By Robb Aldrich
More and more homeowners are investing in air-source heat pumps (ASHPs) to heat and cool their homes and move away from fossil fuels. Manufacturers and distributors have seen double-digit growth in the northeast for the past 5-10 years, and there are few signs of this slowing. Our team at Steven Winter Associates has looked into these systems from many perspectives, and we hope this article answers some questions that we often hear.
First: What is an air-source heat pump?
It’s basically an air conditioner that can operate in reverse. During the summer, it uses electricity to move heat from indoors to outdoors. In the winter, it moves heat from outdoors to indoors.
What do ASHPs look like?
They come in many shapes and sizes, but one of the most common is a ductless, mini-split system. These have one outdoor unit and one indoor fan coil that provides heating and cooling to a space. This indoor fan coil is often mounted high on a wall, but there are also ducted systems, ceiling-mounted fan coils, and floor-mounted fan coils. “Multi-split” systems have one outdoor unit connecting to several indoor fan coils. There’s too much variety to describe here, but the operating principles are very similar.
Can they really extract heat from cold, outdoor air in the middle of winter?
Yes (usually). It’s true that ASHPs have been typically used farther south (they’re pretty standard in Florida). But new technologies are appropriate for colder climates. Many manufacturers list performance down to 5°F, and some go down to -15 or -20°F. In our testing, we have seen them performing down at -24°F.
Of course, the colder it is outdoors, the harder it is for ASHPs to extract heat. So they deliver less heat at lower efficiencies under very cold conditions.
How efficient are these heat pumps in the winter?
That is the big question, and the answer is complicated. Heat pump efficiency is often expressed as a coefficient of performance (COP). COP is defined as energy out (heating) over energy in (electricity). In a study funded by the Department of Energy and in partnership with Efficiency Vermont, we were able to study some ASHPs in homes around New England. Efficiencies varied quite a bit, but we found that heating season COPs averaged around 2.0. That means for every kWh of electricity used, two kWh of heat were delivered. This is twice as efficient as electric resistance heat, but we’d hoped to see COPs closer to 3.
Efficiency depends on many, many variables, and we’re still coming to grips with all of them. See the supporting information for more on this.
Can these systems provide all the heat needed in a home in the northern US?
Sometimes. In many very efficient new homes, heat pumps provide all the heat needed without any problems. In old, leaky farmhouses, the size of the ASHPs you’d need to provide all the heat could be substantial (and expensive). Also, in VERY cold areas (where temperatures stay below -10 or -20°F for extended periods of time) some backup heat may be needed.
In many older homes, people install ASHPs to offset a portion of the heating needs. A ductless ASHP installed in a central living space, for example, can provide cooling all summer. When fall comes, it can heat this space and delay the use of an oil-fired boiler or furnace. When it gets cold enough, or when you really want heat delivered to the bedrooms, you can turn up the central heating’s thermostat. Many people install ASHPs to displace oil or propane heat rather than replace it. Oil or propane use can be reduced significantly, but the fuel-fired system is still there for when you need more heat.
Do these systems cost less to operate?
It depends on energy prices! A couple years ago, when oil was around $4 per gallon, the answer was typically YES. When oil dipped down to near $2 per gallon, it was much less certain. Savings depend on the application, the existing system efficiency, details of the heat pump installation, etc. At current prices, heating with natural gas is usually less costly than using ASHPs. But ASHPs always save energy when compared to electric resistance.
In new homes, we’re seeing a trend where people don’t use any fossil fuels. While ASHP energy costs might be a bit higher than gas heating costs, there’s lots of savings associated with oil tanks, propane tanks, gas plumbing, not to mention the ongoing meter and service charges.
Do ASHPs reduce carbon emissions?
It depends on how you heat now and where your electricity comes from. With electricity that is largely carbon emission-free (solar, wind, hydro, nuclear), heat pumps can lead to significant emissions reductions. If your electricity mix has a lot of coal, reductions are questionable.
If you’d like more information on any of these questions – along with links to other reports and resources – search for “heat pumps” on http://blog.swinter.com/.
Robb Aldrich is a Senior Mechanical Engineer, Steven Winter Associates.