Lennox Heat Pump and Air Handler Periodically Keeps Running Past Set Point


Old 04-02-18, 12:32 AM
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Join Date: Aug 2017
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Lennox Heat Pump and Air Handler Periodically Keeps Running Past Set Point

Last spring I replaced the contactor on my aging (but still reliable) Lennox heat pump and soon after I decided to replace the old thermostat with a new digital one with WiFi (Honeywell RTH6500). In summer, I noticed the entire system was sometimes (but not always) staying on once the inside temp had gotten much cooler than the thermostat set point.

Once in one of these failure modes, If I changed the set point to an even lower temp, then pulled it back to the current temp, I could hear the thermostat click, but everything would keep running. The only way to correct the situation was to cut the system off at the main breaker box and let it reset before turning it back on. Sometimes, things would work correctly for weeks before failing again. I tried replacing the defrost control board, but the symptoms persisted on an intermittent basis.

During the fall and winter, I did not have any problems with the heating, but I have now seen the issue repeating when the system is being set back into cooling mode. The problem is intermittent (though seemingly increasing in frequency) and I do not want the system to run for an entire weekend (or longer), if I need to take a trip once things get warmer.

Does this sound like I have a bad thermostat, bad wiring or some other problem?

Thanks in advance.
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Old 04-02-18, 02:00 AM
Join Date: Feb 2014
Location: USA
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I would suspect the control board. Next time remove the stat and see if it stops.
Old 04-02-18, 09:02 AM
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Control board ? You mean the defrost board ?

I would suspect a thermostat problem. With a heat pump system..... heating and cooling are the same process but in the summer a reversing valve is actuated. Same contactor used for heating and cooling.

Basically everything is the same except for the thermostat program.
Old 04-02-18, 04:43 PM
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Join Date: Aug 2017
Location: USA
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If you mean the defrost control board, I already replaced that late last summer. it did not seem to make a difference in the intermittent process of the entire unit continuing to run past the set point. Last year, I also tried pulling the thermostat face plate off when it was in one of these overrun situations and the unit continued to operate. Some people also suggested wiring, but I would think that would impact both turning the system on and off and the unit NEVER seems to fail turning on at that set point..

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