Heat Pump Primer: Page 4 of 4

Why Electric Heating Finally Makes Sense
Beginner

Inside this Article

A typical heat pump
A typical heat pump can deliver two to four times the amount of energy it takes to operate. It can act as a boiler, furnace, and air conditioner—all in a single unit.
An air-source heat pump
Air-source heat pumps are easy to install and are beginning to rival the performance of ground-source heat pumps, at a fraction of the cost.
Ground-source heat pumps extract heat from the earth using a well, several bore holes, or loops laid in shallow trenches.
Ground-source heat pumps extract heat from the earth using a well, several bore holes, or loops laid in shallow trenches.
Closed-Loop Horizontal Trench
Closed-Loop Horizontal Trench
Closed-Loop Vertical Bore
Closed-Loop Vertical Bore
Closed-Loop Water Source
Closed-Loop Water Source
Open-Loop Water Source
Open-Loop Water Source
The WaterFurnace 502W12 hydronic heat pump
The WaterFurnace 502W12 hydronic heat pump is capable of delivering 150°F water, and is suitable for baseboard radiator systems, underfloor and overfloor radiant applications, and fancoils that transfer heat to a forced-air heating system.
The WaterFurnace 500A11 ground-source system
The WaterFurnace 500A11 ground-source system provides forced-air heating and air conditioning, and can contribute to a home’s domestic water heating.
The outdoor unit of a minisplit air-source heat pump
The outdoor unit of a minisplit air-source heat pump can be small and inconspicuous. The refrigerant lines going to the inside minisplit unit are protected inside of the shown chase.
A minisplit air-source heat pump puts the second heat exchanger inside the house
A minisplit air-source heat pump puts the second heat exchanger inside the house, moving refrigerant (rather than air) between the two.
A typical heat pump
An air-source heat pump
Ground-source heat pumps extract heat from the earth using a well, several bore holes, or loops laid in shallow trenches.
Closed-Loop Horizontal Trench
Closed-Loop Vertical Bore
Closed-Loop Water Source
Open-Loop Water Source
The WaterFurnace 502W12 hydronic heat pump
The WaterFurnace 500A11 ground-source system
The outdoor unit of a minisplit air-source heat pump
A minisplit air-source heat pump puts the second heat exchanger inside the house

ASHP Performance

The best VRF ASHPs operate with COPs that are close to those of GSHPs, and their performance doesn’t change over time due to long-term changes in ground temperatures.

Costs of both GSHPs and VRF ASHPs vary widely depending on available installers and system popularity, but the pricing differences can be dramatic. It is not unusual for GSHP installations to cost $25,000 to $35,000, while $10,000 to $15,000 is more common for the VRF units. With simple installations in places where a lot are installed, VRF heat pump installations can cost as little as $5,000.

Heat Pump Water Heating

Both ground- and air-source heat pumps can be configured to heat water along with space heating and air conditioning. In the summer operation mode, water heating can be almost free—as a byproduct of the cooling cycle. In this mode, heat is extracted from the indoor air, but instead of simply dumping it into the ground or outside air, a “desuperheater” diverts the waste heat for water heating. Only a few heat pumps incorporate water heating, but this will become increasingly common as heat pump advances continue.

Heat Pumps & Photovoltaic Power

If we can reduce space-heating loads—through high levels of insulation, well-insulated windows, and airtight construction—and provide some heat with passive solar design, then it makes sense to provide the small amount of needed heat with solar electricity. And it makes sense to use heat pumps, rather than electric-resistance heat, since heat pumps are far more efficient.

PV systems can allow achieving net-zero-energy use in homes. With the heating loads low enough and using a heat pump to deliver more heat from each kWh, a simple roof- or ground-mounted PV system with net metering should be able to satisfy those needs. For example, a well-built, well-insulated, and well-sealed 1,500-square-foot house in  southern New Hampshire, for example, could require about 9,500 Btu per square foot per year for heating, or about 14.25 million Btu per year. That converts to about 4,175 kWh, which in New England could be supplied by a 3.5 kW PV array.

Most early net-zero-energy homes used GSHPs for heating because they offered the highest COPs. But today’s VRF ASHPs are nearly as efficient, and that efficiency is less likely to drop over time. Plus, because of the significantly lower installation cost of VRF heat pumps, the economics can be far better then with GSHPs, allowing you to invest the savings into a bigger PV system.

PV-powered heat pumps can provide a truly renewable source of heating (and cooling). Avoiding all combustion in the home eliminates the risks inherent with fossil fuel and wood combustion—long-term health problems from air, soil, and water pollution; explosions; etc. This is why more and more, leading energy engineers and builders are now examining VRF air-source heat pumps more closely.

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Alex Wilson is the founder of BuildingGreen in Brattleboro, Vermont, and executive editor of Environmental Building News. He is the author of Your Green Home (2006) and coauthor of the ACEEE Consumer Guide to Home Energy Savings (ninth edition, 2007).

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