Air Source Heat Pumps — Space Heating

(This is an expanded version of an article published by ‘Green Energy Times’ in their Oct. 2012 issue, #16)

By Tad Montgomery

Rosenbaum HP Outside

Marc Rosenbaum’s Fujitsu 12RLS single zone ductless ‘mini-split’ heat pump (

Most people either have not heard of an air source heat pump (ASHP) or do not know how common they are. Most of us own one or more already, because they include nearly all refrigerators and air conditioners.

ASHPs collect heat in one space and deliver it to another through the use of special fluids, a compressor, and usually a fan. The fluids evaporate within tubes in the space to be cooled, and condense in tubes in a space to be heated. We might transfer heat from a house to the outdoors, or the other way around. We also might transfer heat from one part of a house to another in the same way.

The ASHP known as a ductless mini-split is an excellent example. The mini-split is not very large, hence ‘mini,’ but it is also divided into two or more components, with a compressor in one place and condensers or evaporators in up to four other places, hence ‘split’. They can serve both as heaters and air conditioners.

The mini-splits have become very efficient devices that deliver two to three+ times more energy in heat than they consume in electricity. Though this might sound like a violation of the laws of physics, they achieve this because the heat is being moved, rather than produced directly from electricity.

A single compressor for an ASHP, usually placed outdoors, might supply heating and/or cooling to as many as four places in the house. This is where the ductless part comes in.  Ducts are often among the worst problems with regard to a building’s energy efficiency. The ductless mini-split moves liquids instead of gases, and only a short distance.  That is part of the reason why it is so efficient.

There are, of course, down sides. The mini-splits have a minimum temperature rating, dependent on model, and if the outdoor temperature drops below it the heating system switches to conventional electric resistance heating or a backup heating system is required. This minimum temperature varies from unit to unit and can be as low as -10°F. Different models also have different warranties.

Air source heat pumps are not appropriate for every situation. One example is trying to heat a leaky, old Victorian house with an ASHP.  One client of mine wanted this until she learned that five units would be needed at a cost of $25,000.  The first step in such a situation is almost always to button up and insulate, then explore alternative heating systems.

In the right situation mini-splits have the potential to reduce energy usage dramatically while increasing comfort. These systems have been used in Asia and Europe for decades, and now that energy prices are climbing in America we will be seeing a lot more of them here, too.

Rosenbaum HP Indoor

Interior component of Rosenbaum’s Fujitsu mini-split (above painting)

Marc Rosenbaum of Energysmiths installed a mini-split in his 1,600 sq. ft. house on Martha’s Vineyard in 2012 to replace a high efficiency boiler (see photos). He has monitored the system extensively and reports that he reduced his heating energy consumption by 75%. His plan is to install photovoltaic panels at his house in order to cut his reliance on nuclear and fossil fuel. He also installed an ASHP hot water heater, but that will be the subject of a later article.

In the future we expect these ASHP units to become even more efficient. For example, some commercial applications of mini-splits capture heat in one area of a building where cooling is needed and bring that heat to another area where it is needed for space heating.

Rebates: Efficiency Vermont & PSNH sporadically offer rebates for ASHP’s.  A representative for PSHN queried by Green Energy Times emailed: “For 2013-2014, all 4 (NH) electric utilities proposed heating and cooling system rebates, including a rebate for mini-split / air source heat pumps. We think this technology is extremely efficient and works great with homes heated by Oil or LP FHW (liquid propane forced hot water – eds).” The NH Electric Co-Op offers rebates up to $2,000 for mini-splits. Western Mass. Electric $150-500 for “Central AC/Heat Pump.”  For up-to-date information on rebates and incentives contact Home Energy Advocates.

Models: Two ASHP space heater models are Mitsubishi’s ‘Mr. Slim’ and the Fujitsu 12RLS.

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