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John21 04-11-2013 10:23 AM

Lennox G51MP Gas Furnace Problem
I have a Lennox G51MP Gas Furnace since 2005. Recently I've been getting a hint of gas smell from the air vent closest to the furnace during the period from when the furnace heat cycle is initiated by the thermostat to when the air blower is turned on. This indicates to me that there is a communication of gas between the fire box and the air plenum. Perhaps the heat exchanger has developed a crack but it could be something else! Has anyone experienced this problem? What was your out come?

yuri 04-11-2013 10:41 AM

next to impossible. if the heat exchanger was cracked then the pressure switch would trip and you would get flame rollout and safety switch tripping. I have that exact furnace. chances are something has fallen down on to the top of the exchanger and is burning. I have seen everything from kids toys dropped into vents to ham sandwiches left by builders fall down there. best to get a Pro to check it anyway.

John21 04-12-2013 03:38 PM

Lennox G51MP Gas Furnace Problem
Hi Yuri
Thanks for your response.
"then the pressure switch would trip" Would this be the Combustion Air Proving switch's you're talking about?
Out of curiosity could you explain the term "flame rollout" for me? I can see that the flame rollout switch's are proving flame is established.
Something burning on the heat exchanger is not the problem. There is 10 feet of 8 X 14 ducting plus another 9 feet of 5 inch tubing.
I'm waiting on a Pro.
What should a heat exchanger set me back installed?

yuri 04-12-2013 05:52 PM

I have that exact furnace and there are thousands of them in Winnipeg and we have not had one heat exchanger failure. It has a 20 yr warranty. If it was cracked then when the main blower comes on high pressure air will go thru the crack and disturb the flames and roll them back and hit the rollout sensors on the sides of the burner box. You are probably seeing the flame sensor. If it is really badly cracked then it may have a hard time proving the pressure switch on startup as it will suck air thru the crack. Let me know what he finds as we have not had any failures yet.

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