Burner intermittent, now not starting

[Joblack]

I have a Napoleon BGD36CF natural gas fireplace, approximately 13 years old. It has been trouble free up until a couple days ago when I came home to a cool house. I started with replacing the low batteries in my wall thermostat. Then I began troubleshooting….
The pilot always remains lit
The wall thermostat relay works fine, 1 ohm of resistance as read from the fireplace valve, wires disconnected, switch in ‘heat on’ position
The thermopile reads 670 millivolts tpth to tp, thermostat switched off.
With the wire reconnected, switching the heat from off to on the reading between tpth and tp drops from 670 to 150 millivolts.
No change from gently tapping the valve with a screwdriver handle.
I did a little cleaning around the pilot wire brush/vacuum.
Over the past couple of days it was very intermittent, the thermopile would read low when it shut off, now the thermopile reads well and the fire won’t start. Leads me to think something in the valve was on its way out.
I am suspecting that I may need a new valve, model W725-0025.
Thank you for any thoughts on what else I can try to resolve this

 

[bigealta]

Thermopile is usually the fastest cheapest and easiest to replace. It's worth a shot before replacing the gas valve.
I don't know about your specific unit but in general that's the deal with gas appliances.
The millivolt readings can read ok but the thermopile can still be bad or sporadic.

 

[Millbilly]

Thermopile is usually the fastest cheapest and easiest to replace. It's worth a shot before replacing the gas valve.
I don't know about your specific unit but in general that's the deal with gas appliances.
The millivolt readings can read ok but the thermopile can still be bad or sporadic.

I've had good mv readings on thermopiles that were bad

 

[Joblack]

Thank you both for the replies. I will look into ordering a new thermopile and possibly a valve. I can find the parts online but may have a 2 week shipping wait if I can’t purchase locally.
The thermopile looks pretty straightforward to replace and I have read recommendations to also replace the thermocouple while I am in there.
I am a mechanical person, are there any tips or tricks to know before replacing the main valve if it is required?

 

[Lennox65]

Thank you both for the replies. I will look into ordering a new thermopile and possibly a valve. I can find the parts online but may have a 2 week shipping wait if I can’t purchase locally.
The thermopile looks pretty straightforward to replace and I have read recommendations to also replace the thermocouple while I am in there.
I am a mechanical person, are there any tips or tricks to know before replacing the main valve if it is required?

Have you tried removing the switch wires from the valve and jumping the TH and TH-TP terminals? If you are testing the millivolt output correctly, 670 with the wall switch off is like a brand new thermopile. Dropping to 150 could be due to a bad switch or loose connection.

 

[Joblack]

Thank you for the suggestion, yes, I had tried jumpering out the thermostat directly.
I did a little more searching and found that for my model of valve, SIT 820 Nova, I need the ‘On’ voltage to remain above 175 mv. I increased the pilot flame size to increase the voltage output and the fireplace is working for now. I have exhausted my local options to purchase locally and am ordering a new thermopile online today.
Thanks again to everyone for your assistance

 

[Joblack]

An update, following intermittent operation after increasing the pilot flame, the new thermopile has arrived ahead of schedule. I replaced it (tools required- 11mm wrench and Phillips screwdriver), reduced the pilot flame and now read 650 millivolts wires connected heat off and 230 millivolts wires connected heat on with consistent operation.
So nice to have my fireplace back