Fan blower motor of Bryant furnace keeps running after flame is off

Question

Fan blower motor keeps running in winder after the room temperature reaches the setting. Occasionally, especially in the case that the outside temperature is very low, the motor stops as expected. But most of the time, it keeps blowing air. However, it functions properly in summer, i.e., when the room temperature cools down to the setting, the motor stops.

The furnace model is Bryant 383KAV. After some online research, I checked that: (1) Fan is set to AUTO in thermostat; (2) Flame rollout limit switch HH18HA452 passes continuity check using multimeter, about 0.7 Ohms; (3) Temperature limit switch HH12ZB170 also passes continuity check using multimeter, about 0.6 Ohms; (4) LED light flashes a few times when the furnace turns on. Then it stays red all the time. I assume no error is detected.

I am confused now what could go wrong. Temperature Limit Switch HH12ZB170 has been replaced recently. Maybe I should manually disconnect each of two limit switches one by one to verify?

Answer 1

Furnace blower fans will continue to run for a bit after the flame shuts off to strip the remaining heat from the heat exchanger and bring it into the rooms the furnace is heating.

Think of it this way - the gas valve (the flame) is controlled by the thermostat, but the blower is controlled by a sensor mounted in the heat exchanger. If the heat exchanger temperature falls within a certain range the blower will run to make use of that heat. You will also notice that the flame will run on its own upon startup without the blower for a short period until the heat exchanger warms up enough to justify running the blower over it and delivering heat.

Answer 2

yes, I understand that part. The issue is that it keeps blowing air until temperature goes down a few degrees lower than the setting and then fire starts up again. My guess is that the switches lose certain sensitivity, so they sometimes work during extremely cold weather (say sub 0) when temperature drop is quick, but not during regular cold, say 30s and 40s.