I have an older Rheem forced hot air furnace which runs on oil. It's controlled by a Honeywell RTH6450 (5-1-1 Thermostat).

When I first started the system this year, I noticed the furnace fan would not shut off (continuously runs). I suspected the fan relay to be at fault so, being it's an older model, I purchased one from eBay. The relay is a Honeywell R8225A 1058. I installed the relay and no change. The system fan will not shut off.

While looking at the thermostat, when the ambient temperature reaches the set temp, the thermostat will show "heat off" but the blower fan will continue to operate. I'm at a loss as to what could be wrong.

Do you think it's possible I have (2) bad fan relays? (the original and the one from eBay). How do I test this relay?

 

Ok, doing more reading and I find that the "Fan limit switch" could be at fault. Going to look into that possibility.

 

Well, system is now operating normally. The culprit was the "fan limit switch" that was not adjusted properly. It took me awhile to figure it out but I now have the system running as it should.