Hot water has long been an intractable energy hog in efficient homes. With the new generation of heat pump water heaters there is finally a more affordable alternative to solar hot water: a technology handicapped by weather, copper prices and high maintenance moving parts.
Older plumbing may use large diameter pipes that leave a lot of hot water stranded in the pipes when you shut off the tap.
Heat pump water heaters use electricity very “efficiently”, and are a good match with a renewable electricity system, like solar PV. It’s a mature technology that does what it promises and is surprisingly affordable. However, heat pump water heaters need specific installation requirements to be a good match for your high performance home.
Best conditions for a heat pump water heater:
- Basement or far enough from the living area that the noise of the compressor is not bothersome.
- Well-air sealed and insulated space
- A buffered space where there is no effort to control the air temperature and cool temperatures are not going to make a comfort problem
- A roommate that emits some waste heat – typically an older boiler or furnace
Worst conditions for a heat pump water heater:
- In a house with point-source heat (heat that is delivered to one spot rather than distributed all over the house)
- In small, enclosed area
- In a basement with no waste heat
- House with two or more showers and a lot of people
A home run plumbing layout uses small diameter pipes because each pipe only serves one fixture. Small pipes keep stranded hot water to a minimum.
A heat pump water heater isn’t for everyone, even if you have the extra renewable electricity to operate it. Efficiency Vermont’s High Performance Home Team has put together a water heating alternative for homes where a heat pump water heater would perform poorly or make a localized comfort problem.
- Well-insulated electric resistance tank water heater, such as the Rheem Marathon water heaters,
- Low-flow water fixtures, particularly the showerheads and the kitchen faucet,
- Low-flow dishwasher and clotheswasher,
- Drain heat recovery, such as the PowerPipe or EcoDrain,
- Low pipe-volume between water heater and water fixtures (a function of the pipe diameter and the pipe length to the water fixture), or a demand recirculation system to get water to the tap fast
A demand recirculation system uses a pump to push the stranded water in the pipe back to the water heater, while delivering hot water to the tap fast. This approach helps conserve water but only does a little bit to save water heating fuel.
In an existing home some of these things may be cost prohibitive, like replacing all the pipe between the water heater and the bathrooms. But they work as a package to ensure that the relatively low “efficiency” of the water heater doesn’t gobble up too much of the annual energy budget. If you have to give up too many of these elements, say you are unwilling to replace your clotheswasher, it’s probably better to go with a heat pump water heater and just operate it as an electric resistance water heater for a couple of months a year.
Water conservation is the easiest, cheapest, fastest way to save water heating energy, and it adds up fast! After you’ve trimmed your water use, then think about how you’re going to heat the water.
While the water conservation approaches above are very important if the water heater is an electric resistance one, they are still a good idea for keeping your water heating energy low even if you are using solar hot water or a heat pump water heater.