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Heat Water With Air

A BuildingGreen Top 10 Product: Rheem HP-50 Heat pump water heater

More than one way to heat water Rheem's HP-50 water heating options include heat pump, electricity, or both. The HP-50 has the longest warranty and lowest dB rating of all heat pump water heaters.
Image Credit: manufacturer

–From BuildingGreen’s 2009 Top-10 Products list

The Rheem HP-50 is an Energy Star-listed, heat-pump water heater with an integral 50-gallon tank. Intended for residential use, the HP-50 operates in three modes: Energy Saver mode uses the heat pump only (energy factor 2.0); Normal mode, for higher demands, uses the heat pump and one of two electric-resistance elements (energy factor 1.5); and Electric Heat Only mode relies solely on the electric elements, functioning like a conventional water heater for particularly high demands. These 75.5″-high, 21″-diameter units have an operating range between 40°F and 120°F and come with automatic freeze/overheat protection. The HP-50 comes with a 10-year limited warranty and is available through plumbing wholesalers. The Rheem HP-50 is not the highest-efficiency heat-pump water heater on the market, but it is the first relatively affordable, integral-storage, heat-pump water heater from a large national company to enter the North American market. It is also the quietest product on the market, rated at 49 dB, and has the longest warranty.

More information:

Rheem Manufacturing Company

101 Bell Rd.

Montgomery, AL 36117

Phone: 334-260-1500

Toll-free: 800-432-8373

www.rheem.com


14 Comments

  1. Dick Russell | | #1

    Situation-dependent economics?
    An air-source heat pump water heater (HPWH) will chill the air around it. This may be fine for an ordinary home in a cooling-dominated climate, but a real problem elsewhere.

    In winter in a heating climate, assuming the water heater is indoors, the heat ending up in the water will be partly the compressor work (source: electricity) and partly heat from inside the house (source: the house's heating system). One could argue that in this situation the heat source might just as well be the fuel that drives the heating system.

    In a superinsulated house, other than in winter when some heat is provided to the house, the HPWH will over-chill the surrounding area. If the area must be reheated by a heating system to mitigate the chill, where is the gain?

    It would seem, then, that any advantage of a HPWH is very dependent on the construction and location (climate) of the house.

  2. Expert Member
    Carl Seville | | #2

    Limited use product
    I agree with Richard's assessment. After a long discussion with the rep at Greenbuild it became clear that they only reach their peak efficiency in moderate to hot climates in unconditioned spaces. The rep said that they see a big part of their market in retrofits for existing water heaters that are in garages. It seems to me like a useful product for limited applications. It certainly isn't anything earth shattering. I see it as sort of a band aid that you can use on only one part of your body rather than a long term solution to any specific problem.

  3. John Semmelhack | | #3

    41 million households...
    Carl,

    These are not limited use products. 41 million households (in 2001) use electricity as their water heater fuel, accounting for 100 billion kWh or 9% of all household electricity use (http://www.eia.doe.gov/emeu/reps/enduse/er01_us.html)

    Nearly all households that use electricity for hot water also use electricity for heating/cooling, either via a heat pump or electric resistance heat. I’m pretty sure a majority of these households are in the southeast, south and southwest.

    Most electric water heaters have Energy Factors (EF) of 0.90 or 0.91. The worst performing HPWH on the market (the Rheem) has an EF of 2.0 (at 68F ambient temperature). Yes, a HPWH located in conditioned space will “rob” the space of some heat during the heating season, but they will cool (and dehumidify) a space during cooling season.

    For a typical 4-person household in my climate (central Virginia) with a standard heat pump (8.0 HSPF, 13.0 SEER) and leaky ducts (~20% duct losses), the net benefit (hot water energy savings + cooling energy savings - added heat energy) of a 2.0EF HPWH in the conditioned space is about 2,000kWh. In Atlanta, with a shorter heating season and longer cooling season than Virginia, the net benefit is closer to 2,500kWh. In Miami, with no heating season and a very long cooling season, the net benefit is close to 4,000kWh.

    You can do the math on energy cost savings for your area…for me it adds up to about $250/yr at current rates. Hardly chump change for a $1,000 product that should last 15-20 years.

  4. GBA Editor
    Martin Holladay | | #4

    Who uses electric water heaters?
    John,
    I'm not sure you're right that "nearly all households that use electricity for hot water also use electricity for heating/cooling." Here in Vermont, where most residents have no access to natural gas, most people have an electric water heater — because electricity doesn't cost much more than propane (sometimes electricity is cheaper than propane). And with electricity you don't need a flue.

    But Vermonters don't use electricity for space heat, as you imply. They generally use oil, supplemented by a wood stove.

  5. Expert Member
    Carl Seville | | #5

    Limited Use
    My point about it being of limited use is that it only works in the heat pump mode fully when it is installed in unconditioned space in warm climates. In conditioned space, which is where most good green buildings put their mechanical equipment, it will provide unwanted cooling in cold months and not work full time in heat pump mode. In cold climates it will not be much more efficient than a standard electric heater. It looks like an excellent retrofit product for warm climates where it is installed outside of conditioned space. At Greenbuild, the product rep said that their primary market is replacement of old units in garages in hot climates. That seems pretty limited to me.

  6. Pete Engle | | #6

    Not all that limited
    Carl, I agree that these may not be the best solution for heating-dominated climates, but I don't completely agree that this is a serious limit on its utility. Perhaps my disagreement is just a matter of degree. There are lots of cool climates where central air conditioning is not necessary, but the A/C manufacturers still seem to make a living selling them.
    These HPDHW units look like a great option for people who live in mixed to hot climates and use electricity for the hot water supply. Sure, that's a limit, but certainly not a fatal one. Lots of people live in these areas.
    Also, I'd disagree that their best application is outside the envelope. For cooling-dominated spaces, these units will supplement the cooling, and they also dehumidify. That makes them very good candidates for indoor installations in warm/humid climates as well.
    I'm not sure they'd be the best solution for my climate (NJ), and I'm pretty sure that if you live in Vermont, you're out of luck. That's the price you pay for living in the woods.

  7. Jesse Thompson | | #7

    Even in VT
    How about a wood-fired heat pump water heater?

    All you back woods Vermont folks can just crank up the wood stove with your free carbon-neutral pile of fuel and let the HPDHW suck the heat out of the air and put it into your DHW. I'll let somebody else calculate the efficiency of it all...

  8. GBA Editor
    Martin Holladay | | #8

    It's called a hot-water coil
    Jesse,
    All you need is a stainless-steel coil in your firebox, connected to a loop of copper tubing to a tank in a closet upstairs. The water circulates by thermosyphon. No electricity necessary.

    I've used one for 15 years. They work great. No need to buy a heat-pump.

  9. John Semmelhack | | #9

    Who uses electric water heaters?
    Martin,

    Alas, I haven't been able to track down the number of households that use electricity for both space heating and water heating. The best I could find was that 41 million households use it for water heating, 31 million use it for their main space heating (with an additional 13 million that use it for supplemental space heating). In addition, there were 58 million households with central cooling. What's the percentage of crossover?...you're right, it's not "nearly all" as I claimed in my first post, but it has to be significant...20 million, perhaps, with electric heat and hot water? My main point (to Carl) is that this is not a limited market.

    In my region (Delaware, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, and West Virginia—and the District of Columbia from the way the EIA slices it), 13 million households heat water with electricity, 11 million on electric heat and 16 million with central cooling.

  10. Alex Wilson | | #10

    Suitability of heat-pump water heaters
    I'm with John on this debate. In cold climates, most people get their Btus of heat from sources less expensive than electric-resistance heat. But even if they heat with electric-resistance, on an annual basis, I'm guessing that the Rheem HPWH would be able to operate in full HP mode for well over half the year, and even it were otherwise robbing electric-resistance heat from the space, dropping its effective EF to 1.0 during that time (worst-case), you would end up with more than a 50% improvement over a standard electric water heater.

    I think in 10 or 20 years, when virtually all electric water heaters are heat pump models, we'll get a chuckle about this debate.

    Here's a link to a more in-depth article I wrote in Environmental Building News (companion publication to GreenBuildingAdvisor): http://www.buildinggreen.com/auth/article.cfm/2009/9/25/Heat-Pump-Water-Heaters-Ready-for-Prime-Time/? If anyone wants to read that and doesn't have a password, e-mail me and I can provide a link that will provide access to the article for seven days: [email protected].

  11. Peter d'Entremont | | #11

    heat pump water heater
    Alex Wilson is on the right track. Even in coastal Maine we have an issue with high humidity during the non-heating season, all two months of it. The dehumidification aspect is a compelling one when coupled with a decent COP.

  12. Travis Thompson | | #12

    HPWH with Solar Preheat
    I'd like to get your opinions on using one of these as the backup to a solar thermal system. I have a single collector solar system on our house. We have a less than ideal solar site but our system performs very well in the summer. The output drops off greatly in the wintertime. I have been thinking of trying one of these out in our house to be the backup to the solar. The water heater is in our currently unconditioned basement but I will eventually finish this space off. Any thoughts on how one of these units would work in conjunction with solar thermal? I can also see a potential benefit to these in basements that require dehumidification, as many do here in North Carolina. I have to run a dehumidifier in our basement all summer, a 500 watt load running for 8 to 12 hours per day. Wouldn't one of these, not only double the efficiency of our water heater but also eliminate the additional dehumidifier load?

  13. Michael Chandler | | #13

    Hi Travis
    The issue with these things is that they move BTUs from hot air to cold water, so in a situation where the solar is not producing enough because it's winter the heat pump water heater would also be at a disadvantage. I'd go with one of the new condensing LPG or NG Rinnai RC98HPi-P or Quietside water heaters for that application.

  14. Thomas | | #14

    Basement install
    My water heater is in the basement currently and I've looked at one of these to move my water heater 20-30 feet closer to my water usage, currently I have a natural gas water heater (chimney and bathrooms are on opposite sides). I think this would be ideal, since my incoming water is 40F and raising it up to the 60-65 that the basement stays on it's own should be a pretty big savings. In the summer it seems you could vent outside air in and out of the heat exchanger to get the most savings.

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