Help with diagnostics on Lennox G51MP

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Old 10-24-13, 12:00 PM
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Help with diagnostics on Lennox G51MP

I have a 2 1/2 year old heat pump Lennox furnace. Once last year and now twice today, it has stopped working and the house temperature falls below setpoint. The LED diagonostic codes show RED SLOW FLASH and GREEN STEADY ON which is described as "primary or secondary limit switch open. Limit must close within 3 minutes or unit goes into 1-hour Watchguard." What is causing this? When I open the utility area door, I smell a faint odor of gas (could be from the water heater).

This door to the small utility area was originally louvered when we had the house built but we replaced it with a solid door (for noise abatement) since both the furnace and water heater are direct vented out the exterior wall. Could that be what is causing the shut down?

A friend tells me to simply flip the on/off switch on the side of the furnace when this happens. That is fine for short term use but I am planning on leaving for the holidays and don't want to come home to frozen pipes. I have called the heating and cooling company who installed it but they have not returned my call yet. I want to be somewhat educated when they call or come out in case they want to sell me something that could simply be fixed.

Thanks for any help!
 
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Old 10-24-13, 12:48 PM
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The main cause of a limit switch tripping is lack of air flow. This can be caused by a dirty air filter, closed supply vents, dirty evaporator coil, blocked/restricted return air grilles. I would start with checking those items. The limit switch trips because it is over-heating and is a safety device. It is usually self-resetting but after a few trips it becomes weak and will not reset.
 
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Old 10-24-13, 10:01 PM
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Okay, thank you firedawgsatx. I did talk to the heating and cooling people and they told me to check the filter. I had replaced it 7 weeks ago and even brand new and clean, there is a lot of suction on it. I may go to a thinner brand. I left the furnace area door open and it has been working fine since then. My boyfriend also checked the outdoor vent and it seemed to be drawing. The furnace is working fine now. Not sure if I want to pay for the service call tomorrow or not... We did put some saran wrap under a few supply vents earlier this year to block the cold AC draft. (these are wooden floor covers and don't have an open/close or directional control) so we will take that off now. Thanks a lot for your help. The repair company suggested replacing a certain part if the filter wasn't the problem. It is good to know there are other factors.
 
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Old 10-25-13, 06:33 AM
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We did put some saran wrap under a few supply vents earlier this year to block the cold AC draft.
Yes, definitely remove the saran wrap as that can cause the issue you are having.
 
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Old 11-19-13, 08:03 AM
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Air Flow - Limit Switch

I am having a similar problem with the limit switch opening.
The limit switch opens at about 133 and then closes about 90 degrees.
It takes appox about 2 minutes to reach 90 then it relights.

My question: does just 5 opening of the limit cause the watchguard to lockout or is there a time period that the chamber can cool and reset limit switch with out signaling watchguard?

Must the limit remain closed for the entire heat period no matter how long?
 
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