Furnace cycling strangely

[COboondocker]

This weekend it was about 10 degrees and we were at elevation (9000'). When I set the thermostat at 61 degrees or lower it cycled flawlessly. When it was set to 62 or above it would run and cycle off normally. When it started to cycle the second time it would run for about 10 seconds and not ignite, the ignitor wouldn't even click, then turn off and never try again. If I clicked the thermostat off then back on it would work just fine again and go through the same cycle. Any ideas why a higher temp would cause this? It's an Atwood 8520.

[wa8yxm]

donn0128 wrote:
Temp cool down. Totally normal. Home furnaces do the same thing.
The box cools, fan shuts off. Residual heat rewarms the temp sensor, fan runs again to cool the temp sensor a second time.

Very good description of how the Furnace in the S&B works, Now what you need to know is that RV furnaces..... Work differently.

IN the RV world the blower comes on BEFORE the burner, and remains on not till the temp drops but for a set time after the burner shuts off. So it's not T-Stat controlled.

My guess is one of two things

The T-Stat has a problem (SUSPECT #!)
The batteries rand down due to the longer run time and the T-Stat control voltage went below minimum causing a lock up.

My T-Stat.. won't do that but my OEM, in thory, could have.

[COboondocker]

dougrainer wrote:

COboondocker wrote:

dougrainer wrote:
I would suspect a bad Ignition module. The fact that it would run the fan for 10 seconds and then shut OFF and never try to restart points to a Module. It takes LONGER than 10 seconds for the Ignition sequence to start. Doug

But it never failed once when I turned the furnace off then back on at the thermostat...And it might've been 15-20 seconds. It was more of an estimate. It would shut off around the time I would normally hear the clicking ignitor.

This is what fools some people. WHY would it function If manually restarted?. That is why you take an educated guess and replace the Module. There is a problem in the board logic. Doug

The board logic thinks there is a 3 try ignition lock out. By resetting the wall tstat, you remove the lock out from its memory

Thanks. I just wanted to make sure I was being clear about exactly what was going on. I'll give that a try. Should I contact Atwood to get a replacement module or is there a better place to go?

[dougrainer]

COboondocker wrote:

dougrainer wrote:
I would suspect a bad Ignition module. The fact that it would run the fan for 10 seconds and then shut OFF and never try to restart points to a Module. It takes LONGER than 10 seconds for the Ignition sequence to start. Doug

But it never failed once when I turned the furnace off then back on at the thermostat...And it might've been 15-20 seconds. It was more of an estimate. It would shut off around the time I would normally hear the clicking ignitor.

This is what fools some people. WHY would it function If manually restarted?. That is why you take an educated guess and replace the Module. There is a problem in the board logic. Doug

The board logic thinks there is a 3 try ignition lock out. By resetting the wall tstat, you remove the lock out from its memory

[COboondocker]

dougrainer wrote:
I would suspect a bad Ignition module. The fact that it would run the fan for 10 seconds and then shut OFF and never try to restart points to a Module. It takes LONGER than 10 seconds for the Ignition sequence to start. Doug

But it never failed once when I turned the furnace off then back on at the thermostat...And it might've been 15-20 seconds. It was more of an estimate. It would shut off around the time I would normally hear the clicking ignitor.

[dougrainer]

I would suspect a bad Ignition module. The fact that it would run the fan for 10 seconds and then shut OFF and never try to restart points to a Module. It takes LONGER than 10 seconds for the Ignition sequence to start. Doug

Long shot would be a bad Hi temp sensor. The furnace is in its cool down cycle and the fan shuts off, but because the Hi temp switch is bad, closes again to let the fan run for that 10 seconds and then shuts down again. BUT, that would NOT stop the next called for heat Cycle.

[theoldwizard1]

COboondocker wrote:
When it started to cycle the second time it would run for about 10 seconds and not ignite, the ignitor wouldn't even click, then turn off and never try again.

The "never try again" is normal after a failure to ignite. Not sure why the igniter does not try to light after the first time.

That 10 second window is pretty short for doing diagnostics, but that is what you need to do. During that "window", use a 12V test light to check for power at both sides of both gas valves and the 12V feed to the igniter.

[COboondocker]

donn0128 wrote:
Temp cool down. Totally normal. Home furnaces do the same thing.
The box cools, fan shuts off. Residual heat rewarms the temp sensor, fan runs again to cool the temp sensor a second time.

But it never turns back on. I let it drop down to 50 before flipping the switch off and back on at which point it ran perfectly. And why would it do this at 62 degrees above and not below?

[donn0128]

Temp cool down. Totally normal. Home furnaces do the same thing.
The box cools, fan shuts off. Residual heat rewarms the temp sensor, fan runs again to cool the temp sensor a second time.