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Community and Q&A

Hot Water Heater Options

stolzberg | Posted in General Questions on

We’re putting in new hot water heater for major reno in NW Massachusetts  and my contractor discovered that 80 gallon heat pump hot water heater won’t fit in our basement, which is about 70 inches high to open floor joists.  So they want to put in a 40 gal. heat pump hot water heater in tandem with a 40 gal. electric water heater, putting the set temperature lower on the electric unit so it will act primarily as a storage tank, just kicking on when high demand on the heat pump water heater.

Is this reasonable assumption?  Will the electric water heater hold the temperature of the hotter water entering it from the heat pump water heater or will it turn on a lot and draw much power, defeating the purpose of putting in heat pump water heater?

For what it’s worth, we put 2 inches of EPS under the slab and are planning to spray foam the walls of stone foundation, so hoping basement will stay relatively warm.  Would wrapping the electric water heater in extra insulation help much?

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    The second tank won't act as a "storage tank" for hot water from the heat pump water heater (HPWH), because the water won't circulate between the two tanks. If you are able to setup a small circulation pump to circulate water between the two tanks, then it would work, but you'd also increase your standby losses (in addition to the power the circulation pump will need to run). If you go this route, keep in mind that the water needs to be drawn off of the TOP of the HPWH tank, then enter the TOP of the electric water heater, with the water drawn off the BOTTOM of the electric water heater and returned to the BOTTOM of the HPWH. That way you keep the water temperature most balanced between the two tanks. This arrangement would have the HPWH doing most of the work of heating the water, with the electric water heater acting primarily as a storage tank, but the electric water heater would cycle if you have a high demand for hot water.

    With the electric water heater in series with the HPWH, but with a lower setpoint, you reduce the runtime of whichever is the "second" water heater in the chain, but only while hot water is flowing. All the rest of the time, both water heaters will cycle normally and a lower setpoint on the electric water heater just means slightly less cycling and standby losses due to the lower temperature.

    With both water heaters in parallel, they both cycle normally, and the lower temperature setpoint just acts to reduce the average temperature drawn off of both water heaters. You would still have both water heaters cycling, and slightly lower standby losses from a lower temperature setpoint on the electric water heater.

    If your goal is to have the HPWH do most of the heating, then you want the first option with the circulation pump. You'd probably be better off with two HPWHs though in terms of long term operational efficiency.

    Bill

  2. Expert Member
    AKOS TOTH | | #2

    Provided they are plumbed in series this works. I've done something similar for my folks with a tankless heater to avoid draw delays and cold water slug issues. The buffer tank barely ran. Only ran if there was no hotwater use for a long time to make up from standby losses. Key to this working well is to set the HPWH to run hot and install the mixing valve on the output of the resistance tank to bring down the temperature.

    In your case, the resistance tank might run if there are large hot water draws but these should be few and far in-between.

    Maybe slightly less efficient than a single 80 gal unit but not by much. You can help a bit by wrapping the resistance tank in an insulation blanket. Plus even the 80 gal unit will most likely run in resistance mode when there are large draws to speed up recovery.

  3. dickrussell | | #3

    When I read about HPWH installations, one of the first things that comes to my mind is the ultimate source of heat that the pump is moving into the water tank. Part of that heat is the electrical input of the pump, but a lot more (hopefully a high COP) is from the surroundings. In the OP's case, that would be the air in the space around the installation but ultimately that heat comes from the ground around and under the foundation and to some extent conducted and radiated downward from the floor above. But the foundation and slab will have been insulated, retarding inward flow of heat, and anyway heat won't be conducted in from the ground until the air temperature down there is below that of the ground, which in NW MA might be on the order of 50 F. Eighty gallons of water weighs 666 lb, and heating that, from well water temperature at 50 F or perhaps municipal supply at even 60 F, up to 120 F will draw around 40,000 BTU from the pump. At a COP of even 2.5, 60% or 24,000 BTU will come from the basement surroundings. Spread over a 24 hour day, that would be 1,000 BTU/hr. One can play with the numbers, including effective surface area for heat conduction, but it does seem that the basement would tend to run in the low to mid 40s much of the time. However, if the heating system for the house also is located in the basement, losses from that to the ambient air down there could provide much or nearly all the heat that the HPWH puts into the water. In practice, what is the heat balance typically seen?

  4. kevinhenry | | #4

    For what it's worth, the Eco2 Systems (formerly Sanden) SANCO2 heat pump water heater with an 83-gallon tank is only 68 7/8" high. Because the heat pump is a separate unit, rather than sitting on top, the tank is shorter than it is on integrated HPWHs.

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