MAIL: Heating Choices

Outdoor units of air-source heat pumps.
Outdoor units of air-source heat pumps.

I enjoyed reading the article entitled “Platinum PV” in HP158. It was interesting to see where the designers didn’t agree with the LEED standards. I would like to take issue with another of the design decisions made in this house—the choice of an electric furnace instead of an air-source heat pump.

The article mentions that it was less expensive to add PV modules to cover the electricity of the electric heat than to install a heat pump. While this may be the case, it doesn’t take into account the electricity used to power the heat. Electricity is used in real time—the utility isn’t storing our PV-produced power for us to use later. This electric heat unit will likely run mostly at night and during cloudy weather. This means that it will be using electricity largely produced by coal, nuclear, or natural gas—not PV-produced electricity.

So this design choice has a heating system that uses more electricity than a heat pump. And this electricity is most likely coming from conventional (nonrenewable) energy sources. It seems that a Platinum-certified LEED house would strive to use less energy overall (even if offset by PV production) and certainly less from fossil fuels. Installing an air-source heat pump would have met both of these goals and provided air conditioning for the home.

Matthew Huffman • Swoope, Virginia

Comments (2)

MontanaGreen's picture

As is so often the case with systems engineering, "It all depends..."

The mix of generation sources that provides energy to the house, depends on the supplier (utility) to which the house is connected. And it also depends on the time of day, and season of the year.

Although the utility may not store the energy from the PV modules, that energy displaces the generation sources used by the utility, at the time the PV modules supply it. Then, the home heating system takes energy from the generation source mix at the time it is needed.

So, the generation mix during sunny days, compared to the generation mix when heating is needed, may be a factor.

If you want to ignore that comparison, and assume the same generation mix during both periods, then the PV has completely replaced the slice of generation that would have been used by the heating system, with energy that was produced by the PV system. Thus the system of electric furnace and PV modules, can be added to the grid with no net increase in generation. Home is heated, with no net impact on generation.

Richard Schmidt's picture

Mr. Huffman is correct. I'm an engineer, energy consultant and owner of PV, solar hot water and heat pump systems in my home. This basic science is demonstrated by years of data and analysis on my home. I have five electric (kWh) meters and fuel oil metering to provide data on my various energy supplies on a daily basis.

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