ASK THE EXPERTS: Heat Pump in Garage?

Beginner
Heat-pump compressor
Heat-pump compressors, like the one for this minisplit unit, belong outside, not inside a home or garage.

In colder climates, could an air-source heat pump be put in an attached (insulated, but not heated) garage as a way to improve its operating efficiency?

James Carrow • via email

Air-source heat pumps (ASHPs) work by extracting heat from ambient air and need an ample supply to work effectively. Installing an ASHP in your garage would suck out all of the warmth from the air captured there, effectively turning that space into a freezer, colder and colder until it would be impossible for the heat pump to extract heat effectively. Installing an ASHP inside a garage can void your manufacturer’s warranty and may cause permanent damage to the unit.

So while you want to avoid installing the ASHP in an enclosed space, installing one to replace an existing inefficient heating system, such as a furnace or boiler, can be a very good investment. I live in upstate New York, where many people are finding it harder to afford to heat their homes, especially with electricity, oil, or propane.

In my area, minisplit, hybrid, and geothermal heat pump systems are popular. Minisplits are the least expensive and easiest-to-install systems. A hybrid system is an air-to-air heat pump installed with a 92% or higher efficiency propane gas furnace. Geothermal heat pumps have the highest installed cost, but offer the greatest savings. Ground- or water-source heat pumps qualify for a federal tax credit of 30% of the installed cost.

Keep in mind that installing any heat pump is not a typical do-it-yourself project. Some manufacturers do not honor any warranty on equipment installed by homeowners or purchased on the Internet—so even though the online price may look great, you may end up paying a lot more to repair or replace equipment if it’s not properly sized and installed. 

If you heat with electricity, oil, or propane, you should look at heat pumps as a way to reduce your heating cost. It would be wise to make energy-efficiency improvements before upgrading your heating equipment.

Bob Zima • Certified Radiant Professionals Alliance radiant heat instructor

Comments (3)

Gordon Prince's picture

I have come across a heat pump that is designed to be installed inside the home. It uses an external source of air -- which everyone in Canada with a house built in the last 20 years has. We all have air exchangers that pump external air into the house and exhaust stale air to the outside. The internally installed heat pump proposes to use the air that is being exhausted by the air exchanger as its source of air to extract heat from. Since it is extracting heat from room temperature air, then after the heat has been extracted from it exhausts it (as the air exchanger was about to do), the manufacturer claims the operating efficiency is better than having the unit work with below freezing air, as is often the problem with external heat pumps.

I'm wondering if the air exchanger has access to a great enough volume of air for this to work. Also I'm wondering if there's something missing from the manufacturer's logic that I'm missing. Any thoughts?

Fred Golden's picture

Hi,

I was reading about a installation of a high efficiency make up air unit that is designed for a University Library, where it would take in about 2,000 CFM outside air, cool it, blow this into the building. At the same time, 2000 CFM of exhaust air was used to help cool the outdoor coil rejecting heat with the 76F indoor air. This will help provide the outdoor coil with some cooler air and enhance the efficiency of the unit. However you must consider the design CFM requirements of a 5 ton A/C system - heat pumps are nearly identical.

A 5 ton heat pump will move 2,000 CFM indoor air flow, and have about 6,500 CFM outside air flow. In the cooling mode, typical air temperature measurements would be 77F inlet, 57F outlet (+/- 2F). For the outside coil, typical inlet might be 95F with 110 - 115F outlet temperature.

During the heating mode, air flow stays at 2,000 inside and about 6,500 outside, until some frost develops on the outdoor coil, lowering the air flow a bit. Typical is 66F inlet and 96 - 106F discharge air on the indoor coil, while the outdoor coil is taking in outside air temp, say 30F and cooling that air by about 20 - 30F. The number of cubic feet of air inside the typical garage is only enough to supply a 5 ton air conditioner for a couple of minutes, and then it will be repeating a trip through the condenser, and cooling by another 20 - 30F. So the garage would quickly be cooled well below 10F.

You can direct the heated air discharged from the home ventilation system to the airstream intake of the heat pump outside unit. This might enhance the efficiency, however with only about 100 CFM to 200 CFM discharge airflow, and the heat pump taking in 6,000+ CFM while running, any 50F - 65F air blowing in the direction of the heat pump will not significantly raise the overall temperature of the air going into the heat pump. Also that moist air leaving the home might have an adverse effect on the heat pump outdoor coil. Because of the excess moisture, it might require more frequent defrost cycles, thus not saving nearly as much energy as you might hope for.

It is also important to note that the heat pump will produce less heat on a colder day. Thus a 1825 square foot home like mine that might need 30,000 Btu's of cooling per hour on a 100F day might have a heat load of 40,000 Btu's per hour on a 15F day, and require a 48,000 Btu (4 ton) heat pump to keep up with the heating demand, and still need to run the 10 KW (34,000 Btu) back up heater from time to time on a very cold day. Much better to run the 14 SEER 3.5 KW heat pump and collect 38,000 Btu's on a 15F day than to run the 10 KW electric heat and only make 34,000 Btu's of heat. So my heat pump will quickly cool my home in the summer, while it will take all night to warm it when it is 20F outside.

Dave Reichert's picture

James' question does raise a valid point. Since the ability of heat pumps to provide heat to indoor space is limited by the temperature of the air around the outdoor coil, couldn't the efficiency or outdoor temperature range for heating be improved if the "outdoor" air was warmer? The theoretical answer is obviously yes, the practical answer is not as easy, and Bob Zima's response explains why.
Perhaps Bob would comment on this proposed solution. Position or vent the outdoor unit so that it is able to draw air from the garage space and reject the air, after heat is removed, to the outdoors. The garage would then become an air "prewarmer" for the heat pump. The practical problem with this arrangement is that the garage will still become colder as it needs outdoor makeup air to replace the air drawn over the coil. And if the garage is attached to a house, then the garage/house wall would move more heat towards the cold side. I suppose if your garage is the size of an airplane hangar a small heat pump wouldn't have much noticeable cooling effect :-)
Other low grade heat sources could be considered ... exhaust from warmed space or the outlet of an air-to-air heat exchanger come to mind.
With the relatively high cost of geothermal or the need to install and run backup heating when outdoor temperatures restrict the heat pump's abilities, there's a fair amount of dollars that could be invested to extend the air-to-air heat pump's range before exceeding the cost of those alternatives.
I thought the question deserved a little more thought, Dave Reichert

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