Atwood furnace repairs

[Jiminsav]

anyone on the board know how to fix a Atwood 8525 furnace?
I get 3 lights on the control board which means flame sense..where is this sense thing?

[bdosborn]

Manual Clicky

[Jiminsav]

why, thank you, Mr Osborn..has a much better pic than anything else i found.

[bdosborn]

Cool.

Bruce

[kludge]

Is the burner lighting but not sensing the flame?

Here's the normal startup sequence...

1.TH calls for heat.
2.Fan turns on.
3.Sail switch closes, pre-purge timer starts (clears out any stagnant propane in the combustion chamber)
4.TFI (trial for ignition, gas valve turns on, spark ignition)
4a.Some type have HSI (hot surface ignition). Some have spark ignition. If you have HSI - I don't think you do - there will be a warm up time for the HSI before the valve turns on)
5.Flame sense (sometimes it can be hard to sense a flame with the spark ignition going (EMI) so they shut it off and sense for flame without spark)
6.Run, retry, or Lockout.
Some units can have multiple (like three) trials for ignition if the first is not successful.
An unsuccessful TFI will usually be followed by a post-purge and Lockout (Manual intervention required to try again.)

Part #24 is the flame sensor/electrode.

The tip of the electrode hangs out into the flame right at the burner.

It senses a flame by "flame rectification". Basically the flame is where the chemical change from fuel/air to water/CO2 is happening and when the electrons move between atoms it creates a region that acts like a weak diode.

A small AC current is passed from the electrode to ground to sense the weak rectification.

If the wire is bad, it won't work. If the electrode is not positioned properly in the flame it won't work (usually means it's bent or too far away from the burner). If the electrode is touching the burner (shorted) or some other part of the furnace it won't work. If the flame is lifting off the burner (wrong fuel/air mixture) it won't work. Those are the cheap/easy fixes. If the controller is at fault (bad component in the sense circuit) then you'll need to try another controller$$$.

Do ourself a favor and get a long 1/4" drive extension for your 1/4" drive socket wrench for disassembly.