Heat comes on with "fan on" but not "auto"

[akozols]

Hi All,

My first post on the forum. First a little background on me. I have a electronics background and currently am a field tech working on high speed printers. I can usually fix things around the house(washer, fridge, dryers) after researching on the web how things are supposed to work.

I've come across a problem that has me stumped. My sister in law has a Goodman heater. When the thermostat is set to FAN ON, the heater will come on and work normally, but the fan will stay on when the heat goes off(normal). If she puts the thermostat on AUTO, the heater will come on for about 15 seconds then turn off. I wasn't familar with heaters, but it sounded like a thermostat problem, so I installed a new one. This did not help, still has the same symptom.

What I'm trying to do is to search the web to learn how a heater works and all the components that are involved. It's hard to fix something until you learn how it's "supposed" to work.

Any ideas? Any help would be greatly appreciated.

Thanks,

Al

[paul52446m]

Hi All,

My first post on the forum. First a little background on me. I have a electronics background and currently am a field tech working on high speed printers. I can usually fix things around the house(washer, fridge, dryers) after researching on the web how things are supposed to work.

I've come across a problem that has me stumped. My sister in law has a Goodman heater. When the thermostat is set to FAN ON, the heater will come on and work normally, but the fan will stay on when the heat goes off(normal). If she puts the thermostat on AUTO, the heater will come on for about 15 seconds then turn off. I wasn't familar with heaters, but it sounded like a thermostat problem, so I installed a new one. This did not help, still has the same symptom.

What I'm trying to do is to search the web to learn how a heater works and all the components that are involved. It's hard to fix something until you learn how it's "supposed" to work.

Any ideas? Any help would be greatly appreciated.

Thanks,

Al

This would not be the stat. All the stat does is a make and break switch. How old is the furnace? 80+ or 95%? Up flow or down flow?
Model No.? Right now the first thing that comes to mind is a limit switch opening before the blower starts. Normally a limit would not open that fast,
but it could be getting weak. If the blower is running all the time then the limit would not open up. Get back with me. Paul What does the code lite tell you?