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Lennox furnace g26q3-75-2 doesn't keep up


Jan 1, 2023
Model Number
More than 10 years
Running into an issue with my furnace. It's a Lennox g26q3-75-2, with a Honeywell SM8670k continuous retry
Will not get the house up to 72 deg. The blower will continue to run, but the burner won’t stay on long enough to heat the house - it goes off after a few minutes, then stays off. The blower pretty much never shuts off. The colder it is outside, the worse it does.
If I turn the thermostat down to 67, it seems to improve – it will cycle somewhat normally, and shut off normally.

About week ago: We had a cold snap with high winds. The heater wouldn’t start at all, so I checked the intake for snow, and sure enough it was blocked. I cleared it out, and then it started.
Thought everything was fine, but then it started not working well – I could see the automatic spark come on and try to light, but the pilot wouldn’t come on. In addition, it wouldn’t get the house any warmer than about 67 deg.
Had a local furnace tech come out, and he pulled the burner access cover off, and then it lit. He cleaned just a few leaves out of the intake, and now it seems to have cured the pilot lighting problem.
Still, however, it seems to come on just fine, heat for a few minutes and then the burner shuts off.

I need to figure out why it the burner is shutting off.

I have checked for obvious obstructions in the intake and exhaust
Cleaned the flame sensor.
New batteries in the thermostat
New filter.
All registers open.
No blockages on returns.
Status code for flame sensor indicates 6-9 microamps for flame sensor current.
I have not checked the AC fins, as the intake side is hard to get at – I want to exhaust all other possibilities first. Am currently working on a way to get a borescope in there to see.

Just did a timing on it:
Turned the furnace on: Burner came on and lit just fine, ran for 6 minutes, then went off for 10 minutes. Main blower continued to run, but seems to go to low speed when the burner shuts off, and back to high when the burner starts.
Burner came on again, ran for 4 minutes, then turned off again.
Thermostat was set on 70 deg, but it’s thermometer never got above 69.

I am a bit suspicious that the whole systme might just be dirty - will look at the fan blower and ductwork next.
Alos,possibly a bad thermostat - how do I know when it is actually calling for heat? It will say "heat on" even when the burners are off, so I assume it is calling for heat, but the furnace is preventing it. Honeywell 4 wire thermostat.

What else can I check? Thanks in advance for your help.

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