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Hybrid heat-pump hot water heater cooling effects

LenMinNJ | Posted in PassivHaus on

We have a GE hybrid heat pump hot water heater in the non-partitioned 1100 square foot basement of our pre-certified PassivHaus home in northern NJ.

We’ve found that when it’s operating as a heat pump, it exhausts enough cold air that it actually cools the basement significantly.

The first and second floors each have their own ductless mini-split heat pump heads. The basement and dormered attic don’t have heating or cooling, but they do have ERV vents and returns. During the Summer the whole house, including the basement, top to bottom, measured within 2 degrees. But this winter we’re seeing 5 or 6 degrees lower temperatures in the basement.

Some of that could be due to our not-quite-final sealing of envelope penetrations in the basement. But some of it is due to the heat pump mode.

Proof? When I switched the hot water heater from hybrid mode to resistance-element-only mode the temperature down there rose a couple of degrees.

I bought a small 6 cubic foot chest freezer that my wife wanted, and placed it next to the hot water heater, thinking that its heat exhaust might offset the hot water heater’s cooling effect. It’s not helping. Apparently, the freezer’s too small or too efficient to have any effect.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Len,
    Your observations make perfect sense, and are well-known to most green designers. For more information on this topic, see:

    Heat-Pump Water Heaters in Cold Climates

    Heat-Pump Water Heaters Come of Age

  2. Dana1 | | #2

    In summer in most of N.America east of the continental divide a good fraction of the heat drawn from the basement is in the form of latent heat- the heat of vaporization taken when condensing moisture out of the air. In winter there is much less moisture in the air, and hence, a more pronounced drop in temperature with very low volumes of condensate being produced. This is compounded by the lower incoming water temperatures that demands a higher operating duty-cycle for the water heater than in the summer months. Most homes use more hot water in winter than in summer too.

    But is this a problem? Is the basement now too cool in winter for your intended purposes? If yes, reducing the total amount of hot water usage will help, starting with the easy stuff such running laundry on "cold" or "warm" instead of "hot", taking showers (with low flow shower heads) instead of tub-baths, etc. which can add up to a 10-25% reduction in hot water use.

    Taking it a step further, one could install a gravity-film type drainwater heat recovery heat exchanger downstream of the showers (the bigger the better, about $600 for a unit 50%+ return efficiency through EFI, if you open an account with them to get the wholesale price: http://test.efi.org/sites/default/files/power_pipe.pdf ), which is another double-digit percentage reduction of hot water use for most showering families. Then, installing an uninsulated greywater treatment tank in the basement downstream of the heat exchanger returns another fraction of the tepid water heat to the basement and provides reusable irrigation or toilet-flushing water (if you want to take it that far- a side benefit of using treated greywater for toilet flushing is that the toilet tank won't be chilled below the dew point of the room air, so it never "sweats" even during sticky-humid summer weather.)

    The economics of drainwater heat exchangers vary widely with energy costs, but is always cost effective on a lifecycle basis (at 25 years, when the performance has degraded to something like 80-85% of it's day-1 efficiency.) The economics of greywater treatment vary widely, but makes more sense in locations where water is scarce &/or expensive, and you have uses for the volumes of treated greywater. If you can collect both the bathing (tub or shower) & laundry greywater it will be something on the order half or more of your total hot water usage. Letting it stagnate to basement temperature means at least 2/3 of the total energy that went into heating the hot water ends up back in the basement, and only about half of that energy was extracted from the basement air.

    The combination of drainwater heat recovery & greywater storage/treatment makes the net basement heat extracted by the hot water heater close to energy neutral (or even energy-positive), since you're not removing heat from the basement then (literally) letting it run down the drain.

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