Heat Pump Water Heaters - a good choice for your project?

by ‐ Tags: alternative energy, building science

Heat pump water heaters have been gaining increasing notice recently and can yield cost savings, but aren't always the best option in all cases.

Heat pump water heaters have been gaining increasing notice since GE highlighted their product in a commercial featuring snow monkeys and after the major home improvement stores started carrying them alongside their more traditional water heaters.  What most people notice about them first is either their size or their price tag—both are bigger than most other water heaters. 

 

What are they?

Heat pump hot water heaters use basically the same technology that is found in heat pumps that heat and cool buildings in many of the warmer parts of the country. They use electricity to heat the water and are therefore considered an alternative to other electric water heaters on the market, if a certain building does not have access to a gas line.

The reason for their larger size is that most heat pump water heaters are hybrids containing two water heaters in one.  One of these heats the water using a heat pump that harvests heat from the surrounding air and uses it to heat the water. The second one heats water with electric resistance heating elements, which is what conventional electric water heaters use.  The heat pump side of the water heater is about 2.5x more efficient that the electric resistance and typically is in use 50-100% of the year with the electric resistance filling in when the heat pump is not providing sufficient hot water.

Cost and Savings

Costs have been coming down slowly, though most are about 3x more expensive than conventional electric 50-gallon water heaters.  Even with the higher cost, most heat pump water heaters will pay for themselves within several years.  Savings over a conventional electric water heater are in the hundreds of dollars per year, though this will vary depending on electricity costs and usage patterns. They are so efficient that they are the only electric water heaters eligible to earn the Energy Star label.

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Pros and Cons

Given the potential for savings, it may seem like a no-brainer to install a heat pump water heater, but they aren’t always the best option.  They require about 700-1000 cubic feet of air around them, which would be a space large enough to hold 700-1000 milk crates worth of air.  Certainly larger than the average mechanical closet!  They also need a space that is tall enough to fit their above-average height if it’s a hybrid model.  Add-on units, which attach to an existing water heater, add about 2.5’ of height to the existing tank. 

There has also been some concern that if they are in a space that is actively heated, they are stealing heat that has already been paid for in space heating costs.  Plus, as they heat, cold air is generated as a waste product—which can be an added benefit in the summer months but a challenge if it's dumping cold air into a conditioned space in colder months. Ideal locations, depending on the climate zone, include garages and unconditioned basements.  Some manufacturers recommend attics also, but I’m not keen on having 50 or so gallons of water over my living space with the potential to freeze and burst. 

I have an add-on heat pump unit installed on a water heater in an unfinished basement at my house in Virginia.  The heat pump provides hot water 100% of the year with no need for the electric resistance coil to kick in.  There has been a marked decrease in my electricity usage and it helps to keep the basement a little cooler and drier over the hot, humid summer months. 

They have also been utilized in at least several large multi-family projects in Virginia with success.  It took a little extra planning to determine how to provide enough air while keeping the mechanical closets a reasonable size, but the details were worked out an now residents are enjoying substantially lower utility bills and the property managers have a neat feature to point out to potential tenants.