How do I wire my furnace humidifier so it comes on any time the fan is on?


I have an Aprilaire 600a humidifier from a few years ago and currently it's wired into my furnace with two control wires labeled "W/G" and "Cf". The "W/G" is connected to the W terminal on the furnace (call for heat). The "Cf" is connected to "C". My understanding is that any time call for heat occurs, W is shorted to C, which is detected by my humidifier and it enables the flow of water.

I would like the humidifier to run any time the fan is on, not just when there's a call for heat. The fan control is determined by the "G" terminal. Is there a way to change my wiring to do this?

Edit: Furnace model is Bryant 925t. Per the manual:

The HUM terminal is a 24 VAC output, energized when the gas valve relay is operating during a call for heat.

This seems like it wouldn't help since it's only available during the call for heat.

Edit 2: There is also a "EAC-1" terminal that is energized "whenever the blower operates". However, this operates at 115...

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You've asked several questions here.

Checking if it's 24V AC

First of all, by far the vast majority of furnaces use 24VAC for control wiring. However, it's always better to be sure rather than make assumptions, so there are several ways to do this:

Look at the spec sheet or manual for your model Look for labels on the control board, probably where the transformer lines come in Look for labels on the transformer itself Use a multimeter to measure between C and R terminals


Since you mentioned you have a 9GMXT furnace, it has a 24VAC dedicated humidifier terminal. The manual states:

The 24 VAC HUM terminal is energized when the low pressure closes during a call for heat. [..] Connect a 24 VAC humidifier to the 24 VAC HUM terminal and C screw terminal strip on the control board thermostat strip.

It's rated at 0.5 Amps max. The Desert Springs motor is 24VAC 4W (I had to go check on mine) which is 0.17 amps, so it's perfectly...

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I want to install a thermostat/humidistat combo that will allow for my furnace fan to kick in when the humidity drops.

In the summer, the heat will never run, and thus my humidifier will never run since currently, that is the only way to run the humidifier.

Q: Is it possible to wire up my furnace fan, to kick in only when the humidity in the house gets below the desired setting? - I do not desire to have the fan run 24/7, only when humidity needs a boost.

What parts are required to do this?
Please recommend which thermostat will perform this job.

Details so far:

At my thermostat location:
- there is a red wire, white wire, two black wires, and another white wire.

I have a GeneralAire 1042 humidifier.
Furnace is Vancouver made Airco 8300 Turbo (25 years old).

The 25 year old humidistat upstairs - was connected to both black wires. It no longer works, previous owners say the white wire is broken...

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I was looking for some help in wiring my Furnace Humidifier to the furnace and Humidistat. It would be nice to use the Upstairs programmable Humidistat to control the humidity but also only have the humidifier come on when the furnace is running.

I currently have it set up so a separate humidistat attached to the cold air return duct switches the humidifier on when the humidity drops below the level that you dial in. The problem is that the electric motor on the humidifier runs even when the furnace is not on. Also I have to go downstairs and adjust the humidistat every time the temp changes ( living in Alberta in the winter this happens daily).

These are the products I am using

-I have a NOVA programmable thermostat that inside has 2 terminals you can connect to for humidity control.

-I have a Keep rite 92% efficient furnace

- I have a desert spring drum style Humidifier

Below are a few diagrams from my owners...

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A little while back, I wrote about my first year using a Nest thermostat and since then several people have asked me for details on how I wired up my whole-house humidifier. I hesitated for a long time to post much detail, since it'd be really easy to completely fubar your HVAC, humidifier and Nest all in one fell swoop. But, after much mental consternation, I've decided to provide some detail, but with this really big disclaimer:

I am not an electrician, an HVAC specialist, a trained Nest consultant or frankly anyone you should trust. I provide this information solely as a reference implementation and provide no warranty that it's accurate or even safe. You should consult an electrician or specialist before doing anything you read here. Incorrect wiring can cause massive damage to your HVAC system, or even result in fire.

In determining how to wire things up, I relied on advise from the good folks at The Thermostat Forums, and you can also look to the DIY...

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1 Answer

Our aprilaire humidifier is only 4 years old and it does not seem to be working. We have static in the house and when I did test mode it never drained water like it was supposed to. Any ideas what I can do...

Hope this helps, check and see if humidistat is set to 35% humidity and that bypass damper on humidifier is in the open position or winter position.(humidistat is usually located on cold air return duct near furnace). Next, replace humidifier media pad, remember to replace media pad (filter) annually, you also may want to clean any lime deposit out of humidifier housing with a solution of warm water and white vinegar before installing your new media filter.Remember to turn power off at side of furnace when cleaning humidifier and replacing media. Before installing new media turn power back on to furnace, set humidistat to the on position, turn furnace thermostat above room temp., and inspect humidifier to see if water is running from top,(you may want to...

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Although all furnace-mounted humidifiers work under the same principles, two basic types are the most common.

The humidifier may be a flow-through type, which is shown here and is the subject of this tutorial, where the water flows through the humidifier and drains out. Or it may be a reservoir type, which uses a reservoir of water and a rotating drum to introduce the moisture necessary for humidified air. (A third type is known as a "steam humidifier" is not discussed in this tutorial.) flow-through humidifiers, you don't have to worry about possible bacteria growing in the reservoir's standing water, which can cause illness or humidifier fever.

Flow-through humidifiers are often mounted to the air return duct and connect to the hot-air supply off the furnace through a humidifier supply takeoff duct, as shown in this photo. The supply takeoff bypass duct diverts some heated air to the humidifier by taking advantage of a natural pressure differential...

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neglia_admin November 19, 2012

As the outside air becomes cold and dry, so too does the air inside our homes. So how do we achieve a more comfortable environment? By improving our home’s humidity! Ensuring that you have the right level of humidity in your house can make the cold winter months a lot more comfortable and enjoyable for you and your family. At Carson Dunlop, we are often asked a variety of questions about furnace humidifiers. The most commonly asked questions include:

“Do I need a furnace humidifier?” “Do I have a furnace humidifier?” “How do I maintain my humidifier?”

Do I Need a Humidifier?

If your house is new, you may not have a humidifier. However, you may not need one because the foundation and wood framing are still drying out, releasing moisture into the air. In addition, new houses are “tight”, which means the air within them hangs around for a while before being replaced by dry exterior air. The air is around long enough to pick up...

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Best Furnace Humidifier

Like other humidifiers, a furnace humidifier increases the moisture in the air. But unlike other humidifiers, it adds moisture within your entire home. Designed as part of the home’s HVAC system, and often installed on the ventilation ductwork that is near to a furnace. Furnace humidifiers can only operate when the furnace is running.

Furnace humidifiers are able to humidify all the air in multiple rooms when the indoor air is relatively hot. Using a furnace humidifier plays a prominent role in winters or during cold months, because most homes will turn on the heating system which dries out the air. As furnace humidifiers have advanced with technology it has become safer to install and use, and cheaper to maintain.

With so many different brands and types of furnace humidifiers on the market, it can be difficult to make an educated choice when trying to buy this product. To help you with your decision, we’ve showcased our favorite furnace...

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Locate your humidistat.
Your humidistat may be located near your thermostat or mounted at the furnace on the plenum (sheet metal box). Set humidistat:
Set to [OFF] OR [0]
Move the humidistat dial to a "higher" setting until you hear a "click." This click indicates the current level of humidity the humidistat is sensing in your system. Set bypass damper (if equipped).
You may see the works open/closed, summer/winter, or it may not be labeled at all.
Set damper lever perpendicular to duct to "close" or set to "summer" setting.
Set damper lever parallel to duct to "open" or set to "winter" setting.

Important Reminder: Please read the section "Should I changed my water panel?

How do I know if my humidifier is working?

Typically, most humidifiers are installed to run when the furnace is running in "heat" mode. With this in mind, you should be able to see water draining from the humidifier while the heat is running or for a short time...

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The pictures in this section illustrate how to disassemble the humidifier. Without the use of a volt meter to verify that power was reaching the solenoid valve and fan motor, my only option was to visually inspect the various parts for any sign of damage that would cause the humidifier to fail. I opted to first inspect the fan motor and control board since the symptoms indicated I wasn't getting any power to the fan or solenoid valve.

Disassembling the Humidifier

1) First, UNPLUG THE HUMIDIFIER from the power source.
2) The humidifier body is easily removed by pulling down on the plastic locking tab (photo #1) and swinging the cover up and toward you while lifting slightly. This will release the humidifier body from the rest of the assembly.
3) Notice the four screws in photo #2 (and also in a closeup in photo #3). They have been loosened so the plastic frame that holds the guts of the humidifier (fan, motor and control board) in position can be removed.

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Before you do anything else, turn the furnace power off. Most likely your furnace has a main switch, make sure it is in the off position.

Remove the furnace cover.

Locate the connection diagram on the inside of the cover. I will talk about it in a second. For now I want to focus on the overall connection diagram for the booster fan.

If you take a look at the fan diagram below you will notice some of the parts I've been talking about, booster fan, relay, blower motor, etc.

One thing to keep in mind is that all the info here refers to MY furnace, your connection diagram might be different but the theory behind all this is the same.

OK, let's dissect this diagram.
The dots represent connection points in our circuit, they can be on the controller board, in a junction box, etc. More wires can go to those points, I've shown only the ones we care about.
Let's start with the blower motor. Like I mentioned before,...

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