Converting a central air unit to a heat pump?

JRP3

Member
Sep 17, 2007
205
I've been wanting to add a heat pump to my house and have been looking at either a mini split or a central unit. I currently have a central air AC unit attached to my oil furnace which is really overkill. My house is small and in the woods so I only use AC for a few hours a few days of the year. Most of my heating is with wood. I've been wondering if I could replace the external compressor unit with a heat pump and use the existing lines and indoor heat exchanger which would make installation easier and keep costs down. Is this possible?

I also wondered about modifying the AC unit to run in reverse like a heat pump but the little bit of research I've done makes it seem as if it would be difficult but can be done.
 

Brian26

Minister of Fire
Sep 20, 2013
512
Branford, CT
I saw you installed a Mr Cool in your other post. They make a whole house unit for $2800 with the same quick connect fittings. Really impressive cold performance as well. Full heat output to -5 degrees and 78 percent at -22.

 
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peakbagger

Minister of Fire
Jul 11, 2008
5,629
Northern NH
I've been wanting to add a heat pump to my house and have been looking at either a mini split or a central unit. I currently have a central air AC unit attached to my oil furnace which is really overkill. My house is small and in the woods so I only use AC for a few hours a few days of the year. Most of my heating is with wood. I've been wondering if I could replace the external compressor unit with a heat pump and use the existing lines and indoor heat exchanger which would make installation easier and keep costs down. Is this possible?

I also wondered about modifying the AC unit to run in reverse like a heat pump but the little bit of research I've done makes it seem as if it would be difficult but can be done.
[/QUOTE

Yes in theory its heat pump cycle but in order to get cold weather operation the compressor has to be configured differently. The density of the refrigerant vapor varies enough with temperature, that a heat pump compressor has to be run at variable speed to deal with the varying density. An air-conditioning compressor does not need that capability. Part of the big efficiency gain with a minisplit is there is no ductwork. AC ductwork normally is installed in an unconditioned space and the a combination of air leaks and poor insulation means the cooling does not get to the room. Even if you put in superefficient heat pump that loss is still there.
 
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JRP3

Member
Sep 17, 2007
205
I saw you installed a Mr Cool in your other post. They make a whole house unit for $2800 with the same quick connect fittings. Really impressive cold performance as well. Full heat output to -5 degrees and 78 percent at -22.

Yes I've looked at that but if I can use the line set and exchanger I already have in place that would save time and money.
 

JRP3

Member
Sep 17, 2007
205
AC ductwork normally is installed in an unconditioned space and the a combination of air leaks and poor insulation means the cooling does not get to the room. Even if you put in superefficient heat pump that loss is still there.
My ductwork is all in the basement which always needs more heat in the winter so losses are not an issue, actually a benefit. As I said cooling is not a problem, I'm looking for heat. It sounds as if trying to run the a/c in reverse would be inefficient, what about hooking a heat pump up to my existing lines and heat exchanger?
 

begreen

Mooderator
Staff member
Nov 18, 2005
85,058
South Puget Sound, WA
My ductwork is all in the basement which always needs more heat in the winter so losses are not an issue, actually a benefit. As I said cooling is not a problem, I'm looking for heat. It sounds as if trying to run the a/c in reverse would be inefficient, what about hooking a heat pump up to my existing lines and heat exchanger?
If the existing system uses a different refrigerant than no. Clearing and cleaning the lines would be very hard and the current refrigerant would need to be captured. Same thing for the heat exchanger.
 
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JRP3

Member
Sep 17, 2007
205
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semipro

Minister of Fire
Jan 12, 2009
3,882
SW Virginia
... what about hooking a heat pump up to my existing lines and heat exchanger?
No on the heat exchanger, maybe on the lines.
The existing copper line set might be adequately cleaned using a solvent made for this purpose. Its not cheap though.

I'm considering doing something similar where I'd replace my existing system with the Mr. Cool system referenced above using the existing copper lines. The lines run to my attic inside an interior wall. Frankly, I'm worried more about condensation and mold occurring on potentially uninsulated lines in the wall than I am about cleaning the existing lines.
 

Highbeam

Minister of Fire
Dec 28, 2006
17,988
Mt. Rainier Foothills, WA
Looks like a great opportunity to replace those old lines with new. You'll get an entirely new, clean, system made for the job. I can see wanting to reuse the electrical system that feeds the heat pump but not the refrigerant lines that have weird gunk/oil/R22 stuff in them plus might be corroded or leak when exposed to the higher pressures.
 

woodgeek

Minister of Fire
Jan 27, 2008
4,256
SE PA
Another couple factors....often the CFM required for a Heat Pump in heating mode is higher than the AC. So the ductwork may be undersized for heating.

The previous owners of my house (and the neighbors in an identical house) did not put in central air. I started with large ductwork and a HP. The neighbor put in those small 'high velocity' ducts and AC. 12 years later, I scrapped the oil boiler 9 years ago, the neighbor took an oil delivery yesterday.

The company that installed their AC.... also services and sells oil boilers and sells them oil. Just sayin'.

Other factor... most people that put in HPs also put in electric backup. The coils are CHEAP, but you need hefty wiring for that separate from the air handler wiring. I think I my wiring to the attic has a 100A, 240V breaker. !!!
 
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