Control Nest Fan when fireplace is on?

What I’m looking to do is automatically turn on my HVAC fan only when my fire place is running. My thought is to watch the temperature in the room and if its above, say 80 degrees, and the outside temperature is below 40 degrees, the fan would turn on for 30 minutes to circulate the air in the house. I have Nest Manager and I have CoRE, i’ve been playing with both of them, but cannot figure out how to do it. The “for 30 minutes part” is where I’m getting hung up. I can turn the fan on given those conditions, but figure out the best way to turn it back off.

Well, just in case anyone searches for something similar and lands here, there solution I came up with was to use a recursive “Follow Up” piston to accomopany the primary piston.

The primary (type basic) looks for the room temp to be greater than 80 and the outside temp to be below 40. This turns the fan on. A second action then runs a “Follow Up” piston.

The follow up piston checks the temperature, if it is < 80 degress it sets the fan to Auto. Else, it calls itself again with a 5 minute delay.

So what happens is, temp rises above 80, fan turns on, the follow up piston then checks every 5 minutes to see if the temp has fallen below 80, when it is, it turns the fan back to auto.

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How are you getting this to work? When I set my nest fan to auto after it had been called to run, it just continues running until it’s times out

Have you used the “Wait” function? i have several CoRE aplications that turn on > then wait 30min > then turn off…

My (gas) fireplace has 2 switches. One for the fan which is 120v with neutral. The other is a switch that simply open/closes a connection to the thermalpile.<—more on that later.

If your setup is the same, the fan comes on when the temperature of the fireplace reaches a certain temp. I believe mines is hard set for 120 degrees. You can Install a Smart Plug for the fan switch and now you have way of telling ST if the Fireplace is on/off. If wattage is greater than…I actually installed a Smart Switch to the fan switch for a more advance setup…(See Thermalpile below).

Now for the thermalpile; Since these work off of sensing small voltages across radiant heat, such as the pilot, regular Smart Switches will not work. What I did was use the fan on/off to a 120V to 24V converter to a 24V mechanical open/close relay. I used this to open/close the circuit to the thermalpile.

Before the safety police get me. This is actually safer, IMHO, than the dumb switch. I have multiple redundant rules to make sure the fireplace does not come on when no one is home nor in the room. I also have it setup where it auto shuts off once the room reaches a certain temp.

With the dump switch, the family just let it run all darn day without a care in the world for safety or cost.


as far as I can tell the Nest Tstat has a built in fan timer… you can set only certain predetermined times. When the fan comes on… it runs the timer for whatever time was preset and runs until it times out.
Even setting the fan to auto after it has started the timer doesn’t cancel the timer…

Looking for another solution.