Lennox HP Defrost Not Working


Old 02-20-10, 01:19 PM
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Exclamation Lennox HP Defrost Not Working

Hello, new lady here to the forums here, work IT, tech, etc..

I have a Lennox 3 Ton Heat Pump (HP26-411-1P) that I have recently twice caught all iced up (inside and outside of the HP coils).

The first time was on a weekend that it was rainy and the next time was on the following weekend when it was very humid.

Understanding how a HP works, I just switched it into A/C mode and let it run for 15 minutes to defrost and thaw out both times. Although it was still putting out warm air inside, after defrosting it was putting out MUCH warmer air. Thus, electric switch valve is working.

So, this tells me that the defrost mode is not working - also haven't heard nor seen it in defrost mode lately.

Recently I flipped the breakers and opened it up outside. I located the defrost board and the sensor. Both the sensor, held in place by a clamp, and the defrost board with all its wires look okay, but we all know looks can be deceiving.

The part number I got off the defrost board was 7221018-100 and when I Google'd two websites came up with a match:

americanhvacparts $89

acfurnace $90

Which site (or other) and is $90 a good price for the defrost board?

I couldn't find a part number for the sensor though. ???

So, what are the chances it is the defrost board vs. the sensor and which should I replace first - the defrost board?

Thanks for any and all help/advice!

I have since learned that the HP26-411-1P unit I have is one of the early units, thus it has a Defrost Relay. Could this be stuck? Would that keep it from going into defrost cycle? I guess I need to go out there and tap on it some.

Last edited by AWEstun; 02-20-10 at 02:11 PM.
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Old 02-20-10, 03:01 PM
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Qty Cat # Part # Description

1 78H11 78H1101 THERMOSTAT

1 34M63 LB-101263A LB-101263A DEFROST CONT
Part is a Substitute for 68J8401(68J84)

$13 and $64

Shop some more.
Old 02-20-10, 03:35 PM
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Thanks for those part numbers and prices.

I have been able to determine that I have the earlier model of HP26-411-1P, which had the defrost relay (separate from the Defrost board). It's about the size of 2 match boxes stacked and has a clear case one the relay. Can someone tell me part number please?

Also, which would you replace first: Defrost Board, Defrost Relay, or Thermostat Sensor?

The lowest price I am finding on the 78H1101 Thermostat is $25 (+ shipping) - is there better?
Also, I guess I could take off the thermostat, seeing as the circuit isn't working as-is, put the thermostat in a ziplock bag, put it in the freezer for a couple of hours, then check for continuity across it.

The lowest price I am finding on the LB-101263A DEFROST CONT is $90 (+ shipping) - us there better?

Last edited by AWEstun; 02-20-10 at 04:34 PM.
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