3rd Gen Nest STAT how to control humidifier

[MP42]

We have an April air humidifier with its own humidistat on the return air duct, but I’d like to know if my third generation nest thermostat can be wired to control the humidifier.

[meder]

Looking at the same thing.

Top post on Amazon for the Aprilaire 600M says you can do it.

Gloria Hagenbarth

Reviewed in the United States on March 19, 2018

...I want to provide some insight to people who may want the same thing, which is a humidifier completely controlled automatically by a (3rd gen) Nest, and for it to run regardless of the heat being on or not. Our house is only a couple years old, and the furnace is fairly efficient, so it doesn't cycle heat long enough for the humidifier to get the RH level in our home to the desired levels.

I ended up purchasing the relay that many of these diagrams have, and I didn't even need it. I'm not sure what the purpose of the relay is, but as you can see in the attached picture, all the wiring in our system was done on the 24VAC terminals. This eliminates the need for the transformer, the relay, and the humidistat. Wiring this way allows the Nest to completely control the humidifier. When the Nest calls for humidity, it sends 24VAC on the * wire, so hooking that up to the humidifier solenoid, and supplying the common wire to the other completes the circuit when called. Wiring this way also requires 6 wires to the Nest, so I ran 18/7 wire by taping it to the existing 18/5 wire and pulling it through the wall to the basement....

[Tommarchetti]

do you have a pic of the wiring for the humidifier/nest  connection?

[Tommarchetti]

did you get an answer on this question, I have the same issue,

[meder]

Hi Tommarchetti,

After some research we connected the AprilAire 600M humidifier (brown in my case) trigger (not power) wire to the star (*) connection on the Nest 3rd gen just as described and updated the "pro" settings to recognize the humidifier.

It works as described.

Through the nest app on phone/tablet/pc I'm able to see the water drop(s) display under the temp display along with the word "Humidifying" above the temp display.  There is a % measure for the "Inside Humidity" below the temp sensor(s).

Under Settings -> Options -> Humidifier there is a toggle and sliding scale to set humidity % along with "Quiet Time" settings for time control.

To setup the parameters for humidifier control on Nest 3rd gen: 

Settings -> continue -> continue -> humidifier -> set type -> continue - on the star (*) humidifier wire symbol I used settings:

Humidifier

Hum Only

Use Fan

This allows you to set the humidity level to distribute without heating.  Heating will come on as needed by the temp sensor.  

Keep in mind that the humidity sensor is primarily in the Nest 3rd gen head unit so that's were you will be most "accurate" throughout the house. 

Hope that helps.

[Tommarchetti]

Thanks for that input. DO you know what terminals on the Nest wire block the two wires to the humidifier are attached?

[meder]

Hi Tommarchetti,

There are two yellow wires from the AprilAire 600M without any additional markings:

One yellow wire attached to "COM" on furnace.

One yellow wire attached to star (*) on Nest 3rd gen. 

Try that, let us know how it works. 

[Tommarchetti]

thanks

[jnunziante]

Very helpful. About to install an Aprilaire 600M to my Carrier unit with Nest gen 3 thermostat. Once completed, I shouldn’t need the humidistat any longer, correct?  And what about the relay and the transformer that come with the humidifier?  I assume the Nest now provides the 24V needed and controls the fan to come on whenever RH drops below a set amount and the humidifier turns on / right?  Thanks. 

[meder]

Once completed, I shouldn’t need the humidistat any longer, correct?

Correct.  There is nothing connected to my old humidistat and control panel anymore.  The nest was installed next to it and wires were all transferred over to the Nest. 

And what about the relay and the transformer that come with the humidifier?

The relay built onto the Aprilaire 600M is wired to the control board on your furnace.  I'm not aware of a transformer, but don't think it is required as the needed voltage for the Nest control panel is provided by the furnace power supply.  I think the transformer was used to power the old humidistat and its controls perhaps. 

I assume the Nest now provides the 24V needed and controls the fan to come on whenever RH drops below a set amount and the humidifier turns on / right? 

Correct. The furnace control board provides the 24v power to the Nest which is just a control panel and humidity/temperature monitor.  When you set the Nest to say 40% humidity it monitors and sends a signal to the furnace control board to turn on the furnace fan and activate the humidifier relay to pump water through the element. 

[jnunziante]

Thanks!  Very helpful.

The question I'm now thinking through is, do I want the humidifier to run independent of the heat or joined to it?  Said another way, if humidity levels are low, do I want the humidifier to be able to run with the fan but NO heat?  Or do I want the humidifier to run only when the heat is also on?

For a few reasons, I'm leaning toward the latter (humidifier can only turn on when A) relative humidity (RH) is low, and B) heat is also on), but want to make sure that I don't inadvertently have the heat running more than necessary.  Basically, I want temperature/heat to be the 'primary' function.  So, ideally, I'd want the following:

1. If temp is low but humidity level is above the target RH I've set, then have the heat turn on with no humidifier;

2. If both temp and humidity are below the targets I've set, have both heat and humidifier turn on;

3. If temp is at or above the target temp, but humidity is below the target RH, then do nothing - I don't want the humidity level to dictate when the heat turns on, and I don't want a low RH to trigger the humidifier and fan to run without heat.

Does that make sense?  

My understanding is that regardless, the wiring should be the same: one wire from the humidifier's solenoid valve to Nest's "*" connection, and the other solenoid valve wire to the "C" on the furnace board - no extra relay or transformer.  I believe I can dictate what comes on/off and when via the Pro settings on the Nest.  Or maybe what I'm looking for does require a relay somewhere in the sequence?

Thanks,

Joe

[meder]

Joe, 

Some good considerations re: humidifier vs. heating settings and I have asked myself the same questions thinking that relatively warmer than room temp humidified air will carry better through the vents than room temp humidified air.  

I have not seen a way in the Nest app to set the humidifier to control along with the heat setting. 

ie. Just tested by turning the humidifier to 50% and the water flowed along with the fan kicking on.  My house is 67F with the thermostat set at 65F so the heat did not also turn on.  Obviously turning the temp to 69F turned the furnace burners on also. 

Given all this I think that the humidifier control acts independently of the heating control and they cannot be paired.  

Someone else may have other experience with this or a modification to couple humidity and heat control.  There may be a setting on the furnace software to kick on the heat control when the humidity control triggers.  Figuring out your relative heat index spread would be important ie. every time the humidity control triggers, turn on the furnace +2 degrees hotter than current level or something like that.  

//Marcus