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Old 01-16-2013, 02:09 PM   #1
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Lennox heating problem


(this unit was with the house when my son bought it and heat is only problem at this time, cannot afford new unit)

Make : Lennox
Model : G40uh-36a-070-03 no diagnostic lights flashing
new prove switch acting same as old switch

Had this problem last winter and A/C tech put in new prove switch and it worked till it quit a few days ago.

when the t-stat is calling for heat, the air inducer starts and runs and after a few minutes the unit still does not ignite. Then i play with the prove switch and pull a manual vacuum and unplug and replug the prove switch wires and it will finally start the ignitor and fire up, it may run for days or till heat cycle ends.

When the combustion air inducer starts running, is it suppose to create a vacuum to hold the prove switch open or closed so the furnace will ignite ? While it is running, I placed a dime sized piece of saran wrap over the pressure port and it would not stay, that tells me the air inducer is not creating vacuum to hold switch o/c. Can this fan be removed and be cleaned so it will pull vacuum. or am i missing something... help please

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Old 01-16-2013, 03:06 PM   #2
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Lennox heating problem


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Originally Posted by howardc View Post
(this unit was with the house when my son bought it and heat is only problem at this time, cannot afford new unit)

Make : Lennox
Model : G40uh-36a-070-03 no diagnostic lights flashing
new prove switch acting same as old switch

Had this problem last winter and A/C tech put in new prove switch and it worked till it quit a few days ago.

when the t-stat is calling for heat, the air inducer starts and runs and after a few minutes the unit still does not ignite. Then i play with the prove switch and pull a manual vacuum and unplug and replug the prove switch wires and it will finally start the ignitor and fire up, it may run for days or till heat cycle ends.

When the combustion air inducer starts running, is it suppose to create a vacuum to hold the prove switch open or closed so the furnace will ignite ? While it is running, I placed a dime sized piece of saran wrap over the pressure port and it would not stay, that tells me the air inducer is not creating vacuum to hold switch o/c. Can this fan be removed and be cleaned so it will pull vacuum. or am i missing something... help please
Yes that's how it works. The pressure switch (or prove switch) senses trhe vacuum created by the fan. It verifies that the fan actually came on and that your vent pipes aren't clogged. The entire fan housing can be removed but I'm not sure you can take that apart, so you'd have to clean it together.

However if it needed cleaning I would wonder why it works after jiggling wires to the pressure switch. Jiggling the wires shouldn't improve your venting so I'd honestly look at the switch itself. Those pressure switches can get stuck so that may be whats happening, then when you giggle it or pull a manual vacuum you break it free & it works until it gets stuck again.

As far as your saran wrap test I wouldn't judge too much based on that. Those motors only pull about 1-2" Water Column vacuum which is a very small amount of pressure. It may not hold something up to it. The only way to really test it is with a manometer.

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Old 01-17-2013, 09:40 AM   #3
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Lennox heating problem


Thanks for the reply.
Son says it will run the cycle it starts with and not come on again until one of the wires is unpluged and repluged to the prove switch. after i replace the switch and it does the same thing Is it possible the sure light control board may be bad.
thanks again.
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Old 01-17-2013, 10:21 AM   #4
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Lennox heating problem


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Originally Posted by howardc View Post
Thanks for the reply.
Son says it will run the cycle it starts with and not come on again until one of the wires is unpluged and repluged to the prove switch. after i replace the switch and it does the same thing Is it possible the sure light control board may be bad.
thanks again.
It definately could be the board. Especially if it isn't flashing a code when it isn't working. I would think it would either be that or the pressure switch. Probably your best bet is to get the pressure switch. It won't be too expensive & is fairly easy to replace. If it's the board that's going to be much more costly. So I'd try the cheaper option first.
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