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-   -   Heat Pump Defrost Question (http://www.diychatroom.com/f17/heat-pump-defrost-question-87443/)

zootjeff 11-22-2010 04:30 PM

Heat Pump Defrost Question
 
I have a Goodman 14 SEER 410a 2.5 ton heatpump.

How does this unit know when to defrost? Does it have a set schedule based on temperature or does it some how sense it's icing over?

I'm also curious, how does defrost work? Are there actual heating coils in the heatpump or does it somehow heat up using the compressor? I notice the compressor is running during the defrost cycle..


-Jeff

Marty S. 11-22-2010 05:21 PM

Jeff there's two kinds of defrost boards and I don't know which the goodman uses. One type has a sensor and it engages defrost every 60 or 90 minutes of run time when the sensor is below 28 degrees.
The other type is called on demand. It has several sensors which act together and start the defrost when needed instead of using a set time.

When a HP goes into defrost it's actually acting as an air conditioner,but the condensor fan motor is off. It transfers the heat in the house to melt the frost outside. Usually they are wired to bring on the back up heat at the same time so you don't feel several minutes of very cold air.

zootjeff 11-22-2010 05:30 PM

Quote:

Originally Posted by Marty S. (Post 538900)
Jeff there's two kinds of defrost boards and I don't know which the goodman uses. One type has a sensor and it engages defrost every 60 or 90 minutes of run time when the sensor is below 28 degrees.
The other type is called on demand. It has several sensors which act together and start the defrost when needed instead of using a set time.

When a HP goes into defrost it's actually acting as an air conditioner,but the condensor fan motor is off. It transfers the heat in the house to melt the frost outside. Usually they are wired to bring on the back up heat at the same time so you don't feel several minutes of very cold air.


So in defrost mode, that means the backup heat will actually heat up the indoor coils quite a bit, making it easier for the heatpump to defrost.. Is there any issue overheating the indoor coil or does it only help the process?

beenthere 11-22-2010 05:44 PM

Quote:

Originally Posted by zootjeff (Post 538903)
So in defrost mode, that means the backup heat will actually heat up the indoor coils quite a bit, making it easier for the heatpump to defrost.. Is there any issue overheating the indoor coil or does it only help the process?

The aux heat coils don't heat up the indoor coil. if they do. Its installed wrong.


Goodman uses time and temp defrost on all of their heat pumps. Including the 2 stage units.

Marty S. 11-22-2010 06:01 PM

Quote:

Originally Posted by zootjeff (Post 538903)
So in defrost mode, that means the backup heat will actually heat up the indoor coils quite a bit, making it easier for the heatpump to defrost.. Is there any issue overheating the indoor coil or does it only help the process?

On a gas or LP furnace that is correct. Indoor coil sees the same warm air in emergancy or aux mode so no it's not going to harm it. Electric heat has the elements above the evap coil so it's strictly tempering the air.

beenthere 11-22-2010 06:25 PM

Electric aux heaters aren't before the heat pump. So they don't heat the coil.

If they were before the coil. Then they would harm the heat pump. If they were on at the same time as the heat pump in heating mode..

zootjeff 11-22-2010 06:41 PM

Quote:

Originally Posted by beenthere (Post 538933)
Electric aux heaters aren't before the heat pump. So they don't heat the coil.

If they were before the coil. Then they would harm the heat pump. If they were on at the same time as the heat pump in heating mode..

That is why I asked the question about my setup. I have a 2 stage gas furnace and a heatpump not electric backup heat.. The coils are underneath the downflow gas furnace. If they HP calls for backup heat during the defrost cycle, it will turn on the furnace and heat up the coils. Is this a problem? It is basically like running the HP in AC mode in the summer when it is 100F inside the house, but the outdoor fan doesn't turn on....

beenthere 11-22-2010 06:43 PM

If the heat pump and furnace both run for heat at the same time. You bet it will damage the heat pump.

Marty S. 11-22-2010 07:38 PM

Quote:

Originally Posted by beenthere (Post 538953)
If the heat pump and furnace both run for heat at the same time. You bet it will damage the heat pump.

Been a long day there boss? He's asking about defrost mode.

Marty S. 11-22-2010 07:49 PM

Jeff look at the wiring diagram. W from the defrost board goes to W1 in the furnace which brings on the gas while the HP is in defrost. That's how it's designed to work. You can disconnect W at the defrost board if you want but there will be cold air coming out of the registers , in defrost, instead of tempered air.

zootjeff 11-22-2010 08:00 PM

Quote:

Originally Posted by Marty S. (Post 538992)
Jeff look at the wiring diagram. W from the defrost board goes to W1 in the furnace which brings on the gas while the HP is in defrost. That's how it's designed to work. You can disconnect W at the defrost board if you want but there will be cold air coming out of the registers , in defrost, instead of tempered air.


Some people recommend you don't hook up the W line to the outdoor unit, then in defrost mode you'll get cold air. If you are trying to make your gas bill zero for the month, then dealing with the cold air during defrost is probably ok. I'm wondering if you are always better off just using the backup heat during defrost so that it shortens the defrost cycle. You can extract a lot more heat from 100 degree air than you can from 65 degree air, then your heat pump won't run as long..

The thing that bums me out is that my thermostat doesn't know when the defrost cycle occurs.. If it completes a defrost cycle, then the call for heat goes away because it only needed a 1/10th of a degree to heat the house to trip the thermostat, you just wasted a defrost cycle.

I guess if you build up ice in 35 degree weather, it will still be there when the call for heat comes 20 minutes later..

beenthere 11-23-2010 03:29 AM

Quote:

Originally Posted by Marty S. (Post 538987)
Been a long day there boss? He's asking about defrost mode.

Not long. Just screwy.

beenthere 11-23-2010 03:31 AM

Quote:

Originally Posted by zootjeff (Post 538997)
The thing that bums me out is that my thermostat doesn't know when the defrost cycle occurs.. If it completes a defrost cycle, then the call for heat goes away because it only needed a 1/10th of a degree to heat the house to trip the thermostat, you just wasted a defrost cycle.

The ice is gone for the next heat call. And you get better/more heat quicker.


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