Main burner won't light anymore. Before was intermittent and fixed by shaking surround. Thermopile: low volts when switch on, good volts off.

[Mcburger]

Hello, I have a natural gas fireplace called the Heat-N-Glo FB-GRANDLP, which is a millivolt system.

(I can't find its manual but I found a similar model's (I think) FB-IN and FB-GRAND).

It has two control options, a simple on/off switch located on the decorative painted metal surround and a remote receiver box.

The remote receiver box quit working a few years ago. We've just been using the on/off switch. A tech told us the receiver box is defective.

A problem we had was the main burner shut off randomly while using the fireplace, but it was just an annoyance since the burner would turn back on by itself if we shook or pushed the fireplace's decorative metal surround.

Now the main burner won't turn on at all. Pilot light is still burning though. I'm a pretty big newbie at fireplaces but did some research and testing.

I tested the on/off switch and remote receiver box and both are good. They have continuity according to my multimeter.

Using diagnostic methodology described in this forum post, I tested some electric things that are needed to turn on the main burner's gas valve. I described in detail but I'll italicize anything that you might already know as an expert, so you don't have to read them.

The fireplace's gas valve has 3 electric terminals, one marked TH, one TP, and one TH-TP.

The thermopile is connected to the TP and TH-TP terminals. The power switch and remote box are connected together to the TH and TH-TP terminals.

The thermopile generates around 500 mV when the power switch is off which is okay. When I turn on the power switch the mV generated by the thermopile drops into the single digits.

I also completely unplugged the switches and tried a paperclip to bridge the TH-TP and TH terminal like a switch that's turned on, but the main burner still doesn't light. I didn't measure the voltage while trying this but can if you want me to.

Based off this info (And I may be wrong as a newbie), the main burner is not turning on because the gas valve isn't opening.

The gas valve is not getting enough voltage to open. (Unless it's somehow stuck closed. Can that suck up all the power that the thermopile can generate resulting in low voltage readings?)

The problem is not caused by a defective power switch, which is confirmed by my multimeter and the fact that paperclip can't open the gas valve either.

Can I say that the cause of my main burner not turning on can be these possibilities?:
1. defective thermopile
2. bad gas valve
3. problems inside the electric circuits controlling the gas valve, excluding power switches and thermopile

I'm not sure if these are the correct diagnoses or how to narrow it down without just replacing parts. But I've found a webpage that says my thermopile is defective if the voltage is low while in circuit(switch turned on).

I can accept this advice and replace the thermopile first then gas valve next (in order of cheapest to most expensive), but I'm wondering why the fireplace used to be fixed by shaking the metal surround? Is the problem somewhere else?

A hypothesis I have is that it was a separate issue. And now another issue has occured, preventing the fireplace from turning on at all but unrelated to the issue fixed by shaking the surround.

Ultimately should I trust the website's advice that my thermopile is bad?

I'm real sorry for the long post, I really wanted to get a lot of details in it. And I really appreciate your volunteer time.

Search term/extra info: the control panel is a Heat-N-Glo 783-926

 

[DAKSY]

Check to ensure the thermopile connections are tight at the terminal block, if they are. I’d say that the thermopile is toast.

 

[Lennox65]

If the switch wires were loose, shaking the surround could have caused them to make proper contact and allow the circuit to be completed. That would explain the intermittent operation.
Separately, the thermopile is a wear item that loses its effectiveness over time. I usually replace the entire pilot assembly because the thermocouple is also a wear item and usually fails along the same timeline as the thermopile.

 

[Mcburger]

Check to ensure the thermopile connections are tight at the terminal block, if they are. I’d say that the thermopile is toast.

Thank you so much. The connections are tight so sounds like thermopile is bad.

 

[Mcburger]

If the switch wires were loose, shaking the surround could have caused them to make proper contact and allow the circuit to be completed. That would explain the intermittent operation.
Separately, the thermopile is a wear item that loses its effectiveness over time. I usually replace the entire pilot assembly because the thermocouple is also a wear item and usually fails along the same timeline as the thermopile.

I really appreciate the advice. It's much more expensive to replace the whole pilot assembly than just the thermopile but sounds like a good idea. 👍

 

[Mcburger]

I want to update this post, the fireplace is fixed and the thermopile was defective. The fireplace guys also added a screw that was not installed into the electrical terminal. They thought screw might be needed to make good electrical contact with the gas valve. And be sure to fully tighten your electrical connections for your switches and thermopile.

Thanks a ton for helping me guys, you're great. I would have tried to replace to get myself but we decided to hire the fireplace guys this time and they also clean/inspect the fireplace