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Is a hybrid heat-pump water heater worth it in a basement in the SE?

Brad22949 | Posted in Mechanicals on

Ultimately I am trying to figure out the most cost-effective / green (using a weird “embodied energy/cost of system vs.cost of photovoltaic to offset/energy use from a major coal burning region” rule of thumb) to heat my water.

I live in central Virginia where winters can run from brutal to mild and summers are pretty much always hot. I am considering a hybrid heat pump hot water heater placed in my basement (the only reasonable place for it at my house site) and use it in resistance only mode during the heating season (this point was over looked earlier!) and in the heat pump setting during the cooling season.

So does the energy savings for 6-8 months of the year as heat pump offset the cost of buying a hybrid system to use the electric resistance the other 4-6 months? Should I just buy an efficient electric resistance water heater and use the savings to increase my photovoltaic arsenal? Is a propane condensing water heater in the question providing my electric cost (dirty though they be) and need to deliver propane and obtain a tank?

I asked this question earlier in the year as part of a larger question (and all other parts of the question were answered well) and so there was some confusion as to my question. In reading that question my first reaction would be that I was an idiot to try to heat the house with an indoor heat pump (were do you think your getting the “heat” from? are you trying to create a perpetual motion machine when really you would be heating water with the waste heat of the mechanics?).

Yeah well… as my wife says “I can’t read your mind”. I thought one would note my question of using a HYBRID not just a normal heat pump and that I wouldn’t be using the heat pump aspect in the winter.

As a reflection on that question I did install a radiant floor system in the finished basement of this not yet completed house, with the intentions of using solar hot water (I learned most of my solar/green design info in the late 90s (from books written in the 70s and a lot of old hippies) and thought active solar beat photovoltaics any day; I learned otherwise after installation of my system). I am still considering using this already installed system with the water heater in question. If I got with a hybrid heat pump can it’s resistance element keep up with heating my domestic water and heating needs (probably off-set with a mini-split and tiny efficient (outside air vented) wood stove (maybe the Hearthstone Tribute or Lima).

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  1. GBA Editor
    Martin Holladay | | #1

    A heat-pump water heater can save energy, even if it steals space heat during the winter. Whether it saves enough to justify the higher purchase price depends on how much you pay for the water heater and your local electricity rates.

    The price of PV modules has dropped to the point where investing in PV is often the best measure to invest in. Again, this depends on your local electricity rates. But it's safe to say that a PV array will last longer than a heat-pump water heater.

    The following two articles include examples of payback calculations and savings estimates that will help you make your decision:

    Heat-Pump Water Heaters Come of Age

    Solar Thermal is Dead

  2. kevin_in_denver | | #2

    It sounds like you are using a minisplit heat pump for winter space heating. I've done the rough math for this configuration and found that using a minisplit to feed heat to the heat pump water heater is more efficient than using electrical resistance heat for your hot water.

    In this calculation, it doesn't matter if it's PV electricity or grid electricity. You would use about 50% less energy than resistance heat. If you look at a 30 year period, you would need to replace the first HPWH after 15yrs, and the second one at the 30 year mark.

    If you are just two people that take quick showers, then this savings might not be enough to justify 2 HPWHs versus one $900 Marathon that should last at least 30 years.

    The value of the summertime cooling is significant and factored into that calculation.

  3. Expert Member
    Dana Dorsett | | #3

    The math for heat pumps works at today's PV ~$4-5/watt all-in cost for grid tied systems but by the time your GeoSpring fails it may have flipped.

    A couple of weeks ago Jigar Shah (founder of Sun Edison) was quoting an installer in Texas who expected to be selling residential roof-top PV at $1.65/watt ( including inverter, before subsidy) in 2014 due to recent innovation in low-cost low-labor racking systems, and streamlined permitting & inspection in some Texas locations. Panel pricing in volume is about 70 cents/ watt today, but that is widely anticipated to be cut by about half by 2017 (some say only a third), despite an uptick in price of silicon. This is due in part to multiple competing thin silicon technologies that should be in full production by then, cutting the both the raw silicon and processing costs dramatically. A local MA company 1366 Techologies just received large venture funding to build a large production facility for it's proven thin-silicon casting process, and Twin Creeks sold the IP for it's proton exfoliation process last year to GT Advanced Technology, a large company in Shanghai with a real stake in the PV materials & processing biz. Either process would cut the silicon in a panel by 75% or more, and are processes capable of high automation.

    If, as anticipated, panelized PV is hitting under 25 cents/watt in 2025 (about the time your heat pump water heater croaks), and the red-tape issues surrounding PV are cleaned up (currently complicated by utility, state & local requirements), spending the grand on more grid-tied solar may have a better lifecycle IRR than another heat pump water heater if you have the real estate for it.

    In the meantime, I would expect the all-stainless construction of AirGenerate's AirTap heat pump water heaters would render a lifecycle on the order of 25 years rather than 12, and when the labor costs of replacement is factored in, may be the better deal this week. It's more expensive than a GeoSpring, (but not 2x the cost), but it should last as long as a Marathon, and use less than half the net power. (The ATI-80 has 80 gallons of storage, but the ATI-66 is plenty for most people.)

    If most of your hot water use is showers and that's where your capacity concerns lie, a drainwater heat recovery unit can roughly double the apparent capacity of any of them for well under a grand if you get them wholesale through EFI (rather than retail through Home Depot, or direct from the manufacturer.) But you need at least 4- 5' of vertical drain downstream of the shower to use one. And since it works via heat recovery, the apparent efficiency of the hot water heater soars on showering draws (but not during batch draws such as tub-fills, since both the drain and hot water need to be running at the same time to make the exchange.) From a raw economics point of view it won't pay for itself very quickly for low volume users with low-cost water heating, but may pay for itself up-front in reduced hot water heater sizing for high showering-volume users. (A 50-60 gallon tank instead of 80-100 gallons, and higher net efficiency to boot.)

  4. Expert Member
    Dana Dorsett | | #4

    FWIW: The Hearthstone Lima is really nice & Euro-modern & all, but it's 3x the price of the Tribute (you could buy a mini-split with the price difference!), and tested a few percent lower efficiency than the Tribute. ( It's also rated at about 50KBTU/hr, which is on the big side for a high-R home, to the Tribute's more manageable 36K.)

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