I installed my first heat pump water heater about eight years ago for a client whose roof was not appropriate for the solar hot water system I wanted to use. Since then I’ve installed many more heat pump water heaters and, as long-time GBA readers will not be surprised to hear, not a single solar hot water system. Having mostly killed the solar thermal market, heat pump water heaters continue to grow in popularity and by now most of the people I talk to are at least aware of the existence of this technology, even if they’ve never seen one or don’t quite understand how they work.
Basements are a good option, but not always available
Until recently, all of my installations had been in basements here in upstate New York. There are several obvious advantages to locating a heat pump water heater in a basement. The heat pump dehumidifies a normally damp space while it also makes efficient use of waste heat from a boiler or furnace during the heating season. During the non-heating season there’s enough air volume and ground source heat in the basement to keep the heat pump happy. And fluctuating basement temperatures don’t affect anyone’s comfort upstairs where the thermostat is.
There are occasionally disadvantages to a basement installation, but these are mainly a concern for the unlucky installer who might have to get a large, heavy tank down a rickety flight of stairs, through a narrow door, or in a worst case, might have to dig a hole in the floor with a pick and shovel and line the hole with EPDM rubber in order to get an extra two feet of headroom for a tall tank in a short space.
But where do we put a heat pump water heater when there’s no basement and no boiler? In new high-performance slab-on-grade homes, it’s hard to find an obvious place for a heat pump water heater.
Heat pump water heaters usually have minimum volume requirements, so they can’t be shoehorned into a cabinet the way electric resistance tanks can. And having a heat pump that scavenges heat from conditioned space seems potentially wrong-headed. In the summer not so much—it’s free cooling. But in the winter making hot water means robbing Peter to pay Paul: the house’s heating system (in this case an air source heat pump) will have to work harder because the water heater is pulling heat out of the air. If the COP of the heat pump water heater is 2.5, but it’s being supplied with heat by an air source minisplit, is the combined COP in the winter a little lower or a lot lower?
We struggled with these questions on a recent Passive House project, and my boss Josh Edmonds suggested we try something unorthodox: putting the water heater in the laundry room upstairs instead of downstairs in the relatively small mechanical room. After nearly a year of experience, it turns out this arrangement works well. And if the occupants are willing to modify behavior a bit, it works really well.
Laundry closet install is an efficiency win
In a small laundry room of 70 square feet, standby losses from the clothes dryer, in this case a heat pump condensing unit, can significantly raise the room temperature. We’ve seen 10°F or more with the heat pump dryer running and the door closed. The heat pump water heater on the other hand, can lower the temperature in this space by 10°F or so. It’s worth noting here that water heater COP goes up with room temperature, so the heat pump water heater already has a higher efficiency than if it were located in a 50°F basement.
The picture gets even gets better when hot water demand—showers and baths for example—coincides with clothes drying. Then the COP on the water heater really maxes out, the room temperature stays balanced, excess moisture in the air is removed by the heat pump’s evaporator, and there’s little to no transfer of heating or cooling load to the minisplit.
Interestingly enough, as I was Googling to see if I could figure out how to calculate the COP implications of a heat pump water heater operating in conditioned space during the winter when it’s basically shifting its load onto the minisplit, I found an article in Home Energy by Ian Shapiro of TAITEM Engineering exploring this exact question. It’s well worth reading if you are interested in the technical details.
Shapiro has a couple of interesting conclusions. During winter the efficiency picture for the heat pump water heater in a space conditioned by a minisplit turns out to be roughly similar to the basement scenario once you factor in the higher ambient temperature in conditioned space. It’s basically operating as a two-stage heat pump.
Using Shapiro’s formula and applying rough numbers for our scenario with an air source minisplit operating in heating mode but without standby dryer losses, we calculate an adjusted COP for the hot water heater of above 2.0—or a respectable 200% efficiency. But if we shift load and scavenge standby heat from the dryer, the COP rises significantly, to between 4 and 5. Those are the kind of numbers we really like to see.
Some installation details
We have since built two other houses with a similar arrangement, and so far occupants are happy and efficiencies look good. We are careful to set expectations ahead of time by emphasizing that heat pump water heaters are about as noisy as refrigerators and by designing with that in mind so that if possible the laundry/mechanical room does not communicate noise to a bedroom, for example.
Manufacturer guidelines vary, but all heat pump water heaters have minimum volume requirements for air. One option we tried in our most recent project is to combine the mechanicals and laundry in one space. Normally this space might be a bit too small for the HP water heater’s volume requirement, but because we left the mechanical room ceiling open to the truss service cavity above, the heat pump has access to a much larger volume of air. And some new models can also be ducted, so that supply and return are potentially in separate rooms. In that case the water heater itself can be located in a much smaller space.
One last lesson we’ve learned the hard way: the mechanical room is critically important real estate and it should have enough space to house bulky equipment and allow easy installation and service. Usually this comprises the electrical panel, the ERV/HRV, the well pressure pump, possibly water filtration, condensate, and maybe even the water heater, distribution manifold, and washer and dryer. Mapping out appliance footprints, duct paths, pipes, etc. ahead of time pays big dividends later.
-Norm Farwell specializes in energy efficient mechanical systems at Simple Integrity in Cooperstown, NY. Photos courtesy of the author.