Furnace Model: G50UH-36B-090-03
Furnace Internal Diag/PCB: White-Rodgers 97L4801
Thermostat:White Rodgers Model: 1F80-261

Furnace has been working without flaw since I purchased the house in Dec 2004. I set the thermostat low to save on heating bill while away. When I increased temp setting on thermostat the following symptoms began.

In heating "auto" mode when heating is called for (set temp higher than ambient temp shown on thermostat. )

1. Smaller fan starts up as usual.
2. I can hear gas valve solenoid actuate and gas is introduced into manifold and ignited.
3. After gas is ignited for about 10 seconds (guestimate) the gas shuts off but the smaller fan remains on.
4. As long as the thermostat "set temp" is higher than ambient temp and heat cycle is set to "auto", the unit cycles. In other words, with the exception of the burner not staying lit, the furnace (I think) seems to work normally.

I have switched the main power off to "reset" the system to no avail. I'm unemployed (what a shock) currently and on a tight budget. I'm mechanically inclined so I wouldn't be afraid to tackle this with some educated coaching. Any assistance would be appreciated. Thanks very much.
Bill T

 

Understand first that I am a complete amateur here for advice also. But, search for how to clean furnace flame sensor. The flame sensor sits in the way of the flames. If it doesn't sense flame it shuts off the gas. It needs to be cleaned occasionally and cleaning is simple. Don't mistake the ignitor for the sensor though or you'll just break your ignitor.

Search first for how to clean the sensor and see if that sounds like your problem.

 

Thanks for the info. You can't be more of an amateur than I. I do appreciate your suggestion. Easily found the flame sensor and cleaned it. There was a bit of residue but "scotchbrited" it just the same....to no avail. Still wondering if I did some harm to the furnace PCB when I set the temperature so low.

Thanks again.
Bill T

 

The blower door has a viewing window. Check for flashing lights while the system is acting up. On either side of the blower door should be a legend of how to read the diag lights. post results here.