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Air-Source Heat Pumps in Homes: Step #2 – Pay Attention to the Envelope

This is part of a series; see the first post here.

This shouldn’t be news to anyone:  In most homes, insulation and air sealing are the most effective ways to improve comfort and reduce heating and cooling costs.

This holds true regardless of heating systems or fuels used. So why is it emphasized even more when talking about heat pumps and electrification? Four reasons.

1.  Heating System Capacity and Cost.

Say your home has a design heating load of 60,000 Btu/h.[1] If heating with fuel, you’ll need a furnace or boiler with a capacity of at least 60,000 Btu/h. These are easy to find. (In fact, you may have a hard time finding heating systems with capacities lower than this.) Air-source heat pumps, on the other hand, have smaller capacities. I don’t think you’ll find an ASHP with heating output of 60,000 Btu/h at cold winter temperatures. So to meet this load, you’ll need multiple ASHPs. And that gets pricey.

Even if you are not talking about multiple heat pumps, a 3-ton[2] heat pump is quite a bit less costly than a 5-ton heat pump. Costs of heat pumps scale more dramatically than costs of boilers and furnaces. So lower heating loads → fewer, smaller heat pumps → lower upfront costs.

Spray foam insulation

Spray foam in an attic – one of many ways to insulate and seal.

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Air-Source Heat Pumps in Homes: Step #1 – Be clear about goals

This is part of a series; see the first post here.

Building new homes that are all electric makes TONS of sense. I’ve written about that before. Electrifying existing single-family homes, however, is not necessarily straightforward. Many state and utility programs in the Northeast[1] offer hefty incentives for air-source heat pumps (ASHPs), but fuel-fired systems are often left in place and used as the primary heating system. Clearly, when that’s the case, carbon emissions are not reduced much. Other programs are pushing completely electrifying homes and removing fossil fuels, but these programs are not gaining all that much traction.

This may seem obvious, but it’s important to consider homeowners’ goals and desires when installing heat pumps in homes. I don’t necessarily see this considered by policy makers and electrification programs, and I think it’s a big disconnect. Programs and policies are focused on the big picture (appropriately) and generally want to reduce/eliminate fossil fuels to help meet carbon reduction goals. What homeowners want can vary like crazy.

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Air-Source Heat Pumps in Homes: Soup to Nuts

As I’ve written in several posts [1], electrification is all the rage. Modern air-source heat pumps (ASHPs) are fantastic technologies, and they can provide reliable, efficient, clean, affordable, and sustainable heating and cooling when done well and in the right application. I worry that these caveats are too often glossed over. I’ve also seen really bad heat pump installations, and I think it’s easier to screw up a heat pump than a boiler or furnace.

ASHPs on the exterior of a home

Three ASHPs at a home

This post is an overview of the process I suggest for good ASHP installations. I’ll be doing a post for each of the steps here, but this first post is an outline of the whole process because I believe the whole process is really important. The focus here is on homes, mainly in colder climates, and I’m not talking about VRF systems. I use the generic second person, so “you”  can refer to different people in different steps.

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