ASK THE EXPERTS: Heat Pump in Garage?

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 (2)

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?

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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