Heat Pumps

You may have heard about heat pumps - also known as mini-splits.  Perhaps your neighbor has told you how great they are to cool down the house in the summer and supplement heating in the winter.

There are three main types of heat pumps

  • Air Source - This is the most common - it transfers heat between your house and the outside air. Today's heat pump can reduce your electricity use for heating by approximately 50% compared to electric resistance heating such as furnaces and baseboard heaters. High-efficiency heat pumps also dehumidify better than standard central air conditioners, resulting in less energy usage and more cooling comfort in summer months.  Some homes use ducts and some are ductless. A ductless version is called a mini-split.
  • Water Source - Water source heat pumps are not very common. They dissipate heat by way of water instead of air and require well, lake, or other water source access.
  • Ground Source - Ground source or geothermal heat pumps take advantage of thermal energy stored underground, transferring heat in a similar manner to air source heat pumps. Due to the constant temperature of the ground, they offer much more efficient operation.

Using electricity to circulate coolant and air, and to compress coolant, is much more efficient than forcing it into a resistive heat system like an electric baseboard or a space heater. Those types of systems essentially function by deliberately wasting electricity—converting its energy into heat at a one-to-one ratio, which is to say 100% efficiency. That might sound pretty good but when you’re using electricity to power a pair of fans, a compressor, and a pump, instead of converting the electricity itself into heat, you can get more than three units of heat out of every unit of electricity, for efficiency rates of over 300%.

Electric devices and systems are, in carbon/climate terms, as clean as the grid that supplies them. So heat pumps aren’t necessarily “clean” heat in and of themselves(though they are still a more efficient use of less-clean electricity than resistive heat systems, of course).

However, here in Vermont, according to Energy Action Network, nearly all greenhouse gas emissions from electricity consumption are due to the fact that we source some of our electricity from a regional system mix through ISO New England.* Since 2010, the renewable proportion of that regional system mix has increased—and, even more importantly, since 2017, Vermont has begun pulling less electricity from that source. Overall, Vermont’s electricity sector is rated the least carbon-intensive in the U.S.

So, the cleaner your electricity is, the cleaner your heat pump is. And if you combine residential solar with a heat pump, you can guarantee you’re using an even cleaner source than Vermont’s grid. Here are some resources from Efficiency Vermont, Button Up and Green Mountain Power and others: