Thermostat kicks on fan briefly before heat

Hi @speadie and @raym64 ,
thank you for the suggestions. You are right about the fact that many traditional thermostats simply give the control of fan to the furnace control board. However, there are more than hundreds of different control boards under different brands with different control logic. Some systems have layers of safety valves and sensors, and some systems do not have, many of our customers are getting the new thermostat to have better control of the old system. Without independent control of the fan and the furnace, our customers may have a very different experience with the same product caused by different HVAC boards. Some advanced features like turn off the AC earlier and use the remaining cooling or heating, or fan operation for ventilation purposes will be impossible if we give up the G wire control. We are actively collecting user feedback to improve the product experience. I feel like the solution could be giving the user the option to leave the fan to the furnace as an advanced setting

All you have to do is not send a signal to close the relay that connects to G during the heat cycle if the user has selected gas or oil as their heat source. Let the thermostat control the fan during the rest of the time (allowing fan cycling, manual control of the blower fan, etc)- but relinquish control of the fan during a heat cycle to the furnace. Why bother asking users what type of heat they are using if you are just going to ignore it?

Thank you, yes please give the choice to the user. To be clear, I’m not saying G-wire control shouldn’t be there, it should. Even dumb thermostats often have a fan switch to operate the fan independently, that’s a good thing. I use the fan cycle feature myself and with my dumb thermostat I used the Fan independently also. What I and others with HVAC experience are saying is that no thermostat needs to or should send a G signal along with heat, especially not by default.
This isn’t even a user feedback suggestion, it’s an HVAC 101 error

I won’t install this version because it will still turn the fan on too early. Here is a graph of the power drawn by my furnace during a recent cycle (the furnace is controlling the fan by itself).

The igniter doesn’t even turn on before 15 seconds. The air in the plenum will be cold and this firmware version is no better for a gas furnace than the first version.

I set my thermostat as “gas - other” and that is working for me. Again, kudos to @Hookdog for discovering that combination of settings.

The very old systems that were not made with “layers of safety valves and sensors” did not have smart thermostats. At that time the thermostats were mostly simple mercury switches on bimetallic coils. The fan control was handled entirely by the furnace. It is safe to do the same today - leave the fan control to the furnace.

Having the ability to run the furnace fan alone with a 4wire thermostat wire was my primary reason for buying this thermostat. I may try removing the G wire for now but I hope this gets fixed before summer.