Lennox G51MP question

Old 11-21-12, 07:42 PM
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Lennox G51MP question

Hello all,

Lately since the cold weather arrived, I noticed that my furnace was "cycling" (I think that's the right term) a lot when it's trying to heat up the house.. The example I told the furnace technician was in the morning the tstat would read 63F and try to reach 68F (set temp).. I noticed that it would raise a few degrees to 66F and then cut out... it would come on soon after and go to 67F and cut out... then it would kick in one last time to hit 68F... I thought my furnace was a single stage furnace and would just continue to run until it hit it's mark? I really don't know anything about furnaces.. Furnace technician gave the unit a cleanse and found a beetle in one of the condensation(?) hoses and said that may be causing the issue; however, I still notice this cycling behaviour... Before I call the furnace guys back in, I was hoping someone would be able to explain whether I indeed have a problem.

Oh, I also changed the batteries in the tstat (honeywell CT3500). and I read some forums where it says I could change the System Type (currently set to 6 Gas or oil forced air)... Which I think is right, because it is a gas furnace?

Old 11-21-12, 07:59 PM
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System Type (Feature Number 4)
System type options are:
1 = Gravity or steam system.
3 = Hot water, high efficiency furnace (90% or better), or single stage heat pump.
6 = Gas or oil forced air furnace (preset).
9 = Electric furnace.
To change your system type:
❑ Press until display shows your furnace or boiler type.
❑ Press Time to move to next feature or to return to main display.

Change this to 3................................................................
Old 11-21-12, 08:08 PM
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Mmmmm. The repairman should have measured the pressure being applied to the pressure switch. If it was out of the proper range, he would KNOW that was a problem. Then if cleaning the hoses restored the correct pressure he would KNOW that he had solved at least one issue.

Guessing is not a good diagnostic method.

Did he observe the pressure switch opening and shutting off the burner?

Are there diagnostic lights on the furnace circuit board signaling a problem? If so, what?

Are there specific things other than the hose that were cleaned or checked by the repairman?
Old 11-22-12, 04:30 AM
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Houston: thanks, I will change this today and see if that does the trick. Seattle: the technician cleaned the fan and vacuumed out the dust and such. He really didn't do a lot other than inspect a few things here and there.. I don't know if he checked the pressure switch.. He did take a few readings with some devices.. He said that the furnace wasn't high limiting if that means something... he thought the furnace was working well... The diagnostic lights are behaving fine... slow flash when not in use, and when it calls for heat, a fast flash...
Old 11-22-12, 07:57 AM
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If hi heat limiting was the problem the blower would continue to run but the burner would shut down until the heat exchanger cooled off. Then the burner would relight. You would probably only notice this problem if you were standing next to unit.

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