Furnace is exhibiting strange behavior and I can't figure out what part has failed.

System is set to "Heat", fan set to "Auto", and temperature set to 74.

Room temperature drops to 70 and furnace will not start.

I move the fan switch to "On". a moment later, the fan starts running and I set the switch back to "Auto". The fan continues to run, and, the furnace heats the house up to the set temperature. Then everything shuts off and will not start again, no matter how cold it gets, until I move the fan switch back to "On".

OR

I move the fan switch to "On" and leave it there. The fan motor runs continuously but the furnace cycles on and off, maintaining the set temperature in the house, even if I run it this way for 8 hours.

Maybe it's the blower motor temp switch but I don't understand this behavior. Thoughts as to what is happening are appreciated.

TIA

 

That problem would suggest an issue with the thermostat. If it has batteries..... replace them.

 

I forgot to post more info about the furnace... at the link.

Goodman Furnace

 

Hi, if you jump out R&W at the Stat will the fan start as normal? The burner should start then the blower shortly there after.
Geo