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cipherman 11-01-2011 02:04 PM

Humidifier Wiring to HAi Omnistat
I have an HAI Omnistat controlling a Trane XE90 and an Aprilaire Humidifier.

Problem I am having is the thermostat only has a single wire for Humidifier control which does indeed activate the solenoid on the humidifier when the humidity is too low. The problem is that it sometimes calls for humidity even if there is no call for heat. I thought the thermostat would call for both Fan and Humidifier in this case; but it only calls for the humidifier. So the humidifier runs without a fan which is useless.

I called the vendor and they said this was normal operation and something would have to be rigged up externally.

My initial thought was to strap the G (FAN) and humidifier wires together but I think that will also kick in the humidifier in the case there the Tstat calls for FAN and doesn't need humidity.

Any ideas on how to accomplish this?

The manual is here:

Thanks in advance

beenthere 11-01-2011 02:24 PM

Add a relay that is energized by the humidity terminal.

cipherman 11-01-2011 02:47 PM

What should the relay switch (I am assuming a NO relay)? I don't think it should switch the fan? Should the relay switch the 24V RC lead to the Fan G lead upon the Humidifier lead activating?


beenthere 11-01-2011 02:50 PM

Run a jumper from the furnace R terminal to one contact of the relay, then run a jumper from the other contact(NO contacts) to the G terminal of the furnace. Run the wire from your stat's humidity terminal to one of the relay's coil terminals, and then run a wire from the other coil terminal to the furnace common.

cipherman 11-06-2011 12:58 PM

Ok, ran into a snag. The Thermostat Humidifier Output lead is 24VAC not DC. So it won't operate a relay. Any other ideas on how to get this application working?

beenthere 11-06-2011 02:30 PM

All of the Aprilaire humidifiers I install, use 24 VAC.

cipherman 11-06-2011 07:33 PM

Right but relays for switching require DC. I did get it working.

I picked up a rectifier from Radio Shack and used that to convert the 24VAC from the Tstat to 24VDC so that I could activate the relay. The relay in turn switches 24VAC from the W Terminal of the furnace to the Solenoid of the Humidifier.

Here are the parts I used:

Rectifier (I picked mine up from Radio Shack)

Elk Heavy Duty 24V Relay

Works like a champ. Now the Omnistat is serving as the humidistat and turning on the humidifier. I decided to only activate the humidifier if the heat is on which is why I am switching 24VAC from W.

beenthere 11-06-2011 07:37 PM

The humidifier is suppose to wire directly to the thermostat.

cipherman 11-08-2011 09:34 PM

Yes, but this particular thermostat only calls for Humidity and doesn't have a setting to call for Humidity if only there is a call for heat. Hence the external circuit.

MisterTFM 12-10-2011 10:48 PM


Originally Posted by cipherman (Post 765387)

Right but relays for switching require DC. I did get it working.

why don't you just buy a 24 vac relay vs. 24 vdc?

Davep1392 11-09-2014 12:34 AM

Aprilaire makes an airflow proving switch (part 4592) that is part of the 24 volt control wire to the humidifier. This is just a pressure switch that closes when the fan is running and thus could allow the humidity signal to pass to the humidifier. The switch installs just downstream of your system fan on the ductwork. When your fan is off the switch would stop the humidity signal from starting up the humdifier.

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