air you don't understand the purpose of the humidifier and you're using incorrectly.

Fan assist humidifiers are problematic and many companies have turned away from them. They may be mounted on the hot side of your duct work, but this can warp the plastic and make it brittle depending on the furnace and the knowledge (or lack thereof) of the installer. Also they're known to leak, have their fans fail, or just generally make a big mess spraying water around.

The drum style is a mid road humidifier that is useful when you don't have a drain in your furnace room, but can plug up and make a mess if you don't have a water softener, or are incorrectly installed. They can be much worse in a drainless room as they can pump enough water if the float fails to flood the base of the furnace and the room.

Finally the flow-through style is the better unit to go with. It has a pad in the center with coated mesh that becomes saturated with water, at the base of this pad is a drainage tubing, so the excess water flows down into your house drain. The humidifier is installed on the cold side of the duct (avoiding the possibility of heat warping of the plastic). Heat is taken from the hot side of the furnace by installing flex ducting to the humidifier that has it's housing mounted on the cold side. The heat passes through the pad evaporating the water (so less moving parts that fail as with the drum and fan-assisted humidifiers).

The whole purpose of the humidity is not to make your house nice and humid, it's primary purpose is to make your furnace work at it's most efficient. Mainly, it is more economical and effective to heat water than it is to heat air. Injecting a little moister into the ducts lets the house heat better. That moisture is passing through the coils and past the burners meaning that anything living is getting cooked to death.

However, if you are using it in the summer a few things are going wrong.

  1. The heat is not being created that is required to evaporate the water very much at all, in fact. Your furnace fan is designed to run at it's fastest speeds during the summer when the A/C is running, so the coolness of the A/C is even helping to reduce the natural evaporating that may occur, and introduced water to the humidifier is more likely to be pulled into your duct work by the suction created by a fast spinning blower, causing rust and mess.

  2. The heat in the belly of your furnace is not killing bacteria and fungus. You're mixing warm air with water in dark cramped spaces. Spaces that often get filled with hair, skin, and crumbs of food stuffs which provide an endless buffet for the microbes and fungi.

  3. Electricity is being wasted on a humidifier that isn't humidifying. The power cost may be negligible, but if like me you live with heavy calcified city water, you're wasting humidifier pads, causing unnecessary scale buildup in the drain and in the humidifier, and running year round just causes a bunch of unnecessary wear on your humidifier solenoid.

  4. The last, and probably most important. It ain't that kinda humidifier. They make the desktop and standup units for a reason, and they actually do their job pretty good. The one on your furnace wasn't made for that purpose, and it'll never work as well as you want. You'd get more humidity into the air by filling your bathtub and sinks with water and letting it evaporate off.

  • Why are you assuming that one would run a humidifier in the summer?! – ThreePhaseEel Oct 31 '17 at 4:00

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