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Community and Q&A

Using supplemental heat pump hot water heater as AC+”battery” during summer

CJ_Los_Alamos | Posted in Mechanicals on

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:

(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

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  1. Stockwell | | #1

    Isn't this a case of treating the symptoms and not curing the disease? Why not fix the problem that is causing your 2nd floor area to be roasting?

  2. GBA Editor
    Martin Holladay | | #2

    The last time this issue came up was in the Comments section of my article, Heat-Pump Water Heaters Come of Age. In Comment #26, Dave Kresta compared the cooling effect of a heat-pump water heater to an air conditioner rated at 3/4 ton.

    In Comment #31, Curt Kinder disagreed with Dave Kresta, asserting that the average heat-pump water heater provides less cooling than a 3/4 ton air conditioner. He wrote, "Comparing an Integrated HPWH to a 3/4 ton AC is a bit of a reach. Under 5 kBtuh is more like it."

    Obviously, the actual cooling effect will depend on the size of the water heater, the temperature of the incoming water, and the daily draw-down.

    By definition, a BTU is the energy required to raise one pound of water (1/8 gallon) by one Fahrenheit degree. If you want to raise the temperature of 20 gallons of water from 50°F to 140°F, you need 14,400 BTU. If all of that energy comes from the indoor air, that's the amount of cooling that will occur every time you heat 20 gallons of water under the circumstances I described.

    If you're a PhD from Los Alamos, I assume that you are familiar with this type of calculation.

  3. Expert Member
    Dana Dorsett | | #3

    The EF test for water heaters presumes a 65F-70F local room temperature. It has to be quite a bit colder than that to make a big difference in efficiency. If it's in a room that's normally heated to 65-70F in winter it'll hit it's numbers, and in the summer in a 75-80F room it'll do even better.

    The standby loss of a gas water heater is substantial. About 20% of the source fuel heat goes up the stack, some of which is lost to the room, and another 20-30% is lost from the tank to the room (at EF test procedure volumes), heating up the room. This represents a real 24/7 background cooling load.

    A HPWH's standby loss is recovered from the room it is lost to, with that heat re-inserted into the water, with a net negative BTU contribution to the room.

    The combined effect of swapping out a lossy gas burner to a net-negative HPWH would almost certainly drop the summertime temperatures in the laundry room area by more than 5F, possibly by 10F, since instead of heating the room 24 hours per day, it's cooling the room whenever the heat pump is running. More than the HPWH just being an quarter ton or half ton (or whatever) air conditioner, getting rid of the gas burner in that location lowers the actual cooling LOAD. The net effect is more than just the cooling power of the heat pump.

    Unless you take really LONG showers, a 50 gallon HPWH is going to be enough, even if it has to run in hybrid mode sometimes. If you need more water heater than that, if the shower is upstairs, or above a basement, a drainwater heat recovery heat exchanger on the drain downstream of the shower(s) gives any tank type water heater substantially more "apparent volume", and cuts the total energy use even more. The fattest and tallest unit that fits is the "right" one, since the installation labor is about the same, and the increased performance of the bigger heat exchanger makes it "pay off" quicker than a smaller one. In the case of HPWH the payoff of the drainwater heat exchanger is in the smaller water heater needed more so than the energy savings. Since the incoming water to the water heater is now substantially warmer than it otherwise, it's less of a temperature lift, with shorter recovery times even if the water heater is run to the point that the shower output is tepid.

    The heat exchanger does nothing for tub fills, since the drain and potable water have to be running simultaneously for the exchange to take place. But since the incoming water to the water heater is now substantially warmer, it's less of a lift, with shorter recovery times even if the water heater is run to the point that the output is tepid.


  4. Reid Baldwin | | #4

    Your discussion of heat exchanging with a (true) hot water heater is confusing. I think you are over-complicating it. As Dana suggested, just get rid of the gas water heater and install a HPWH in its place. A HPWH is a true water heater.

  5. CJ_Los_Alamos | | #5

    Thanks for the helpful information. I should have clarified that my upstairs laundry room also has a gas powered radiant heat system. The 20% stack loss notwithstanding, the gas powered radiant/HW system I have seems to work well and is economical in *winter*, hence I am not sure I am prepared to rip all of that out and go HPWH only, especially with a hybrid that might (frequently?) draw electric. This thinking led me to consider a (non-hybrid) HPHW that is power-cycled like an AC unit during summer, and heat exchanges with the equipment I have already. Based on Martin and Dana's comments, it seems like the next step is to crank some numbers, and see where things come out.
    Thanks for your help!

  6. Expert Member
    Dana Dorsett | | #6

    Does the radiant heat system run off the water heater, or is it separate?

  7. CJ_Los_Alamos | | #7

    It is currently separate. But I did notice combined systems (e.g. HTP's versa-hydro-solar) that look really nice, but are quite expensive. Having it combined would appear to have some energy saving advantages.

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