This thing was 35 years old when I moved in and it wasn’t new then. There’s no digital/computerized anything. When the heater (gas) is running and it reaches temp the burners shut off and the fan continues to run for a time delay. When the A/C is running and it reaches temp the outside unit shuts off and the fan shuts off immediately. If I manually run the fan after the A/C shuts off the temp at one vent will remain below the temp in the house for an hour. It seems like it has to be in the thermostate but I’m not finding anything with a “Fan Delay” option.
I’m guessing the heater operation may be a safety thing where the fan keeps running to purge gas maybe?
No idea, but when I turn off my new A/C the fan keeps running for a while. Our outside unit is quiet enough that I’ve never been able to tell if the fan runs after it shuts off. I’ll try to listen.
Typically, the furnace will not turn on the fan until the heat exchanger gets up to its operating temperature, so there is usually a delay between the burners turning on and the fan kicking in. When the burners shuts off, the furnace controller continues running the fan until the heat exchanger cools down. Without the air from the fan blowing, the latent heat in the furnace would otherwise cause the heat exchanger to get hotter and hotter (at least until all of the heat inside the furnace is transferred into the heat exchanger), possibly causing damage.
The furnace may be old, but this can be accomplished with very simple old-fashioned analog controls. Fancy shmancy digital controllers will often have dip switches or some other sort of control where you can set the time delay.
ETA: As @beowulff said, it also improves efficiency. You’ve already paid to generate the heat that’s left over inside the furnace when the burner cuts off. You might as well send it out to the house where it can do something useful.
As engineer_comp_geek says, it is the normal mode for earlier furnaces. It is accomplished with a “pipe” that extends into the plenum and a dial with movable set points. One to turn on the blower when the plenum heats up and another to run until it cools. The assembly is called the “fan limit switch” . Not so much a safety device as an efficiency device. Plenum over temperature safety is handled by a over-temp switch that usually has to be manually reset.
It’s not the thermostat that does it, it’s the controls on the furnace itself. On really old furnaces it might be discrete components, but on a modern furnace it’s going to be a control board located somewhere in or on the furnace.
Either that or it will be a separate unit like the fan limit switch in the above post.
As far as the cooling side goes, my HVAC guy says it’s an efficiency thing. Says the fan will run for 90 seconds after the compressor shuts off to harness whatever cooling remains in the evaporator coils.
I don’t think my A/C has this functionality. Never looked at it this close before in 35 years (no need). It looks like a jerry-rigged A/C addition to a heat only system. I didn’t think anything about the fan running after the compressor shuts off until I did. Ignorance WAS bliss.
There are two schools of thought on this. One side, your tech’s, says it’s for efficiency to harness the cold on the evap coil.
The other side thinks the fan should shut off with the compressor as it just picks up and evaporates some of the moisture on the coil, hence contributing to the humidity in the house, which of course is not desired.
I’m a retired hvac tech (although commercial equipment) and my own AC control board runs the fan for I think 30, maybe 60 seconds after the compressor shuts off. I with it would just shut off immediately but there is no way to change it on the board.
I CAN adjust the heating delay, 30, 60 or 90 seconds and THAT one does make sense. Might as well get all the heat you paid for as well as help cool down the heat exchanger.
When I run the fan manually you can feel the humidity going up. I’ve been doing 10 minutes, which raises the temp at the vent 10 degrees (10 degrees below the thermostat setting).
