1 or 2 heat-pump water heaters?
I have a two unit building and will be purchasing heat pump hot water heaters and placing them in the cellar. Each unit will likely have 2 occupants. I am deciding between buying a single 80 gallon Stiebel Eltron Accelera 300 (to share between both units) or two 50 gallon GE Geospring hot water heater (one for each unit). I was wondering if anyone had any thoughts on the best direction to go on this. Here are the points I am considering. Feedback on any of these points is welcome.
1) Overall energy efficiency. I believe the 80 gallon is more efficient. It would take less power to heat one big tank than two smaller ones.
2) Human usage factor. When the tenant does not pay for hot water use, does it have a big effect on consumption?
3) Proximity of tank to fixtures. Separate tanks allows me to put one tank about 20 feet closer to the fixture of one unit. The other unit would be unchanged.
4) Brand reliability. I am reading mixed reviews about the Geospring. Stiebel seems to have a better reputation and longer track record making these things.
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Replies
Daniel,
In general, a heat-pump water heater with a large tank is more efficient than one with a small tank, because this type of heater is very slow to heat water. When the heat pump can't keep up, the electric-resistance element comes on, doubling or tripling your electricity cost.
Ideally, you would install two 80-gallon units -- one for each apartment. However, these water heaters will cool your basement, so your plan only makes sense if you have a large basement, or one that tends to run hot during the winter.
For more information, see Heat-Pump Water Heaters Come of Age.
"Human usage factor. When the tenant does not pay for hot water use, does it have a big effect on consumption?" Undoubtedly yes ( although the inconvenience of a slow recovery time may mitigate to some extent). This, and the shorter runs, are compelling reasons for having separate units. Be aware though that unless you live in a predominantly cooling climate the HPWH is simply robbing Peter to pay Paul and delivers little to no financial or environmental benefit to balance against its noise, cost and maintenance.
The all-stainless construction of the AirTap series seems like it would outlast the Stiebel Eltron units.
The energy abandoned in the distribution plumbing may outweigh any advantage that the higher efficiency of a single large tank might deliver if you have to lengthen any runs dramatically, even if you insulate all that plumbing to R4 (recommended). The cheap pipe insulation sold at box stores is only 3/8" wall R2-ish, it's worth seeking out 5/8"-3/4" wall goods (via internet stores, plumbing supply houses, or Graingers.)
The new GeoSpring units are currently being built in Kentucky under much tighter material & quality control than the earlier versions from Asian manufacturing sources. Reviews from 3-4 years ago may have little bearing on what is currently being stocked and sold.
Even in heating dominated climate there can be significant energy use reductions over a standard electric tank, especially if installed in a cellar where the humidity needs to be controlled by dehumidifiers. About half the energy going into the hot water is being drawn from the room, but in a damp cellar much of that is latent energy (humidity), rather than sensible heat (temperature.) If one lowers the humidity in the basement with a heat pump hot water heater, the duty cycle on the dehumidifier drops off substantially, as does the mold/mildew hazard. The dehumidifier in my US climate zone 5 basement uses on the order of $100/year in power most years (at 18 cents/kwh), and would likely not run at all if there were a heat pump water heater down there. The rest of the room-heat going into the hot water during the heating season would be primarily made up by the heating system, at whatever efficiency & cost that turns out to be.