Bryant Gas Furnace - blower sarts and stops

montepix

We have a Bryant gas furnace (model 394GAD I think). When the thermostat calls for heat, the burners start up, then after 45 seconds or so, the main blower. After about 3 minute, the blower stops, but the burners remain on. Then 30 seconds or so later, the blower starts up again. Three minutes or so later, blower stops, burners continue, then after 30 seconds, blower starts up again. Keeps repeating this cycle until temperature on thermostat reached - then whole furnace shuts off (blower and burners). Any ideas?

beenthere

Blower could be moving too much air.
Or burners could be underfired.

montepix

Furnace has been in place for 18 years - worked fine until about 2 months ago. Changed out thermostat to see if that helped - it didn't. There doesn't seem to be any thermal limit switch that determines if main burners are hot enough before starting blower - just a time delay circuit - although, I agree, the system is acting as if there is one.

3 legged dog

Post a picture of the time delay if you can.

montepix

Timer circuit is located on the main control board. It is apparently solid state (according to the circuit diagram). I have ordered a replacement board and will try that. However, I was wondering if there really is an "air low temperature" limit in this furnace or not.

Tator1076

U can hear if the relay is cutting in or out. You may need to check and see if one of the wires is coming off the blower motor

montepix

Yes, the relay is opening and closing.

Tator1076

Its maybe lose wire or worst bad motor. You need to check to see if getting the right volts to motor then ohm the motor out.