Lennox Fireplace Won't Light
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Lennox Fireplace Won't Light
I have a Lennox MPD4035CNM propane fireplace that won't light. Pilot lights, but no fire.
Two winters ago it was working fine, but started to take a long time for the burner to kick on when the thermostat in the remote called for it. I'd see the "flame" indicator on the remote, but no flame for a while and then it would light with a whoosh! By the end of the winter it wasn't lighting at all.
Last year we didn't try it.
After some research of the symptoms, I determined it was likely the thermopile, so I bought one and replaced it with no change. After I got everything back together, the pilot lit pretty quickly, but no burner. I turned the pilot off, and then decided to relight it so I could check the thermopile with a meter. It look a lot of attempts to get the pilot to re-light, but it did eventually.
I checked the new thermopile and it registered 555mv which I believe should be fine.
Based on this thread I'm going to check the safety switch for the glass, but what else should I check?
Two winters ago it was working fine, but started to take a long time for the burner to kick on when the thermostat in the remote called for it. I'd see the "flame" indicator on the remote, but no flame for a while and then it would light with a whoosh! By the end of the winter it wasn't lighting at all.
Last year we didn't try it.
After some research of the symptoms, I determined it was likely the thermopile, so I bought one and replaced it with no change. After I got everything back together, the pilot lit pretty quickly, but no burner. I turned the pilot off, and then decided to relight it so I could check the thermopile with a meter. It look a lot of attempts to get the pilot to re-light, but it did eventually.
I checked the new thermopile and it registered 555mv which I believe should be fine.
Based on this thread I'm going to check the safety switch for the glass, but what else should I check?
#2
It's very rare to see a glass safety switch. I have never worked on a unit that has one and yours does not have it either.
The circuit is very simple. Three basic parts. The thermopile, the gas valve and the switch/control.
Looking at the diagram..... the thermopile connects to TP and TP+TH. You should measure your +325mvDC at this point. Keep your meter connected here. The remote control or "flame on" switch is connected across TH and TP-TH. Activate the remote or flame switch. What does that voltage read now ? If it sags below around 200mv the pilot flame may not be large enough. That would indicate a dirty pilot orifice.
If the burner doesn't light and the voltage stays high...... apply a short directly across TH and TP-TH. What happens now ?
Lennox manual (pdf)
The circuit is very simple. Three basic parts. The thermopile, the gas valve and the switch/control.
Looking at the diagram..... the thermopile connects to TP and TP+TH. You should measure your +325mvDC at this point. Keep your meter connected here. The remote control or "flame on" switch is connected across TH and TP-TH. Activate the remote or flame switch. What does that voltage read now ? If it sags below around 200mv the pilot flame may not be large enough. That would indicate a dirty pilot orifice.
If the burner doesn't light and the voltage stays high...... apply a short directly across TH and TP-TH. What happens now ?
Lennox manual (pdf)
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Voltage across TP & TP+TH with pilot lit is about 580mv. With "flame" lit on the remote it doesn't really change, maybe a 10-15mv drop. With flame shoeing on the remote and shirting TH & TP+TH I pretty quickly get flame. Remove the short, flame goes away.
I hear the wall switch beep when I use the remote, so I believe it's communicating with the wall switch OK. Also, turning it ON via the wall switch produces the same result as the remote - no flame.
What next?
I hear the wall switch beep when I use the remote, so I believe it's communicating with the wall switch OK. Also, turning it ON via the wall switch produces the same result as the remote - no flame.
What next?
#4
Since you are dealing with millivolts..... any loss is unacceptable. The device that turns the fireplace on is just creating a dead short across the TH and TP+TH terminals. Too small a wire, loose connections, corroded connections will all cause lighting problems.
Typically I see a wall switch that just creates a short on the valve and I see a wireless handheld thermostat that also creates a short on the valve. You'll need to diagnose where you have loose connections.
You mentioned wall remote ? That could be a problem.
Typically I see a wall switch that just creates a short on the valve and I see a wireless handheld thermostat that also creates a short on the valve. You'll need to diagnose where you have loose connections.
You mentioned wall remote ? That could be a problem.
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The drop wasn't immediate, it was gradual. It remained well above the 325mv minimum.
There's a 3 position wall switch with ON / REMOTE / OFF. There's also a thermostatically controlled wireless remote with an LCD screen. Similar to this one, but not identical.
https://smile.amazon.com/Skytech-980...dp/B00PKF6H0M/
I plan on pulling the wall plate and checking all connections, but not tonight.
There's a 3 position wall switch with ON / REMOTE / OFF. There's also a thermostatically controlled wireless remote with an LCD screen. Similar to this one, but not identical.
https://smile.amazon.com/Skytech-980...dp/B00PKF6H0M/
I plan on pulling the wall plate and checking all connections, but not tonight.
#6
When you measure the voltage on TP and TP+TH ........ you are measuring the thermopile directly.
Now put your meter on TP and TH. Turn on the stat or switch. This voltage should come very close to the thermopile voltage. Anything lower then what the thermopile measures is loss. Too much loss and the valve won't open.
Now put your meter on TP and TH. Turn on the stat or switch. This voltage should come very close to the thermopile voltage. Anything lower then what the thermopile measures is loss. Too much loss and the valve won't open.
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I pulled the switch from the wall this morning and found that one of the wires had come loose from the back. Reconnected it, lit the pilot and turned it on with the remote. Fireplace lit right away.