Thermopile not producing enough voltage

[msneely]

I have a gas fireplace (Fireplace Xtrordinair 36 DV XL) with a millivolt system. It's around 20 years old.
I'm using a millivolt compatible digital thermostat (White-Rodgers 1E78-140),
but often there is not enough voltage (~459 mV) from the thermopile to operate
the thermostat - it does not open the valve when called for heat.
Sometimes it works if I switch the heat off and on a few times, or wait a few minutes.
I can always start it using the manual switch under the fireplace.
I've tried a different thermostat (Honeywell CT53K), which is mechanical, with no difference.
I've replaced the thermopile and cleaned the gas orifice, which helped a little, but still having problems.
One thing I noticed is that I get more voltage from the thermopile when the glass cover for the fireplace is off.
Then the thermostat works fine. Could the problem be caused by a blockage in the air intake duct?
Is there anything else that could cause low voltage from the thermopile?

Thanks,
Mike

 

[DAKSY]

You are confusing the ThermoCOUPLE with the ThermoPILE.
That being said, is the burner ON when you get that reading,
or is that the reading on the PILOT with the burner OFF?

 

[msneely]

You are confusing the ThermoCOUPLE with the ThermoPILE.
That being said, is the burner ON when you get that reading,
or is that the reading on the PILOT with the burner OFF?

You'r right, I meant thermopile everywhere I said thermocouple.
The reading of 459mV is with the burner OFF, across the TP and TP/TH terminals.
It goes down to about 225mV with the burner ON.

 

[DAKSY]

Those numbers aren't bad. The reading on the pilot COULD be a little higher,
but the burner should stay on at 225mV.
What happens if you pull the t-stat & connect the wires together?
Does the unit come on & stay on?

 

[Millbilly]

Like Daksy said those Tpile readings aren't to bad. How long is your tsta twire run?