I know many who are happy with their heat pumps. And many studies show better average results than we have achieved.
But I do feel an obligation to share my family’s mixed results since I’ve told a positive story so far.
We’ve been very comfortable, even on the coldest days. But now that we’ve been through a heating season, we can do an efficiency comparison for our heat pumps. The results are disappointing — so far the pumps have been so inefficient that the climate would be better off if we had stayed on gas heat.
And that conclusion does not reflect the additional fact that one of the two heat pumps in our home leaked four pounds of refrigerant. Between the inefficiency and the leak, the net climate effect of our heat pump conversion has so far been about the same as putting a typical car on the road for a year.
UPDATE on June 3, 2023: My revised analysis is that the combined efficiency of the pumps is uncertain and if it is low, it may be low due to running on inadequate refrigerant after the leak. Revised analysis here. That doesn’t change the conclusion that the leak makes the installation a net negative for the first few years at least.
We knew enough not to expect cost-savings, but we were hoping to benefit the environment. When one converts to an electric heating system, one expects gas or oil bills to go down and electricity bills to go up. Because we were able to fully disconnect from gas, we also saved the monthly gas connection fee; that helped keep the otherwise-disappointing net cost change to essentially zero. If gas prices had not been so high this season, we might have seen an overall increase.
What surprised us was the way the climate numbers worked out. Both the cost and the greenhouse gas impact of a heat pump conversion depend on how efficient the pump is in moving heat. As it turned out,
our heat pumps were only about 150% efficient, [see revision here] although they were rated 278% efficient. That means that they needed more power to run and demanded more output from the gas generating plants that add power to our grid. (Gas plants are still the “marginal” power source, even though our grid is getting greener.)
Efficiency ratings on heat pumps are like the mileage ratings on automobiles: They represent the results of a particular test which often fails to reflect real world conditions. Research suggests that real world heat pump performance almost always falls somewhat short of rated performance. Research also suggests that results vary widely. Even for the same model pump, results can vary substantially across installations.
I do not fault the particular pump that we bought or our installers who were careful and professional. It may be, however, that we need better rules of thumb to predict how a pump will perform in a particular installation.
Our other disappointment was fully unexpected. Mid-winter, we noticed that our downstairs pump was peaking at a lower power level in each heating cycle. Although peaking lower in each cycle, it ran more of the time and seemed to be keeping up with heating needs, so we weren’t certain how to interpret the changes. But after a few weeks, we noticed that it was no longer keeping up and the downstairs apartment was getting cooler.
A service call revealed that the pump had leaked over half of its refrigerant. This is not an uncommon problem and our installer knew how to fix it, but it took a full of day of work. The original charge of the pump was 7.4 pounds of refrigerant, of which 4.1 pounds had leaked.
The most common refrigerant in heat pumps is a compound called R410A. It is a mix of greenhouse gases which together are over 2000 times more powerful than carbon dioxide. So, the 4 pounds of R410A equated to roughly 4 tons of carbon dioxide.
I have been reviewing the literature on heat pump efficiency and heat pump leaks and exploring with experts the policy implications, if any, of our experiences. For now, I just wanted to document and share our experiences.
- Data and details computations in this spreadsheet.
- Heat pump efficiency studies
- Heat pump leak studies and regulations