I want to have the room humidifier operate when the furnace comes on. The furnace is in the laundry room in a manufactured home. Is there a relay that I can wire into the duplex outlet to run the humidifier only when the furnace is running?

2/5/14 To clarify my question: The furnace is a Colman Downflow DGAT auto ignition 75000 BTUH model. Hunidifier is room size, Holmes ultrasonic that reviews state will run 12 hrs. but it can leave the floor water soaked. If the humidifier is near the furnace, then moisture can be drawn into the return on the furnace and probably won't saturate the floor, if only operating with the furnace on. I don't know if a simple relay or controls are available to operate this task, and I would like to ask for specs and instructions. Thanks for any reply.

  • 1
    How do you keep your humidifier full? (this is something I've been dying to automate my self)
    – virtualxtc
    Commented Feb 5, 2014 at 9:00
  • What is the make and model of the furnace? Some furnaces have controls for humidifiers built in.
    – Tester101
    Commented Feb 5, 2014 at 14:45
  • I wouldn't recommend switching a duplex receptacle in this way, since it could be very confusing for the next owner. Consider using a single receptacle, and good labeling.
    – Tester101
    Commented Feb 5, 2014 at 14:47
  • Are you looking for specific products (which would make this a shopping question)? Or are you interested in the type and specifications of a relay that would work? Are you interested in how the relay should be wired, or do you already know?
    – Tester101
    Commented Feb 5, 2014 at 14:50
  • 1
    You should consider a whole-house humidifier that mounts to and is hardwired to the furnace. These can turn on using the power from the furnace, signal from a thermostat with humidifier output (not all have this) or an air flow-activated switch.
    – gregmac
    Commented Feb 6, 2014 at 2:56

3 Answers 3


You can build a circuit to do what you're asking, though as others have pointed out it's not necessarily the best way.

What you can do is use a 24V AC relay to switch an outlet.

Waring: 120V AC is dangerous and can seriously hurt or kill you. If you're not aware of what the difference between hot, neutral and ground is, or the thought of "exposed terminals" doesn't scare you, stop now. Learn some basic electrical skills, find another way, or enlist the help of a friend.

Get a relay

You need a relay with a 24V AC coil, and which is rated to switch 120V AC (or higher) at 15A (or higher), it just needs to be SPST (single pole, single throw) though DPST or DPDT (double pole, double throw) is fine.

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Mount the relay in a box

You need a box to put the relay in to protect it from being touched (since there will be 120V on the terminals of this thing). There are many types of boxes, from metal junction boxes to PVC project boxes.

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Figure out some way to secure the relay base down in the box (PVC is a lot easier for this).

Connect power

You'll power this box by plugging it into a regular outlet. You need a power cord and a strain-relief connector.

enter image description here

Get a receptacle

Probably the easiest way to do this is to use the female end of an extension cord and a strain-relief connector. You can actually just buy an extension cord and cut it and half, and use this for the power in and out. You can even use one of the fancy ones with the lighted end, and you'll have an indicator if the relay is on or not.

enter image description here

Otherwise you need a normal receptacle which can either be mounted in this box, or attached via a surface-mount box (aka handy box). If you attach it externally, you'll need to use either armored cable (BX), or extension cord wire (SJO) and appropriate box connectors or strain relief connectors.

Wiring it all up

Here's the wiring diagram for your furnace:

enter image description here (Source)

The relay you get will have a pinout on it, something like this:

enter image description here


  • Connect your furnace W and C terminals to the coil of the relay (terminals 2 and 7)
  • Connect the hot (black) wire from the 120V plug to the power terminal of the relay (terminal 1)
  • Connect the hot (black) wire of the receptacle to the switched terminal (3)
  • Connect the two neutrals (white) together.
    • If you have a DPST relay like this diagram is for, you can optionally switch the white wires by connecting to terminals 8 and 6, mirroring the hot wires. It doesn't really matter.
  • Connect all the grounds together, and if you're using metal boxes, the grounds must connect to the ground screw on the box itself.

That's really all there is to it. When the thermostat calls for heat (powers W terminal), 24VAC will power the coil of the relay, which will turn the receptacle on.

You will likely be able to find all of this at your local box store. If they don't have relays, a local trade electrical or industrial supply store likely will, and I hear this internet thing sells stuff now too.

Good luck.

  • Warning: This may void the furnaces warranty. Also, if you install the relay in the furnace (if there's enough room), you'll simply have to run a single cable out to the controlled receptacle.
    – Tester101
    Commented Feb 6, 2014 at 11:05
  • Thanks for this outstanding answer. This helps me immensely with a project in the same vein, though different. One thing I'd like clarification on, though: does the code say about installing relay switches in residential structures? (NEC 2005 is my reference point)? Commented Dec 27, 2014 at 21:01
  • @BenCollins -- as long as the relay is UL listed (NOT just UL recognized /RU), it's fine for any type of structure. Commented Apr 3, 2015 at 16:28
  • @gregmac -- it'd probably be better to suggest a pre-wired relay (such as the RIBU1C, as it's UL listed and poses fewer screwup opportunities than using a UL listed plugin relay + socket pair. Commented Nov 18, 2015 at 3:14

If the two are close enough together, I'd recommend just using an advanced power strip.

Else, a more advanced home automation system would likely be a better way to go. I personally have gone with z-wave, an thus my controller is not only informed when the thermostats tell the furnace to turn on, but can also add a rule to turn on a specific outlet, or external plug. However, if you don't want to invest in a controller module there is another option:

You could use a Belkin Insight Switch with a Belkin WeMo in conjunction with IF This Then That to set up the rule that when the furnace draws more power, the WeMo is turned on.

  • ...uh, this "bad practice" has been done every day in industrial control panels and commercial building controls for a good century now. There is nothing wrong with having a relay in your house! Commented Nov 18, 2015 at 3:12
  • agreed... fixed.
    – virtualxtc
    Commented Nov 27, 2015 at 9:55

Yes, this is possible. I use a relay attached to the fan relay in my air handler to turn on a booster fan across the house. This is the relay I used: http://www.hvacquick.com/products/commercial/Sensors-Transmitters/Relays-Misc-Controls/Greystone-LY-General-Purpose-Relays

  • What type of specifications should the OP be looking for in a relay for this application? Is there a reason you chose the relay you did?
    – Tester101
    Commented Feb 5, 2014 at 14:48

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