Strange Furnace behavior

[jawnd393]

I have a Armstrong Ultra SX 90 furnace. One time last cooling season while running AC, the blower fan would not stop and the inducer fan was running continuously even when the thermostat was not calling for cooling and the compressor was off. I poked around tapping on relays, turning the thermostat up & down. I can't remember all the details but the inducer was running in cooling mode. The only way to stop both fans was to turn off power to the furnace. With power turned back on the fans would both run again. I can't think of any reason for the inducer fan to run in cooling mode. After leaving it shut down over night, it resumed normal operation the next day. Through the rest of the summer and all winter it has not failed again.
John D

 

[turbo6justin]

I had a similar problem recently with the house furnace on the heat setting. I tracked it down to a relay on the circuit board. I have a new board sitting here but it is an intermittent problem and is working for the moment. Mine has had this problem on and off for about a year, though it recently got much worse, that is why I ordered the board. Mine is just a sticking relay. If I tap on that relay if will allow the induicer motor to stop running but when it is acting up it will stay stuck on almost every time the heat demand is made and the inducer is called online. Why did it happen for you on the cooling cycle? Can't be sure but you should be able to work through the wiring diagram and figure it out. There should be two types of problems, one the relay is being called for and is turning on as it should, (which I would think is a problem with the board since it shouldn't be used in the cooling cycle). Two the relay is not being called for and is stuck closed, problem with the relay and thus the board.

YMMV but this was my recent experience, again I was on the heating cycle but yours seems to point toward the board too. Good thing is some boards, mine included, are not as expensive as I remember them being mine was about $100.

 

[rickairmedic]

Your problem is with the board it is the only thing that will cause this to happen in the furnace.


Rick

 

[jawnd393]

I really should get a spare board. An other time it failed and I found the soldered connections for a resistor had filled from heat. I resoldered it and it worked. You can see the resistor with the brown around the leads.

 

[Tdad2]

From the picture, it seems a quite old furnace & circuit board, isn't it ?

 

[jawnd393]

House was supposed to be built in 94 so that must be the age of the furnace.
Furnace is a Armstrong GUK100D20-1B

 

[bonneyman]

That's one of my pet peeves with modern furnaces. Everyone has got an electronic circuit board (usually outsourced overseas), and most of them stink! When you need a replacement, it's OEM or nothing.
It's hard to play furnace doctor from a distance, but, on furnaces I come across that the inducer and blower motor won't stop running, it's a limit control. If a temp limit opens on these new types of furnaces, the logic says "the furnace is too hot!", so it runs the fan (and typically the inducer) in an attempt to cool the furnace. I've even come across this in cooling mode if the furnace is in an attic. Worth a check.

 

[Dkramer]

On most modern furnaces if the furnace sees a limit, it will bring on the blower and inducer fan in an attempt to cool everything off. I have seen this happen in the cooling season on some furnaces located in an attic. It can get hot enough in the attic to cause some limits to open. The times I have seen this happen were when people were away on vacation and not leaving the AC on. They come home to find the blower running. I don't know where you furnace is located, but if it is in the attic and you get HOT summers, it might be something to think about.