turning off auxiliary heat during defrost - need help

neohioheatpumpApril 11, 2012

Hi Everybody,

I need some help. I have a single stage heatpump with demand defrost (3-tons) and a 90% gas furance (100BTU input). This is a dual fuel setup.

I have it controlled by a focus-pro wireless t-stat kit. It works great EXCEPT FOR.

When my heatpump goes into defrost it initiates the gas-furance. Sometimes during a longer defrost their is significant heat left in the plenum when it switches back to heatpump and there is annoying vibration until all the leftover heat produced from the gas furnace during the defrost cycle is gone. I'm sure this vibration is not good for the heatpump. This has been happening since I had the system install 2.5 years ago. It hasn't caused any problems but I'm sure it might take life off the system. Its also annoying hearing the vibration after it comes out of defrost.

This happens because my furnace is single stage and it produces way to much heat then necessary to temper the cool air of defrost and the heat comes from below the coil.

Can I disconnect a wire from my t-stat base by my furance so when it goes into defrost it doesn't start my furnace? I tried disconnecting the aux/E wire. This solves the problem - but I can no longer activate the gas furnace if I were to switch to emergency heat or for when it drops below the balance point that I have set on the t-stat.

Or do I have to disconnect the W wire at the heatpump because I read on other forums that this is energized during defrost?

I'm looking for some type of solution. I can live with a short spurt of cold air during the defrost cycle.

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You understand that the defrost cycle is running the unit as an air conditioner?

If you do not run the backup heat it will blow cool air into the house.

    Bookmark   April 11, 2012 at 3:44PM
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Yes I realize that. The defrost cycles are pretty short most of the time.

I don't like the fact sometimes when it comes out of defrost there is still significant heat left below the coils from the gas furnace. This causes a condition where the air blowing across the coils isn't cooler than the coil from the refrigerant fromt the heatpump in heating mode. It creates a vibration for a short period of time until the heat from the furnace is blown thru the vents on out. The vibration is not good and happens because the heat is not being removed from the coil and is sent back to the outdoor heatpump.

I could live with a little cold air coming out the vents during defrost. I don't have many defrosts to begin with since its demand defrost.

I just want to figure out how to accomplish this.

    Bookmark   April 11, 2012 at 3:54PM
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For an ideal situation, I sounds like you need the blower to run for a few seconds before the compressor re starts in the heat mode.

    Bookmark   April 11, 2012 at 7:51PM
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If the furnace does not operate during the defrost cycle, the defrost cycle is likely to be much longer and cool air from the registers will be for that longer period of time.

    Bookmark   April 12, 2012 at 7:42AM
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So what is the source of the vibration when the defrost cycle is ending? That does not seem right.

    Bookmark   April 12, 2012 at 10:59AM
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"It creates a vibration for a short period of time until the heat from the furnace is blown thru the vents on out."

It really sounds like something is wrong here.

The residual heat from the backup heat should not be bothering the heat pump all that much.

    Bookmark   April 12, 2012 at 11:01AM
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Well the residual heat from the backup lingers for about a minute or 2 when there is a longer defrost. My furnace is single stage and is a 90% trane with 100KBTU's as input. The heatpump is 3-ton (36KBTU's). So when there is a longer defrost, after the defrost is over there is still considerable heat in the plenum. Soon as the heat-pump begins to create heat a hum starts and then goes away. This hum can be quite annoying and is definitely noticable.

Too be honest, I've recently dis-engaged the auxiliar wire from my t-stat base. This accomplishes my goal of not engaging the furnace during defrost. Only problem is if I wanted to use my furnace as in emergency heat mode it won't kick on. I notice that even when it comes out of defrost with no heat in the plenum from backup heat it makes a significant hum for a minute or two.

Is that normal? Could it have something to do with the way my txv was installed on the indoor coil? Even during regular operation I notice a slight constant noise from the indoor coil but this is much more toloerable and not like the hum that occurs when coming out of defrost. I do not get this slight noise during A/C mode.

    Bookmark   April 13, 2012 at 7:46AM
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Did you discuss the problem with the HVAC contractor? What was the response?

Perhaps the coil was never installed properly, or it is defective. A vibration sounds like a mechanical problem to me. I am suprised you have put up with this for 2.5 years.

    Bookmark   April 13, 2012 at 8:56AM
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I haven't discussed with the installing contracting. I'm not sure its a vibration. It might be a very low-register humm. The heatpump works fine. I get good temperature rises from the vents. While running in heatpump mode the very low volume low register humm isn't very noticable and is tolerable. My fiance doesn't notice it but I do. When it comes out of defrost its noticeable to her and me. It doesn't go into defrost that often. It has demand defrost. During cooling there is no humm at all from the coil. This has been the case since day #1.

The coil had a field installed txv. I remember when he installed he mentioned you could place the txv on specific 'positions of the clock' on the tube.

    Bookmark   April 13, 2012 at 10:48AM
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It could be the TXV bypass valve.

When you change from heat to cool the TXV has to either be bypassed or forced full open.

    Bookmark   April 13, 2012 at 10:49AM
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So that is the txv on the indoor coil? Basically that requires an entirely new txt valve?

If I were to have someone do this repair, is it a big deal? Does refrigerant need to be removed during this?

    Bookmark   April 13, 2012 at 1:49PM
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You need someone to come and diagnose the problem before deciding on a fix.

It could be the TXV, it could be a reversing valve.

No way to tell without examining the system.

It is not going to be inexpensive since the system may have to be emptied of refrigerant, repaired, and then vacuumed out and refrigerant replaced.

    Bookmark   April 13, 2012 at 3:49PM
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I"m going to wait and see. It has still been running fine. Its always done this. Its running right now as I write this. The sound it makes while running in heatpump heat mode isn't bad. Not as quiet as the gas furnace or when it runs in a/c but tolerable. I just don't like those defrosts. Maybe I"m being picky. Does anyone else out there care to comment on the noises after their heatpump comes out of defrost make? I'm not talking about the whoosh of the outdoor unit when it switches from cool back to heat.

Does anyone have to comment if their heatpump is quieter in cooling mode than heating mode?

    Bookmark   April 14, 2012 at 8:41PM
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