Heat pumps have been much in the news recently as an alternative to traditional gas- or oil-fired boilers for home heating. I became especially interested in heat pumps two years ago when my gas-fired boiler needed replacing. I knew that a heat pump was the environmentally preferred home heating option, but I also knew that it gets awfully cold in New England, and I didn’t know anyone who could personally vouch for the effectiveness of this heating technology. Many people advocated it but no one seemed to have it installed in their own homes.
At first I thought a heat pump sunk a pipe deed into the earth where it was warmer and then continuously circulated water through the pipe, always bringing warmer water up into the home’s heat pipes and returning the cooled-off water deep underground to reheat. To my embarrassment, that’s hardly the way a heat pump works. Can you tell I never took physics?
The key to understanding how a heat pump works is a law of physics that says heat always moves to cold. Air conditioners and refrigerators both operate because of that law. Heat pumps do the same, only in reverse.
Here’s how I understand air conditioning to work. Inside your air conditioner is a liquid chemical called refrigerant. This chemical has a tremendous ability to absorb heat. Another law of thermodynamics is that if you compress a gas, it gets hotter and if you let the gas expand, it gets cooler. Two essential components of both air conditioners and heat pumps are a compressor (which make the refrigerant hotter) and an expansion joint (which makes the refrigerant colder).
With an air conditioner, the super-cooled refrigerant that has passed through the expansion valve circulates through coils inside the house. Because the refrigerant is cooler than the air inside the house, the heat from the inside air moves to the coil containing the refrigerant. This transfer of heat from the air inside the house to the refrigerant leaves the air in the house cooler.
A pump in the AC system then circulates the warmed-up refrigerant through the compressor which – you guessed it – compresses the refrigerant, making it super-hot. Then the refrigerant moves to coils in the outside-the-house part of the AC system. The temperature of the outside air may be 90 degrees, but the refrigerant will be at an even higher temperature. As long as the refrigerant temperature is higher than the temperature of the outside air, the “heat moves to cold” principal kicks in and the heat of the refrigerant moves to the outside air, thereby cooling the refrigerant again.
To understand how the expansion joint of an AC system works, think about a can of compressed air you use to blow dust off your computer keyboard. If you shake the can, you can tell that whatever is inside is a liquid. But when you squeeze the nozzle, it comes out as a gas. The nozzle is an expansion valve. If you touch the can of compressed air after using it for 30 seconds, the can is extremely cold. The same thing happens with your air conditioning system.
A heat pump uses the same process, only in reverse. Refrigerant can be cooled to a temperature of -37 degrees (i.e., 37 degrees below zero). As long as the outside temperature is “higher” than 37 degrees below zero, the “warmer” outside air will go to the “cooler” refrigerant, thereby warming the refrigerant. The warmer refrigerant than passes through the same compressor on its way into the house. This causes the refrigerant heading into the house-side of the system to become highly pressurized which makes its heat increase substantially. Because the room air inside the house is cooler than the highly compressed refrigerant, the “heat goes to cold” law kicks in, and the heat from the compressed refrigerant warms the room. The now cooler refrigerant passes through the expansion valve on its way outside, cooling the refrigerant to a temperature lower than the outside air.
It’s like when the AC is on, the refrigerant travels through the system clockwise, but when the heat pump is on, the refrigerant travels through the system counterclockwise. Modern heat pumps have highly nuanced settings that give the homeowner considerable control over temperature settings.
A heat pump uses only a slight amount of electricity to run the condenser, the fans, and whatever pumps are needed for circulation so it is very energy efficient.
It all sounds good, but as I mentioned earlier, I’d like to hear from someone who relies on a heat pump for their primary means of home heating.
One thing I did detect in my heat pump research is that in colder climates (like our part of New England) installers recommend a supplemental heating system that can be employed on extremely cold days when the heat pump doesn’t quite do the job by itself. That sounds prudent, but also is concerning.
However, just this week, the New York Times had an article that put this “do heat pumps work in really cold climates” concern in a different context. “Exposing a subzero ‘myth’ about heat pumps as homeowners increasingly embrace them” by Elana Shau laid the blame for much of the doubt about heat pumps on the fossil-fuel industry which would lose enormous market share if homeowners en masse converted to heat pumps. Lying or misleading consumers is such an integral part of American capitalism that it puts this charge beyond doubt in my mind. But misinformation – or “favorably curated information” – cuts both ways, so perhaps this article was influenced by and slants towards those who would benefit from selling heat pumps or, more charitably, from enthusiastic advocates who minimize cold weather performance in their quest to reduce fossil-fuel dependence.
The last paragraph of the Times story does suggest to me that not all doubt about heat pump performance in the very cold has been extinguished:
There are additional steps homeowners can take to make the most of their heat pumps, like sealing air leaks and drafts and improving insulation, said Troy Moon, the sustainability director for the city of Portland, Maine. Homeowners can also keep their existing furnaces as backup for the coldest days of the year, he said.” [emphasis supplied].
I replaced my existing furnace when it no longer worked. If I wanted a heat pump, would I have needed another furnace too, just out of an abundance of caution for really cold days like this weekend? I hate sounding so negative about heat pumps, but I also hate being cold inside my house, so for me, the jury is still out.