Diagnosing Intermittent Heater Lock Outs

[AP114]

I have a 20 year old Snyder-General condensing heater Mod# GUH100A016IN.

Over the last 4 weeks, it has locked out twice. I cannot reproduce the lock out situation.

Here is what I know.
- I notice that there is a problem because the house gets cold
- I go down to the heater and the inducer blower is running
- The diagnostic light is flashing on the White-Rogers #50E47-140 control (just flashing continuosly, no numbered flashes)
- I turn the power off and turn it on again and everything works just fine for a week or two.

Any suggestions on how to troubleshoot this *

[Freon]

If it were a flame sensing issue I would expect a lockout AND nothing running. The fact your inducer is running is not expected on a normal lack of flame lockout.

Since it's intermittent it will be a PITA to diagnose. Here's a description of how your control should - *.*/wrdhom/pdfs/instruction_sheets/0037-4836.pdf

It seems the lockout will vary in length which implies that, after so many minutes, it will try to relight again. So I would suspect when you hear the inducer, it's trying to light the furnace. If you can, observe what happens when you're there. Does the ignitor glow bright red to * Can you hear the gas valve * If you're uncertain how this should look and sound, turn off the furnace and then when you turn it back on, watch as it lights. It is possible you have a flame sensing problem albeit a very intermittent one. You may also have an inducer pressure issue so check the flue to be sure it's clear.

[AP114]

Thanks Freon

The way this heater is works is that the inducer motor runs as long as the T-stat calls for heat, so it will only shut off when the T-stat gets satisfied.
When the T-stat call for heat the signal(W) does two -
1- activates the inducer blower relay and provides power to the WR control
2-the T-stat signal(W) also runs through the pressure swith and the high limit switch to the Th connection on the White-Rodgers control

That tells me that if the pressure switch were to open while the furnace is running, it would look the same to the WR control as if the T-stat were satisfied and not cause a lock out. *

[how]

When you find a furnace that has locked out without a logical diagnostic led code showing it usually means that the board is faulty or you have a poor ground connection anywhere from the board to the house service.

[Freon]

"That tells me that if the pressure switch were to open while the furnace is running, it would look the same to the WR control as if the T-stat were satisfied and not cause a lock out. *"

I agree. The WR control has no idea what is going on with the inducer other than it sets a prepurge time assuming all is well with the inducer (seems crazy but that's how it seems). I assume you have looked at the furnace schematic and see how the pressure switch is wired. Therefore the only thing I can think is the flame sensor is dirty or the WR control is flakey.