Blower won't turn off

Hello,

I am having a problem with my furnace blower that started about 2 months ago. When the outside air unit turns off, the furnace blower sometimes doesn't turn off. Around that time, it happened once or twice a day. Everything was fine for about a month and it happened again on one day then it was fine until a few days ago. When this happened, the fan was always set to Auto on the thermostat. Turning the Air/Heat switch to off would have no effect and the blower would continue to blow air. The only way to stop it was to turn the furnace off at the circuit breaker for more than a minute, less time and the blower would come back on.

Three days ago it started happen again but turning off the power to it would not fix it. I did notice that it was less likely to malfunction during the night. Now it never turns off unless killing the power at the circuit breaker. When the house temperature gets below the set temperature, the thermostat clicks and the outside A/C unit turns off. After about 90 seconds when the blower should turn off, I notice about a 1/2 second drop in the air pressure coming out of the vents and then air coming out doesn't seem to be as strong as when the A/C unit is running.

I've done some research and what I've read seems to indicate that it might be a bad thermostat or fan relay. What would be the best way to figure out where the problem is?

The furnace is a Nordyne G6RA-096C-16. The air units are S3BA-024KA (outside) and C3BA-030C-B (inside).

 

Welcome to the forums.

A/C is one blower speed and heat is another.... usually slower.... speed.
It sounds like the stat is shutting off the blower correctly because you hear the blower slow down.
Go down to the furnace.... when the blower is on when it shouldn't be..... and look thru the blower door to see the control board fault LED. See what it is doing.

It sounds to me like there is a fault in the furnace area. Nothing to do with A/C.

 

Thanks for your reply. It's actually installed in the attic. Unfortunately it doesn't have easy access but I will try to get up there.

 

If you are in a hot zone and the attic is real hot..... you may be tripping a heat safety sensor on the furnace. That would be determined by the fault LED.

 

I don't think the board has an LED. Here is the control board (903106):
https://s3.amazonaws.com/s3.supplyho...m/919943-3.jpg

There is a Flame light and Status light. Would the Status light blink in a certain pattern if there was a problem? I still haven't gone into the attic.

 

Yes. Fault LED = status LED. You need to see the code before you open the blower door as that removes power from the furnace and clears the code(s). You'll find a description of codes on the back of the blower door.

 

Sorry for not replying to this earlier (last year). In the fall, the furnace would go through the heating cycle but the gas would not ignite when it was suppose to. I had the control board replaced and it was fine after that. The repairman said there was some corrosion on it which might have been causing the problems.