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owlyn
MVM
join:2004-06-05
Newtown, PA

owlyn

MVM

[HVAC] Heat Pump Defrost/Aux Question

Should the aux heat kick in when the heat pump goes into defrost mode? The fan stays on, but it blows cold air.

nunya
LXI 483
MVM
join:2000-12-23
O Fallon, MO

nunya

MVM

[HVAC] Re: Heat Pump Defrost/Aux Question

Yes. In defrost mode the HP is working like an air conditioner. It will blow cold air into the house. The heat strips are supposed to come on and counter this.

owlyn
MVM
join:2004-06-05
Newtown, PA
Netgear CM2050V
Netgear RBRE960
Netgear RBSE960

owlyn

MVM

Thanks. So, if the aux isn't coming on, would it be a thermostat wiring issue, or something wrong with the way the heat pump is configured? This is a fairly new unit- 2nd winter for it. Inside and outside were both replaced. I _think_ there are setting switches inside the blower unit that can be used to configure a few things. Maybe the installer got it wrong? It did this last winter as well, but it wasn't as cold as this year, so I couldn't check often enough to be sure. Now I'm sure.

pende_tim
Premium Member
join:2004-01-04
Selbyville, DE

pende_tim

Premium Member

It would help if you gave us a model number of inside unit, out side unit and thermostat.

cowboyro
Premium Member
join:2000-10-11
CT

cowboyro to owlyn

Premium Member

to owlyn
Aux should be called by the HP. The thermostat doesn't know when the unit goes in defrost.

owlyn
MVM
join:2004-06-05
Newtown, PA
Netgear CM2050V
Netgear RBRE960
Netgear RBSE960

owlyn to pende_tim

MVM

to pende_tim
They are both Trane. The HP is an XR-13. I'll have to get the exact model number of the blower unit, but it is a matching unit. Nothing odd. I did get 2K of resistance heat instead of the standard 1.5K. The thermostat is a Honeywell- don't know the model number, but it was replaced at the same time. It is a mid-range programmable. Nothing is two-stage, and the thermostat has to be changed from cool to heat- it doesn't switch automatically by just setting the temperature. It does indicate when the aux heat is on.

pende_tim
Premium Member
join:2004-01-04
Selbyville, DE

pende_tim

Premium Member

If in fact the strips are 2KW units, that is not much heat. Most likely they are 20Kw strips.

If you put the system in in "emergency heat" mode, do you get warm air from the registers? That check will verify that the resistance heaters are working. Just because the light comes on, does not mean that they are actually working and producing heat.

If they are throwing heat in "emergency heat" mode the heaters are present and functioning. If defrost does not turn them on then it may be something in the defrost board or a wiring problem.

Jack_in_VA
Premium Member
join:2007-11-26
North, VA

Jack_in_VA to owlyn

Premium Member

to owlyn
First check that the breaker for the heat strips is on. That would be a separate breaker in your panel and there is also one at the AHU itself. Check the obvious easy things first.

owlyn
MVM
join:2004-06-05
Newtown, PA
Netgear CM2050V
Netgear RBRE960
Netgear RBSE960

owlyn to pende_tim

MVM

to pende_tim
I can tell that they come on when it gets below 22 outside. Suddenly it's blowing hot air instead of warm air, so I know the aux is working. It's just during defrost cycles, and the cold air only lasts a few minutes.

And yes, I meant 20k instead of 15 k.

Tex
Dave's not here
Premium Member
join:2012-10-20

Tex

Premium Member

When the outdoor unit initiates defrost, it brings on the indoor unit heat strips until defrost terminates. It's likely the installers didn't wire the system correctly for this to happen.

pende_tim
Premium Member
join:2004-01-04
Selbyville, DE

1 edit

pende_tim to owlyn

Premium Member

to owlyn
Since this system is new, I would get the installers back to verify and correct the wiring between the inside and outside unit, there may be a problem there if the heat+defrost has never worked. There also could be jumpers on the defrost board or the airhadler may not be set properly to tell the system that resistance heat is present.

On the other hand, if it did work before there may be a failure in the defrost board. This part should be under warranty. Depending on your warranty coverage, labor may not be covered however.

Although the system will work well without resistance heat during defrost, there may be other things going on that need to be corrected.

As I sit here typing this response another possibility just came to mind based on the "Hot air" below 22*. Let me ask a question first:

When the temp is below 22* does the outside unit run?

Outdoor unit should be running along with the heat strips until the outdoor temperature gets down to 10* or less if things are setup properly.

If the outdoor unit is not running at 22* it is possible that the system thinks it is a dual fuel system (gas + heatpump) . If so it thinks that the aux heat and the heatpump can't be on at the same time. This would be consistent with no resistance heat in defrost mode.

I know this is a far out possibility, and could just be a simple wiring error done at install. Get the company back to take a look at it. If it is programmed as a dual fuel, this is costing you money not having the heatpump run below 22* with the strips.

Tim

owlyn
MVM
join:2004-06-05
Newtown, PA

owlyn

MVM

The outside unit is running below 22 degrees. I'm inclined to think they have the jumpers on the air handler set incorrectly. I'll call them. Thanks.

pende_tim
Premium Member
join:2004-01-04
Selbyville, DE

pende_tim

Premium Member

As I said the dual fuel setup was a far out possibility. So it looks like you have a possible wiring error when it was installed or a defrost module problem.
Please let us know the results.

Jack_in_VA
Premium Member
join:2007-11-26
North, VA

Jack_in_VA to owlyn

Premium Member

to owlyn

Re: [HVAC] Heat Pump Defrost/Aux Question

OP did you check to see if the strip heat comes on when you put the thermostat in "Emergency" position? If it doesn't check your breakers. One has to assume they worked at one time since it's not a new installation (2nd winter). What did it do last year?

owlyn
MVM
join:2004-06-05
Newtown, PA
Netgear CM2050V
Netgear RBRE960
Netgear RBSE960

owlyn

MVM

Yes, the resistance heat does come on in Emergency mode. We can smell it as well as feel it. I suspect only half/some of the strips come on in aux mode, as there is no smell. The dust probably burnt off of those at the beginning of aux heating season.

Last year, I think it behaved the same way. It wasn't real cold around here, so there weren't as many chances to check to see if it was happening. It's colder this year, so I happened to be home a couple of times when it happened. I'm not sure that the cold air lasts as long as the defrost cycle. Maybe there's some built-in delay before the aux heat comes on?

Okay, so the next question- how would the HVAC contractor test the defrost cycle aux heat without waiting around for it to go into defrost mode?

pende_tim
Premium Member
join:2004-01-04
Selbyville, DE

pende_tim

Premium Member

There is usually a button on the defrost board in the outside unit what will initiate a defrost cycle.

Jack_in_VA
Premium Member
join:2007-11-26
North, VA

Jack_in_VA to owlyn

Premium Member

to owlyn
said by owlyn:

Yes, the resistance heat does come on in Emergency mode. We can smell it as well as feel it. I suspect only half/some of the strips come on in aux mode, as there is no smell. The dust probably burnt off of those at the beginning of aux heating season.

Last year, I think it behaved the same way. It wasn't real cold around here, so there weren't as many chances to check to see if it was happening. It's colder this year, so I happened to be home a couple of times when it happened. I'm not sure that the cold air lasts as long as the defrost cycle. Maybe there's some built-in delay before the aux heat comes on?

Okay, so the next question- how would the HVAC contractor test the defrost cycle aux heat without waiting around for it to go into defrost mode?

It's possible that only part of your strips are ever needed and yes there is a small delay in the strip(s) being energized and putting out full heat. If the cold air doesn't persist the entire defrost cycle then the heat strips are probably working.

The HVAC contractor should have verified the proper operation of all the equipment.

owlyn
MVM
join:2004-06-05
Newtown, PA

owlyn

MVM

Jack_in_VA,

Thanks. I'll need to observe once more, but as you know, it's pretty darn warm in the east right now.