Understanding defrost mode on a heat pump system


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Old 02-14-16, 12:39 PM
Z
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Understanding defrost mode on a heat pump system

Hi all. Here is what I have:

A Nordyne Gibson 2 ton HP along with electric auxiliary heat coils in the air handler. Honeywell IAQ thermostat, and an ICM CC750 comfort control (effectively turns PSM motor into variable speed).

The system works great except I am not sure what is supposed to occur when the HP goes into defrost mode. I cannot find it in any literature.

I suppose either:

- The indoor fan should turn off with auxiliary heat off during defrost or
- The indoor fan should stay on with auxiliary heat on

What is occurring is that the indoor fan stays on but the aux heat coils do not turn on so it blows cold air during defrost. This seems to bother my Mother-in- Law the most when she is over. She complains about the cold draft.

I cannot avoid the mother-in-law so need to figure this out. And yes the auxiliary heat does work when thermostat is set to emergency heat.
 

Last edited by zoneout; 02-14-16 at 01:16 PM.
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Old 02-14-16, 02:39 PM
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There are two types of defrost systems out there.. timed, an demand. Not sure what type yours is but it doesn't really matter. They're just a different method of triggering defrost is all. They all defrost pretty much the same way.

A defrost session is nothing more than turning the heat pump back into a normal Ac for 10 minutes or so for the outdoor coil to warm up (as it does in the Summer) to melt the ice off. The only difference is that the outdoor fan stops running to speed up the melting.

When a defrost is triggered the fan will stop and you will hear a loud escaping of gas as the reversing valve reverses. the outdoor coil will then heat up, melt the ice and when the coil reaches a certain temp the defrost session will end, the machine will stop and reverse itself back to a Winter heat pump.

The indoor fan needs to be on.

The heat strips SHOULD come on during defrost and if they are not then they are probably not hooked up correctly. You must however have enough power in your heat strips to at least balance out the cold which blows. If you don't then obviously the air will feel cold. I have a 4 ton unit and I need at least a 10Kw coil to break even
 
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Old 02-14-16, 03:14 PM
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Thanks Bob.... good information.

What is occurring is that the indoor fan stays on but the aux heat coils do not turn on so it blows cold air during defrost.
The heating coils should be activated directly from the defrost board. It would usually be a white wire on a W? terminal.

The model numbers of the condensor unit and the air handler would be helpful.
 
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Old 02-14-16, 05:06 PM
Z
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Yes, good info and thanks to both of you. I will fill in some of the blanks but if you say that the indoor fan and heat strips MUST be on during defrost then that clears up the main question I had.

Heat Strips are 10Kw. Condensor is Gibson JT5BD-24KA (2 tons). Airhandler is a Trane unit circa 2002 - dont have the model right now. Defrost board is Nordyne 917178 (equivalent to ICM 302).

The defrost board has both W2 and E low voltage terminals. I would have to check how it is wired but I think W2 is connected and E is not.

Hmmm... you are right it could be working but not enough heat output from the strips to overcome the `air conditioning` cooling that happens during defrost - never thought about that. I will have to see if indeed the strips are on.

Regarding the compressor noise when the reversing valve shifts I have some follow-up questions for you guys once this first issue gets settled. Appreciate the help and advice greatly. Thanks!!!
 
 

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