New GE Heat pump based HWH, something for nothing?

Discussion in 'Water Heater Forum, Tanks' started by John in herndon, May 6, 2010.

  1. John in herndon

    John in herndon New Member

    Messages:
    25
    Location:
    VA
    As I am sure everyone is aware, GE has announced a new HWH based on a heat pump design claiming 2 or 3X the efficiency of a resistance based heater.

    So far so good, EXCEPT for the fact that the unit is sucking the heat out of the ambient air. While this might be fine if the heater is in a garage or outdoor space, it is not so fine if the unit is placed in a basement or indoor heated space.

    The BTUs have to come from someplace and in this case they will have to be supplied by the home heating system unless you live in the tropics or it is summer and you are running the A/C (in which case it will help you).

    There is no free lunch and the laws of physics still apply.

    Have I missed something here? If I have, please comment and enlighten me.

    John in Herndon
  2. nukeman

    nukeman Nuclear Engineer

    Messages:
    711
    Location:
    VA
    Correct. They do work best in cooling dominated climates. Since they are basically an AC unit, they also help with a bit with humidity.

    Even in cooling dominated climates, they still might be better off than resistance heating as you probably will use gas or other fuel that is cheaper than electricity (BTU for BTU) (although you would stll be better off using that fuel directly to heat the water (indirect with HE boiler, HE WH, etc.)). If you are running an electric furnace or electric baseboards, than this would not be true.

    I understand that the operating cost for these things is based on what the unit uses and does not account for heat that may have to be made up. This gives many people a false sense of savings because most of the general public probably do not understand how these work.

    Personally, I will stick with resistance based WHs as they are cheaper to buy and are much less expensive to repair.
  3. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    A heat pump is more energy efficient that direct resistance heating. You are correct that a heat pump system does not CREATE heat...it transfers if from one place to another. In this case, it transfers heat from outside the WH to the water. Since heat pumps are slower than brute force electric heat, I think those GE units are combos....using the heat pump AND electric elements as backup/reinforcement.

    The side effect is the same issue as a refrigerator....it does not CREATE COLD. It transfers HEAT from INSIDE the refrigerator to your kitchen. This heat transfer is I think overlooked in comparing energy usage of refrigerators!
  4. John in herndon

    John in herndon New Member

    Messages:
    25
    Location:
    VA
    That was the whole point of my post -- heat pumps Transfer heat from one place to another and in this case they are taking heat generated by the home heating system. Frankly, a tankless coil on the furnace would probably be more efficient because, in any case, in winter, you are heating the water with the home heating system. Nobody said anything about creating cold -- There is no such thing as cold anyway, it is the absence of heat.
  5. Dana

    Dana In the trades

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    3,027
    Location:
    01609
    A tankless coil is about as inefficient a method of heating water that's still legal in the US. You break even or pull ever so slightly ahead of a standalone fossil-fired tank during the heating season, but keeping a high-mass boiler hot during the rest of the year just to support a DHW load has truly abyssmal efficiency due to wretched standby loss & cycling loss performance at such a low averge load. Indirect-fired tanks do far better than embedded coils, but even they suffer serious cyling loss inefficiencies in the off season when married to high mass boiler. (Post-burn boiler heat-purge into the indirect can improve performance considerably though.) The bigger the boiler the less efficient it is as a water-heating system, but in most homes an indirect is a net gain in annual efficiency, whereas a tankless coil is a net loss.

    You're correct on the efficiency analysis of the heat-pump water heater over some part of the year, but during the cooling season it's a HUGE benefit compared to a standard electric tank, by taking a load off the cooling system. (It's not more efficient that a desuperheater on the central AC or heat pump system though.) In much of the southern US there's a far bigger annual cooling load than heating load.

    But that's only half the story. From a net-efficiency of fuel-to-load point of view, electricity is only ~25-35% efficient in the average fossil-fired grid. About 60% of the source fuel energy goes up the flues & cooling systems at the power plant before the juice hits the primary winding of the first transformer. (Combined-cycle gas powerplant do much better, but that's only bringing up the grid average.) If you're reducing your electricity use for heating water by ~50%, by pulling half the heat from a space heated with even an 80% efficiency fossil- furnace or boiler or even a heat pump (COP >>1) you're ahead of the game- it'll take less source fuel and cost less to heat that water. If you heat with electric resistance (electric baseboards/radiant/whatever), it's a wash during the heating season, but a gain during the cooling season. If your heating fuel is propane it'll usually be cheaper to run a heat-pump water heater than a propane-fired water heater, but you'd have to do the math relative to your actual electric & propane rates to be sure.

    The heat transfer issue in refrigerators is totally NOT overlooked (by regulators & utilities, anyway.) The efficiency of that power used matters even in heating-dominated climates where that compressor-heat is supporting the heating load. In most places heating via electricity is quite expensive compared to other sources. In cooling dominated climates the compressor's heat is an addtional sensible-load to the cooling system. Getting rid of all of the older less efficient refrigerators & freezers and replacing them with even minimum-efficiency newer unit would take a HUGE base load off the grid overnight. (Many utilities have bounties on them- they will come and pick 'em up and write you a check on the spot, since it's cheaper to buy a few million old refrigerators at $50-100/per than it is to build a power plant to support even half the base load they represent.)
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,247
    Location:
    Cave Creek, Arizona
    And HOW are you heating that central unit that you want to put the tankless coil onto? Even then it is NOT FREE HEAT, just because you are running the heating system anyway. That was always a fallacy people used when adding a water heating coil to a boiler.
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