Lennox heat pump and air handler periodically run past set point


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Old 08-26-17, 06:46 PM
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Question Lennox heat pump and air handler periodically run past set point

[size=3]I have a Lennox Model 10HP30-2P, which I am trying to nurse through one more season. [/size]

[size=3]Last year, I replaced the old thermostat with a new Honeywell Digital Thermostat with WiFi and I also replaced the contactor on the heat pump unit. Since those replacements, the unit has run well for the last 9-12 months, but it has recently started running past the set point again and I need some help in diagnosing what may be going wrong. [/size]

This time, both the air handler and the pump keep running, but throwing the A/C breaker in the main panel does cause everything to shut off (unlike before I replaced the contactor). I have also tried pulling the thermostat unit away from the base plate when the system was in one of these overrun conditions and everything continued to run.

Help in diagnosing this problem will be appreciated. I am not a HVAC expert by any means, but I have worked with electronics and computer systems much of my life, so I was comfortable following basic diagnostic procedures and making the replacements mentioned above, but I do not want to want to do anything really dumb while trying to avoid a service call.Thanks in advance.
 
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Old 08-26-17, 07:33 PM
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Honeywell has an option that lets you overcool to dehumidify.

You may be able to correct this issue by turning off this function.
 
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Old 08-26-17, 10:26 PM
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Welcome to the forums.

I have also tried pulling the thermostat unit away from the base plate when the system was in one of these overrun conditions and everything continued to run.
That's a strange problem and since you pulled the stat off the base.... it's not a stat problem. In order to keep the indoor blower running and the outside condensor requires several connections at one time.

It almost sounds like there is a splice/cable that is shorting. The only other thing I can think of is a problem with the defrost board. That could keep the blower and condensor running but usually it would be in the heat mode.
 
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Old 08-27-17, 06:43 AM
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Hello Houston 204,

The system will actually run for hours in this state and the temperature will go more than 5 degrees lower than the set point (which seems only to be limited by time and the capability of the cooling system), so I do not think it would be a built in function of the stat, but I will check the documentation again to see if that could be part of the problem.

Thanks
 
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Old 08-27-17, 06:55 AM
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Hello PJmax,

Thanks for the welcome. Perhaps I can help others with IT related issues in the future.

I agree that pulling the stat from the baseplate should have eliminated the stat as the source of the problem, which is why I made sure to mention having done so.

A wire starting to short after years of working correctly seems unusual, but I guess it could happen due to breakdown of insulation or a rodent. I will also check the wiring immediately behind the stat to see if any of those could be the problems, since they were moved when the new one was installed.

The defrost board sounds interesting. The problem is mostly noticeable at night when the unit should be cutting off quite a bit and instead keeps going and plunging the temp several degrees below the set point. We have also had very high humidity this summer.

How would I go about trying to isolate the problem to this board? Is it located by the air handler system or by the pump (and is a replacement available if that is actually the problem)?

Thanks in advance.
 
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Old 08-27-17, 12:02 PM
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When in A/C mode..... the thermostat switches red 24vac to....
1) the blower on green
2) the A/C contactor in the condensor on yellow
3) the reversing valve on orange (needs power in A/C mode only)
(heat is similar but no power on orange)

You need to go to the air handler with a meter to check for voltage from C (common) to Y and G. With the stat removed it should be 0v.

With no stat in place.... the only stat wire live should be R...... R to C.
Any other wire with power from it to C is a problem.
 
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Old 08-27-17, 01:16 PM
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If you don't happen to have a multimeter to do the checks that Pete mentioned, when the system is acting up, carefully remove the wires going to the G and Y terminals on the air handler. If the system turns off, you likely have a shorted wire in the bundle running up to the thermostat. If the system continues to run, then the problem may be the defrost board, however you have ruled out the wiring as being a possible cause.
 
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Old 08-29-17, 10:08 PM
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Hello PJMax,

Thank you for the suggestions. I have dug a bit deeper into the old unit and noticed 2 things.

First, the area around the defrost board was absolutely filthy and much of the board was covered with insect or cobwebs. I thus decided that it would be worth cleaning things up and checking all of those connections before doing anything more extreme (particularly due to the intermittent nature of the problem).

Once I cleaned the area up a bit and took a hard look, I saw something else which caught my attention. The Heat Pump is a Lennox unit which was installed when the house was built and had only one major repair, which required the dealer to replace the compressor under warranty. I was thus a bit surprised to see a Rheem Ruud 621-83-550K defrost board installed inside the Lennox housing. This board also seems to be one of the simplest designs I have seen (at least in terms of wiring connections). As I said before, I am not an HVAC expert, but I thought that Rheem Ruud systems worked differently than other systems with respect to how they power/control the reversing valve, so does is It actually strange to have one of these boards on a Lennox unit or just a curiosity for someone like me?

Thanks in advance
 
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Old 08-29-17, 10:13 PM
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Hello Bob 14525,

Thanks for the tip. I do have some rather antique meters left over from my fathers workshop, but I may give your shortcut a try if things continue to act up after I cleaned things and checked the connections in the housing itself.
 
 

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