MAIL: Heating Choices

Outdoor units of air-source heat pumps.
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 (4)

Ian Woofenden's picture

Hi Fred,

I appreciate your thoughtful perspective and information.

I'm hoping you'll edit your comment to correct the errors in terminology, since it's confusing and lacking in credibility as written. Most of the times when you say "watt" and "KW" in your comment, you mean "watt-hours" and "kWh". I think the only time you use the terminology correctly is when referring to the 400-watt array.

This is not an idle or picayune distinction. The difference between kW and kWh is as significant as the different between miles per hour and miles (and analogous to those two measures, and in that order). Rampant misuse of the terms only encourages electrical illiteracy.


Ian Woofenden, Home Power senior editor

Fred Golden's picture

I read both replies and have a thought. Say your heat load is 13,000,000 Btu's (per year) and you installed a 13 SEER heat pump. It could generate 13,000,000 Btu's while consuming about 1,000,000 watts.

By using a 18 SEER heat pump, it would consume only 722,220 watts to produce the same amount of heat, but installed cost might be $4,000 - $5,000 more.

13,000,000 Btu's by resistance heat would require about 3,000,000 watts to produce. While a electric heater will have the lowest installed cost, it would have the highest running cost. Yet if one where to dedicate the $5,000 saved installed cost to installing $5,000 more in solar panels and inverters, they will probably produce more power than the heating system can consume in one year, offsetting any energy used for the heating system. This is why figuring out the actual numbers is important for your location, cost per KW for both produced power sold in the peak daytime hours, and what you must pay for power during the non-peak winter and night time hours.

By installing a 400 watt solar system, you can generate about 2 KWh per day or 700 KWh per year (less in real life) and one advertiser at Home Power will sell you 400 watts of panels for only about $500. It seems much more prudent to install a 13 SEER heat pump and spend the remaining $5,000 in upgrading the solar system, rather than to spend it on a 18 SEER heat pump and while only save about 300 KW per year on your heating system. Even spending only as little as $800 to expand a solar system by 400 watts will produce twice the amount of electricity that a 18 SEER system will save in one year. If you use the heating system less each year, then your savings will be even less with the high efficiency system, while the lower EER system will be off much of the time anyway. The 400 watt solar system will keep working all the time.

Some super insulated homes require less than 1,000,000 Btu's of heat per year, and that is easy enough to make with a resistance heating system, and not bother with the cost of installing a heat pump. Spend that $5,000 on a larger solar system, and your solar produced will more than offset the energy consumed to warm the home by electric resistance heaters.

And if you need to bake something while it is 15F out and the home is cold, you are not really consuming the electricity to warm the home, you are using it to bake something, and the home will get warmed up by running the oven.

Another thought. Rather than spend thousands of dollars on insulation beyond R-30, why not spend any extra 'budgeted money' on a active solar heating system. So insulating to R-30 should be considered part of the cost to construct a well built home. If you have in your budget $5,000 more, and have a pick between additional insulation or installing a couple of solar hot water panels and a 120 gallon storage tank, I think that the storage tank and some evacuated tube solar panels would provide more heat to the home than the insulation will retain within the home.

You lose more heat through the required minimum ventilation into the home than is lost through R-30 walls and roofs.

Some countries require a home be built with a solar hot water heating system! We should require that here as well, at least for the lower 18 states!

In my area, normally the winter peak power is supplied by the large hydroelectric dams along the Columbia River.

In New York, much of the power comes from large hydro dams in Canada.

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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