Heatilator GCDC42L troubleshooting

[EEProf]

Heatilator GCDC42L (42", LP gas, glass-sealed logset with outside air via flue)

Gas valve: SRV24034(Heatilator & Quadra-Fire Robert Shaw Valve)
Pilot: Heatilator and Heat-n-Glo Propane Gas Pilot Assembly 4021-733
Hi-Limit: Heatilator High Limit Switch (SRV24967)

Pilot lights, holds, log set fires with wall switch. FP runs then 'poofs out' (I only saw it once, that's the best description I can give). It has stayed on anywhere from two hours down to less than a hour.

Gas tech came out and of course it would not perform, worked perfectly of course.

Thoughts?

 

[DAKSY]

Disconnect the wiring from the wall switch & touch the ends together. What happens?
If it stays on, you may have a faulty wall switch. If the issue occurs without the switch, let us know.

 

[EEProf]

Disconnect the wiring from the wall switch & touch the ends together. What happens?
If it stays on, you may have a faulty wall switch. If the issue occurs without the switch, let us know.

Interesting. I replaced the wall switch a year ago, just a SPST Decora, so that's not it (well, nothing is never 'not it' until the real cause is found).

I think it has to do with heat somehow. Something gets warm and then opens, closes, whatever. I may just put a jumper across the switch terminals to turn it on tonight. We typically put it on at about 6 PM and run it until 10 or so.

 

[EEProf]

I tested the high limit, it closes in at ~350ºF and then reopens at ~250ºF or so...as it should.

Tonite, the FP stayed on about 40 minutes. Seems to be getting shorter, which I suppose means that we may be approaching a hard fail, which is always easier to diagnose.

So now it is either the GV or the pilot assy. Any suggestions on troubleshooting?

 

[DAKSY]

Have you taken a reading on the thermopile?
How old is this unit?

 

[EEProf]

Have you taken a reading on the thermopile?
How old is this unit?

No. What are the failure modes of it? Actually, I am not quite sure what it does (other than produce a current when heated).

The unit is 20 some years old.

 

[DAKSY]

The thermopile should be generating 525+/- 25mV when only the pilot is lit.
When the burner is lit, the milliVolts will drip to around 220+/- 20 & hold steady.
Place the probes of a multimeter set to mV on the TP & TH-TP terminals in order to take
the readings. Report back with what readings you get.