Blower intermittently working

Old 12-31-13, 12:31 PM
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Question Blower intermittently working

Hey all,

My mom called me over to her house the other day to see if I could figure out a problem with her HVAC. She had told me it has been acting up since at least last weekend (12/21 - 22). Regardless, her problem seems to crop up with the blower not kicking on appropriately (won't run consistently on heat, cold or off). It will kick on, seemingly randomly, with the thermostat being on the RUN position but if it's already running and switched to AUTO it'll keep on working.

Now the problem with that is that it won't shut off correctly either, the thermostat will be set at 75 and it'll end up being 90 in the house before I'd have to shut it off manually and then it's a pain getting it to restart again.

Personally I have went downstairs and witnessed the burner fire up and heat the plenum as it's supposed to every time, it'll only fire up for a few seconds though because the blower isn't pushing any air to cool it. I have also went downstairs and jumped the primary limit switch to ensure that it isn't a problem with that and the burner stayed running until I let the override switch out.

In each of these instances, I have only seen the blower operate correctly a few times. Again, when it works, the blower works great so I don't think it's a motor either (leaving thermostat, control board, or wiring). Just curious to know if anyone has some insight into this problem as it's starting to stump me. Thanks for any help!

Lennox G23Q2/3-75-2
Natural Gas
I believe it was installed around 2000-ish
Old ass mercury thermostat, not sure model
Old 12-31-13, 12:56 PM
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The first place I'd start at is the thermostat. You need to determine if the burner shuts off when the set temperature is reached. If the temp goes significantly over the set point then the thermostat is at issue.

When your thermostat calls for heat.... the draft inducer starts first. After prepurge the pilot system lights. Then the burner lights. The blower fan is timed to come on 45 seconds after the burner lights. There is a jumper on the board that determines the "blower off" time.

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