It might be surprising to you that residential space and water heating account for more than 50% of the average household energy and produce more greenhouse gases than the average car. For this reason, Elemental Buildings is investing a good amount of its resources into wall and window insulation to reduce the amount of energy required to heat and cool a home. However, an equally important consideration is choosing the heating system to heat the home.
Recently, I wrote a letter to the editors of Environmental Building News regarding the use of heat pumps in residential homes. While heat pumps have gained great popularity as ‘green’ sources of heating and cooling the home, I am convinced that they aren’t the most efficient method of heating. I thought I’d take this opportunity to expand upon this letter and explain why I’m choosing to go with natural gas for water and ambient heat.
Heat pumps can be used as hot water heaters, or to heat the ambient air inside your home during winter. Some people call it ‘free heat from the ground’, others say you are simply moving (pumping) heat from one place to another. In some ways this is true. Heat pumps work because all objects, both hot and cold, contain thermal energy. Consequently, you can take the energy out of the cold objects (to make them colder) and give them to the hot objects (to make them hotter). A heat pump is an anti-Robinhood. It steals from the thermally poor and gives to the thermally rich.
It might be easier to think about this in terms of an object you use every day – the air conditioner. An air conditioner cools your house by transferring thermal energy from the air inside your house (in effect slowing down how fast the air molecules zip through the air) to the air outside your house. If you ever stand outside the exhaust from an AC outlet, you can feel how hot the air can get.
Of course it isn’t free to run your air conditioner. An AC unit transfers heat from inside to outside by successively compressing and expanding a gas (what used to be Freon) in a closed loop between the inside and outside of the house. The energetic cost is the amount of electrical energy required to run this closed loop. This electrical energy can be compared to the thermal energy transferred between the inside and outside of the home. This is called the coefficient of performance or COP.
The COP for a heat pump is around 3.5. This means that the heat pump transfers 3.5 kilowatt-hours of heat energy into the home for every kilowatt-hour of electrical energy it uses. Sounds pretty good, huh? Compared to an electrical heater it is. The COP for an electrical heater is less than 1. In an electrical heater, electrical energy is converted directly into thermal energy (by running it through a resistor, for example) with a little bit of wasted energy to create things like light (your electric stove glows when it’s hot, right?). So an electrical heater can transfer less than 1 kilowatt-hour of thermal energy for every kilowatt-hour of electrical energy it takes to run it.
So why not use a heat pump? While it seems incredibly efficient, it really isn’t. A heat pump requires electricity to run and generating electricity is incredibly inefficient. You can’t just ignore the wasted energy and greenhouse gases produced by making electricity. In America, most electricity comes from burning coal. This produces steam, which turns a turbine to generate electricity. Only 30% of the energy from burning coal actually gets converted into electricity; the other 70% is released into the environment as wasted heat . But this heat is exactly what we need! If we burned the fossil fuel in our house, we wouldn’t be wasting the heat into the environment.
This is why it still makes sense to use natural gas to heat your home. If you run the numbers, the greenhouse gases produced to produce the electricity to run heat pumps is higher than the greenhouse gases produced by burning natural gas to heat a house. This ignores the additional energetic costs in digging a large hole in the ground as a heat sink and the extra expense in purchasing the heat pump.
One day it might make more sense to use heat pumps. In England, research is being done to run heat pumps without using electricity (article). Secondly, as we transition to using solar and wind to generate electricity, we will no longer be producing greenhouse gases to make electricity. In some areas of the country where renewable resources are more popular (such the Northwest, where > 50% of electricity is generated by hydroelectric), it makes more sense to use heat pumps. Which brings us back to another point Elemental Building believes in – going green requires local solutions to each problem.
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