Solar-PV-Powered Heat Pumps: Why not all homes?

European heat pump provides heat-recovery ventilation in addition to heating, cooling, and water heating.
European heat pump provides heat-recovery ventilation in addition to heating, cooling, and water heating.

Irrational Exuberance about Heat Pumps

The  Passive House and Net-Zero movements  have spawned some irrational exuberance about heat pumps. When you build a home that requires less than 5 BTU/hr per square foot of floor space, you can often heat and cool it with solar-generated electricity using a heat pump. From that reality, some folks are wondering why we all aren’t using solar-powered heat pumps to heat all homes? Why would we continue to use combustion heating systems, even condensing gas furnaces and boilers, when we can heat and cool homes with photovoltaic electricity and without combustion?

Why PV-Powered Heat Pumps Don’t Work on Most Homes

American homes that are ready to be heated and cooled by solar electricity are very few. America’s 90 registered passive houses qualify. There may be 800 homes out of 80,000,000 American single-family homes that are sufficiently energy-efficient. That’s 1 in 100,000 homes, which depend on or could depend on solar electricity for heating and cooling. That is 0.001% of the housing stock. I estimate that retrofitting the average home to need only 5 BTU/hr of heating and cooling capacity per square foot of floor space would cost more than $8 trillion or $100,000 per home. $8 trillion is more that twice the US gross domestic product (GDP) for 2013.

Heat Pumps: Far from Technological Perfection

Heat-pump technology isn’t perfect either. Heat pumps compress refrigerant to 550 psi, a wearing task. Heat pumps rely on homeowners to keep their coils clean. Acid rain and chemical air pollutants can corrode heat-pump coils shortening their life. For these reasons, heat pumps may or may not be a sustainable technology.  A few years ago, a Lennox representative in Texas told me that heat pump and A/C systems were lasting less than ten years there because of neglected maintenance and coil leaks.

I wish all American homes were ready for PV-powered heat pumps. For now, we need to forge ahead with insulating, air-sealing, and shading our homes to minimize energy consumption and load. Until homes meet the passive-house or net-zero standard, we either continue using combustion for home heating and grid-based electrical generation or we learn to live without heating and cooling.

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