Humidifier Control

Recently had a whole house humidifier installed on our furnace. My thermostat has the ability to control a humidifier. What I noticed is it will only work when the unit is calling for heat. Turning on the fan will not turn on the humidifier if it is needed. Trying to figure out a way to control the furnace fan to use a separate humidistat, or humidity sensor and use HE to control all that. The thought being to be able to run the humidifier even if heat is not needed.

1: On my thermostat there is the G terminal which is the Blower control. I know that if I put the fan in ON mode the fan is controlled from that terminal, or so I assume. If the unit is calling for heat is the G terminal still the fan control? If I put a NO relay contact from the RH to G terminal will this allow me to control the fan without affecting anything else? I'm not real conversant on HVAC stuff.

2: I have a Sage Doorbell Sensor. If I tie that into the W to C terminal would that work to tell me if the unit is calling for heat. Or alternatively from G to C would tell me if the fan is running when the unit is calling for heat.

3: Then I can use a smart plug to turn the humidifier on/off based on my humidistat and some other factors.

So the net result is Fan On based on Sensor, OR turn fan on by my relay if humidity is too low. Then energize smart plug to turn on humidifier.

What I really need to know if my thoughts on connection to the furnace terminals is correct. Certainly don't want to damage the furnace.

Make sure that your humidifier and specific installation are intended to be operated fan-only. According to my April Aire manual, hot water supply is recommended for fan-only operation.

The Principle of Operation also suggests that water flows when the blower motor is running (and humidity control calls for humidity). This particular product line seems to do what you want already, though I have not tested to confirm ... maybe I will the next time that I run the fireplace insert.

Yes and No. The humidifier fan runs when the unit is calling for humidification, which is controlled by the output on my thermostat. That output only comes on when heat is being called for. It does not come on when just the furnace fan is on by itself. The humidifier wouldn't really work if the furnace fan was not running. So my point here is to be able to turn on the furnace fan without heat being called for and thus run the humidifier, which has it's own fan and water valve.

Ah, I missed that part.

Generally speaking, there should be no issue with applying a dry contact relay to force the fan on (RH switching to G). It would be emulating what the thermostat is doing.

I believe that my humidifier starts to flow with a simple solenoid. If yours is similar, then you may be able to apply a simple Zooz Zen16 or Zen17 solution. They accept 24VAC power and control of dry contact relays.

If neither the air nor the water are hot, then I still have concerns about this solution performing as hoped.

It is connected to Hot Water. Probably won't work as well with no hot air, but should help.

My unit has both a blower and solenoid. They are controlled by 24 VAC from the humidistat, or thermostat in this case. There is 110VAC to the unit which actually powers everything. I could use a relay, but by jumpering the 24VAC input on I can do basically the same thing by turning the 110VAC on/off. Especially since I already have a spare smart outlet.

One of my main concerns, is whether the W and G terminals are tied together in the thermostat when it is in heat mode. If that is the case then putting power on the G will also put power on the W turning on the heat when not needed. I read somewhere that some thermostats do that. Just can't figure out on mine. Guess I'll just have to put a meter on and do some testing. I wish I could control the thermostat directly from HE, but doesn't seem possible. I can from ST, but hate to set that up again just to control the thermostat.

1 Like

If your thermostat has a mode to toggle the fan between Auto and ON, then you can try that. Let it run for a bit and see if the furnace also starts. One of our former houses was set up this way. I turned on the fan while cleaning to send airborne dust to the filter, and suddenly the house got warm. I almost forgot about that.

It sounds like your system is substantially different than mine, so I am pretty much of out my competence zone. I found this image, though, for anyone that doesn't speak typical thermostat wiring language:

After talking to some HVAC folks and doing some testing I did find out that the G and W terminals are not tied together in any way. The G is only used for manual, or ON, operation of the fan. When the thermostat calls for heat the W terminal comes on and then the control board in the furnace handles the fan. It actually fires the burner and heats up before the fan comes on and then reverses the process when the heat goes off.

So here is my plan. I'm going to use a Zooz ZEN17 unit to control everything. I can use the 2 inputs for detection. One will detect when heat is called for and the other when the fan is running in ON mode, ie the G terminal. I probably don't need the fan detection, but it will make it a little more failsafe.

Then using HE logic I can determine the humidity setpoint based on outside temperature and forecast temperature. I will use one of the relay contacts to turn on the humidifier when I need humidity. It will only operate if the unit is calling for heat or alternatively I will turn on the fan when I need humidity if it is not calling for heat. I will also turn the fan off, if I had it on, when it calls for heat so I won't be overriding the auto fan control.

Or at least that is my plan. Probably be some tweaking and changing as time goes on.

This topic was automatically closed 365 days after the last reply. New replies are no longer allowed.