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.
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.
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.
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).