Using supplemental heat pump hot water heater as AC+”battery” during summer
Hello Green Building Advisor,
I have a situation in my home where the (gas) hot water heater is on the 2nd floor in an area that is roasting (80F+) for six months of the year. In fact, the 2nd floor hot water heater is in the laundry room adjacent to two bedrooms that also tend to overheat.
I am intrigued with the idea of using a heat pump water heater as an AC unit with a “battery” (the tank) that heats during the day and which I can heat exchange with my (true) hot water heater for showers in the evening/morning.
However, I can find little information on the web about the potential of a heat pump hot water heater as an AC unit. Basically, heat pump water heater websites and brochures don’t characterize the air cooling potential of the unit. I did ask Steibel-Eltron, and they pointed out that their accelera is not designed for rapid compressor cycling that happens when an AC unit is thermostat controlled. So, rather, I would be looking at putting the heat pump on a timer, say, noon to 8pm, mid April till mid October. Because the heat pump water tank isn’t used for household water, I would let the temperature swing to arbitrarily low and high values. I assume that, during this time, the high temperatures would be sufficient to heat exchange with my primary tank.
I assume there are some good reasons why a heat pump water heater–which heat exchanges with a primary water tank–isn’t advertised or developed as an AC unit with “battery” pack. Here are some (novice) thoughts that I came up with:
(1) Law of diminishing returns. If a heat pump hot water heater was really effective a lowering room temperature, then it’s efficiency might drop as (i) the room temperature lowered and (ii) the tank water temperature rose. I would be interested to see some figures/simple calculations on this. I assume that heat exchanging with the primary tank might mitigate this effect.
(2) Cost. $2-3K for marginal AC and marginal savings for just 6 months of the year may just simply not be worth it. The investment might just not pay itself off. Along these lines, however, it’s worth pointing out that cost could be reduced if the heat pump immediately heat exchanged with the (primary) tank, along the lines of this gizmo here: http://www.phnixexp.com/product/Smart-Heat-Pump-Boilers.html
(3) Heat pumps are so energy efficient, just purchase a split air AC unit and forget the heat exchange with your primary tank.
(4) The cooling potential of a heat pump hot water system is too low to provide significant AC. This statement would seem to contradict some blog statements that I’ve read from folks who actually do appreciate the cooling from their heat pump hot water heaters.
(5) Investing in a PV system will give you much greater bang-for-the-buck. This argument is somewhat incidental in Los Alamos, NM where due to the the laboratory’s max energy needs during the day, net metering is the clear way to go (irrespective of how you heat your water or cool your home).
The above 5 arguments *against* my idea seem possible, but none are so compelling as to completely kill the idea from consideration (at least in my novice opinion). I would be interested to hear what green building experts have to say, for and against, about these issues.
Thanks in advance,
Chris Jeffery, PhD
Los Alamos, NM
GBA Detail Library
A collection of one thousand construction details organized by climate and house part