Air-Source Heat Pumps

Reader Contribution by Paul Scheckel
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Air-source heat pumps are not exactly new to the world of air conditioning and the HVAC industry, and they are very different from ground-source heat pumps (their respective names refer the source of the heat to be gained and used). What is new however is the surge of interest in using air source heat pumps for heating in addition to cooling. This is primarily due to advances in technology that make heat pumps effective for heating in very cold climates (down to -15F), while also delivering exceptionally efficient cooling performance.

An air source heat pump is a modern appliance that extracts heat from one place and moves (or pumps) it to another, much like a refrigerator removes heat from its interior and out into the kitchen. Moving heat is much more efficient than generating it, so heat pump heating is more efficient and costs less than half as much to operate compared to conventional electric heat. Because it’s a pump, it can move heat in both directions – from indoors to out, or outside to inside. The happy result is that very high-efficiency heating and air conditioning are available through this single, integrated heating/cooling appliance.

Heat delivery, use of interior wall space, and the ‘quality’ of heat is similar to a single, central, wall or floor-mounted gas heater. There is also a small, quiet, outside unit. During the past two years, my company (Shelter Analytics) has been focused primarily on delivering energy efficiency services to condominium associations. The quiet operation and low profile of air-source heat pumps have made them ideal for the close-living environment and space constraints of condo owners in addition to single family homes.

Single or multi-head air source heat pumps are also called “mini-splits” or “ductless” air-conditioning and heating units. They are different from central heating and cooling systems because there is no duct-work for heating or cooling distribution. A single outdoor unit serves one or more indoor units which deliver conditioned air directly to the space (or zone) in which they live.

Cold-Climate Heating

Currently, only single head systems (one indoor heat delivery unit) are available for cold climate heating, limiting their use to small dwellings, large rooms, or homes with open floor plans. Of course, a more efficient building envelope (good insulation and minimum air leakage) makes any heating system more effective. In fact, an efficient home equipped with solar panels and an air source heat pump can use solar energy for active heating and cooling.  By late 2014, “multi-head” cold-climate heat pumps will be available offering greater flexibility. To be clear, multi-head systems are currently available that provide efficient heating and cooling in moderate climates where temperatures are consistently above the 0F mark.

The magic of a heat pump happens when a refrigerant (typically R-410A), is circulated through a copper tube within a closed loop between indoor and outdoor units. When the refrigerant is compressed, it becomes colder and able to absorb heat. As it warms, it changes phase from liquid to gas, ready to be compressed and cooled again. Smart controls tell the heat pump which direction to work, so that heat can be absorbed from, and delivered to, the right place (indoors or out) depending on your desired indoor temperature.

Ducts in a typical home are generally leaky and uninsulated. Efficiency gains with a heat pump are increased because there are no ducts through which conditioned air must travel. Additionally, ducted systems are difficult to split into zones, but a single or multi-head heat pump allows you to condition only the room, or rooms, desired.

There are currently only two cold-climate heat pumps available in the U.S. they are the Mitsubishi M-series and Fujitsu RLS2H, but more are likely on the way. It will be important to size units appropriately for your home and climate by having a heating contractor determine the heat load of your home.

For more energy efficiency and renewable energy solutions for your home, please read The Homeowner’s Energy Handbook: Your Guide to Getting Off the Grid by Paul Scheckel.