Lennox inducer motor cycles on and off

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Skitch987

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Howdy folks, this is my firs post, so thanks in advance for any advice!
Here's the issue:
My furnace really struggles to get the house up to temp, with relatively mild outdoor temps (freezing or so). Here are the steps it goes through:
  • Turn up the thermostat, it calls for heat.
  • Inducer motor comes on like it should
  • Igniter starts
  • Burner starts
  • Blower motor comes on.
  • Furnace runs fine for 8-12 minutes, then burner and inducer motor shut off. Blower stays on.
  • After a few minutes, Inducer motor attempts to start – it cycles 3 or 4 times, then stays on.
  • Burner comes on, like normal,
  • Repeat.
I’ve replaced and temporarily run without the filter, checked the flame sensor, temperature limit sensor, vacuum switches. The furnace does not seem like it is getting abnormally hot.

My guess is that something is telling the control board to shut the inducer motor off, and as soon as it goes off, the vacuum switch tells the valve to shut the gas off.

Any ideas what sensor inputs might tell the control board to shut inducer motor off? Am i barking up the wrong tree?

Lennox G26Q3-75-2
Honeywell continuos retry ignition moduel
ICM289 control board
 

Fitter30

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The pressure switch think it's dropping out. It takes 5 faults before locking ojt and showing on the burner control. Would check the air inlet for blockage, the burner condensate trap, and hoses to the pressure control for moisture. For a test when flue fan comes on pressure switch can be jumped and should start the burner. Fan has to be running to jumper.
 

Skitch987

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Thanks, what you say makes sense. There was some very mild blockage on the top of the grate between the intake tube and the furnace. I also re-plumbed the intake to remove a 90 deg fitting, though the length + 90s were still well within spec. But, the intake was largely free and clear. Pressure hoses were dry. Had already pulled the rubber caps off the exhaust and burner condensate traps - they were full of water, (maybe 1") but not high enough to go through the drains, and not excessively full of debris. I've already checked the exaust for blockage.
I will wait and see if my re-plumb did the trick, then jump the pressure valve per your instructions and see what happens.
Would a weak inducer motor cause this? Seems unlikely, since the motor isn't making bad noise or getting hot, but maybe dirty fins or something?
How long should the burner run without shutting off, assuming the thermostat hasn't reached it's setting and is still calling for heat?
 

Fitter30

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Thanks, what you say makes sense. There was some very mild blockage on the top of the grate between the intake tube and the furnace. I also re-plumbed the intake to remove a 90 deg fitting, though the length + 90s were still well within spec. But, the intake was largely free and clear. Pressure hoses were dry. Had already pulled the rubber caps off the exhaust and burner condensate traps - they were full of water, (maybe 1") but not high enough to go through the drains, and not excessively full of debris. I've already checked the exaust for blockage.
I will wait and see if my re-plumb did the trick, then jump the pressure valve per your instructions and see what happens.
Would a weak inducer motor cause this? Seems unlikely, since the motor isn't making bad noise or getting hot, but maybe dirty fins or something?
How long should the burner run without shutting off, assuming the thermostat hasn't reached it's setting and is still calling for heat?
Possibly could always blow it out. Theres probably a gasket between fan housing and heat exchanger more than likely it will be torn when removing the fan.
 
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