Do Heat Pump Water Heaters Make Sense In a Passive House?

Treehugger recently covered a townhouse in Brooklyn designed to the Passivhaus standard that included a heat pump water heater (HPWH). Unlike regular electric water heaters that convert electricity to heat, a heat pump water heater has a compressor, much like in a fridge, that moves heat from the air to the water. This is purported to use less energy.

But as the saying goes, there is no such thing as a free lunch. In my high school physics class, I was taught that it takes a British thermal unit (BTU) of heat to raise a pound of water one degree Fahrenheit (actually I was taught that it takes a calorie of heat to raise water one degree Celsius) but any way you measure it, the heat has to come from somewhere.

That heat is pulled out of the air, and in a regular house, there is a lot of it to spare. But I wondered as a thought experiment: What happens in a Passivhaus design which is essentially a thermally sealed environment? Every BTU or calorie has to come from somewhere, and if the heat is coming out of the air, then it has to be replaced (at least in the heating season). I decided to put the question to the hive mind of Twitter and see what the experts say.

The responses came from all over and were fascinating.

An early and sensible response was to use a split system where the condenser is outside and the great outdoors can provide lots of heat.

This is the condenser of a Sanden CO2 heat pump that connects to the unit in the photo at the top of the post.

There are lots of advantages to this, especially in a very quiet Passivhaus design–air source HPWH are noisy.

Alas, those Sanden splits are really expensive, and as engineer David Elfstrom points out, it is much more common in North America to install the unit inside.

Elfstrom then confirms my thought experiment, that the heat does have to come from somewhere and be replaced, but there is a big benefit in summer because it cools and dehumidifiers.

I was thrilled when Wolfgang Feist weighed in: He is the co-founder of the Passivhaus movement. He notes that we are not talking big numbers.

Outside of the Passivhaus world, where Nate Adams lives, these are small and trivial issues. Adams actually got quite angry that anyone would suggest that you shouldn't put an HPWH inside, though even he finally added a caveat that they should not be in very small rooms. And as Gregory Duncan points out, when you are really counting every BTU, it does make a difference.

In the end, I believe Duncan and Kelly Fordice had the best explanations.

Most Passivhaus designs now are heated with air source heat pumps (ASHP) so when the HWHP sucks any heat out of the interior, then it piggybacks on the ASHP which sucks the heat out of the exterior air. Since both devices have a high coefficient of performance (the ratio of useful heating compared to resistance heating) there is still a net gain over a straight electric hot water heater.

Add that to the obvious cooling season benefits, where it cools and dehumidifiers while delivering hot water, and it appears that heat pump hot water heaters are a year-round win.

Many outside the Passivhaus community might think that worrying about a few BTUs is truly a waste of energy, especially when you can just toss another solar panel on the roof. I will reiterate that this was a thought experiment, where I am trying to understand where the BTUs are coming from, and because the best way to get to zero carbon is to go after every watt, calorie, joule, and BTU to reduce demand. Then we can worry about supply.