Party Walls

Air-source heat pumps are a booming business. In the Northeast, manufacturers report that sales of residential systems have increased by 25-35% per year over the past 5-10 years. We’ve seen more and more systems being installed in existing homes (to provide cooling while offsetting oil or propane used for heating) and into new homes (often as the sole source of heating and cooling).

We’ve looked into these systems often, and from many perspectives. I’m planning a series of posts, but, for now, here are the answers to some basic questions we receive from clients.

First, the basics: What is an air-source heat pump (ASHP)?

It’s an air conditioner that can operate in reverse. During the summer, it moves heat from indoors to outdoors. In the winter, it moves heat from outdoors to indoors. We helped NEEP (the Northeast Energy Efficiency Partnerships) to put together a market assessment and strategy report on ASHPs. The early sections in this document (see p. 12) outline the different terms and types of heat pumps (ducted/ductless, split/packaged, mini-split, multi-split, central, etc.) Unfortunately, different people can use the same term to mean different things, but hopefully the NEEP Northeast/Mid-Atlantic Air-Source Heat Pump Strategies Report can help clarify things.

Indoor section of heat pump.

Outdoor section of a heat pump.

Can these systems really provide all of the heat needed in a home in the Northern US?

Generally, yes, though you certainly have to choose the right equipment and install, operate, and maintain them properly. In the interest of helping people select the right equipment, we’ve been helping NEEP develop performance criteria for cold-climate ASHPs. This is an ongoing process, but NEEP has made a good start. See the cold-climate criteria as well as a list of equipment that meets the criteria here.

Are these heat pumps really efficient?

Well… often they’re not as efficient as one might hope (or as one might infer from ratings or literature). This is a big, contentious question, and I’ll dig into it more in future posts. Through a partnership with Efficiency Vermont and the DOE Building America program, we were able to perform some detailed monitoring of ductless heat pumps in New England homes. In short, we found seasonal coefficients of performance (COPs) averaged around 2.0; a report is available on the Building America site. Our tests showed that, in fact, heat pumps generally did deliver the promised capacity (i.e. heat output) at cold temperatures.

Do you recommend/specify ASHPs in homes and apartments?

Yes, very often. Even if efficiencies are not as high as one might hope, they are still pretty good. And with the heating loads in homes and apartments getting smaller and smaller, a seasonal COP around 2.0 is not necessarily a deal breaker. Heat pumps can still compete with oil and propane for seasonal heating costs, and if loads are very small, is it worth bringing natural gas to a home? Monthly gas utility fees can be more than the heating savings.

For some super low-load homes (like the Passive House below in Connecticut), one ductless heat pump provided all heating and cooling. These systems generally cost $3,000-$5,000 installed. The measured seasonal COP in this home was 2.0, and the total annual heating cost was $260. What other system would make sense?

This Passive House utilizes only one ductless heat pump for all of its cooling and heating.

There are still cases where I recommend more conventional systems (boilers, furnaces, etc.), but the performance and versatility of heat pumps seems to be improving. More manufacturers are making good cold-climate systems, and they’re making many more types of systems. From our evaluations, we’ve also learned some design and installation practices that can boost efficiency. More to come on this topic in my next post…