Location of heat-pump water heater?
tonyparr2
| Posted in Energy Efficiency and Durability on
I read the piece on heat pump water heaters with interest. The only options for location suggested are a garage or basement. As a recent immigrant from Europe, I’m used to a water tank being in the roof space, and our house in California has a large and spacious attic that could easily accommodate a tank (and through gravity probably improve water flow to bathrooms and kitchen below). Since the roof space gets very hot, isn’t this the ideal place to put such a heater? Are there any problems with this?
Thanks,
Anthony
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There are many locations in the US where water heaters are commonly installed in the attic, often in areas with warm/hot climates and no basement. (Texas, Arizona, etc.)
With a heat pump water heater in a warmer parts of California you would be getting some benefit from the higher temperatures in the attic, both in terms of standby loss and heat pump efficiency.
In California there are often local building codes that prescribe where and how a water heater is installed so that it doesn't become a hazard in the event of an earthquake, but most of that has to do with fire hazards from gas-fired water heaters becoming disconnected from the gas lines. You may be required to secure it in a fashion that it can't tip over during an earthquake, but a heat pump water heater may not even have that requirement. You would have to ask the local building department.
Anthony,
There are certainly areas of California where winter temperatures drop low enough to freeze any plumbing in your attic. I would be cautious of installing a water heater in this location unless I knew how low the temperature drops in January and February.
If you get a frozen pipe that bursts in your attic, your house will be flooded when the temperature warms up.
This isn't useful information, but is just a fun physics fact: Putting the water heater in the attic will actually result in slightly lower water pressure, even if the pipe lengths are the same as with it somewhere lower. The difference is tiny--0.2% for an example I calculated--so it's of no practical importance in most cases.
The only time I know of that this matters is if you shut off the flow of a shower at the showerhead. If the cold and hot water pressures are different, you can get backflow into whichever is lower pressure while the showerhead is off. Then when you turn the water on you get a vastly different temperature for a bit until the backflow is flushed out.
The reason for this effect is that the density of cold and hot water is different by about 1%. The hydrostatic heat in the cold water pipe going up to the attic means the tank is at a lower pressure than the incoming water line. Then more hydrostatic head in the pipe coming down from the water heater in the attic regains only 99% of the pressure lost going up.
The hot/cold pressure difference also happens if the tank is below the outlet elevation, but in that case the hot has the higher pressure.
The 0.2% pressure difference I calculated came from assuming a 20 foot height of the tank above the faucet, a 50 psi water pressure, and a 50 C hot water temperature. (Anthony, welcome to the US and our crazy mix of units!)
What is the operating temperature range of the heat pump water heater ? Some of them will cut out at hot temperatures, which would preclude operation in a hot summer attic.
Most heat pump water heaters (HPWHs) are over 6 feet tall, and must be operated standing up, just like a refrigerator.
This requirement disqualifies a lot of attics.
And oh yeah, water heaters are preferred in the basement because the most common failure mode of all steel tank water heaters is leaking. And they are guaranteed to leak within 8 to 25 years.