
Musings of an Energy Nerd

Split-System Heat-Pump Water Heaters

Japanese ‘Eco Cute’ water heaters will soon be available in the U.S.

A Japanese company called Sanden is selling its split-system heat-pump water heaters in Australia. In regions where temperatures stay above freezing, hot water tanks are often installed outdoors. [Photo credit: Why Wait Plumbing Services, Gold Coast, Australia —]
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Heat-pump water heaters are a type of air-to-water heat pump. Almost all heat-pump water heaters sold in the U.S. extract heat from the air in the room where the water heater is located, transferring the heat to water in an insulated tank.

In Japan, heat-pump water heaters are configured differently; they have an outdoor compressor rather than an indoor compressor. (The outdoor units of these heat-pump water heaters resemble the outdoor units of a ductless minisplit.) This type of air-to-water heat pump is called a “split system” heat-pump water heater; it is designed to extract heat from outdoor air rather than indoor air.

The main advantage of locating the compressor outdoors is that these water heaters don’t lower the temperature of the indoor air during the winter. If this type of air-to-water heat pump has a large enough capacity, it can be used not only for domestic hot water, but also for hydronic space heating.

The Daikin Altherma

A few models of split-system air-to-water heat pumps are available in the U.S.; perhaps the best known of these appliances is the Daikin Altherma. Daikin manufactures Altherma units that have a large enough capacity to provide space heating as well as domestic hot water.

While Daikin Altherma units generally perform well, they have a few disadvantages:

Japanese Eco Cute water heaters

In Japan, most split-system air-to-water heat pumps use carbon dioxide (CO2) as a refrigerant rather than the R-410A refrigerant used by the Daikin Altherma. Carbon dioxide has at least two advantages over R-410A: it enables an air-to-water heat pump to produce hot water at a higher temperature — as high as 194°F — and it has a lower global warming potential. While the global warming potential (GWP) of R-410a is 2,000, the GWP of CO2…

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  1. Keith Gustafson | | #1

    I have not had a heat pump die in service yet, but most modern units I thin would not get serviced aside from a relay or board

    CO2 is a common welding gas so I do not think that long term CO2 units would have a service issue

  2. User avater
    Dana Dorsett | | #2

    I'ts a chicken & egg thing...
    CO2 is cheap & available, but refrigeration techs in the US have no training or experience with CO2 refrigerant systems, nor do they have the necessary equipment. CO2 refrigerant systems operate at a much higher pressure than HVC or CFHC systems, and the CO2 is never in liquid form during any part of the refrigeration cycle- it's an always gaseous system. It's just different from the paradigm that precedes it.

    I'm sure 95% of the reefer-techs out there could learn how to deal with these heat pumps in under a week, but until there's a reason to buy the tools & get the training, so for the time being it will be a specialty skill.

    I'm personally pretty happy that small CO2 heat pumps have finally made it to the US. They have more than a decade of experience with the EcoCute systems in Japan, and coming onto a decade for similar systems in Europe. CO2 is a much more environmentally benign refrigerant than any HFC or HFO solution.

  3. User avater
    John Semmelhack | | #3

    Sanden's CO2 refrigerant connections
    Sanden's refrigerant systems are a "packaged" system sealed at the factory...the tech does not make any refrigerant connections. The connections that bridge the "split" between the water tank and the outdoor unit are all plumbing (water).

    In theory, with quality control at the factory the sealed refrigerant system should be a non-issue over the life of the outdoor unit...similar to refrigerators, window AC units and our current crop of heat pump water heaters....of course, we've seen some major issues over the past couple of years with refrigerant leaks in a couple of different heat pump water heater models (namely the first gen. GE and the AirTap units, both of which were supposedly made in the same factory. GE seems to have the issue under control now that they've developed their own unit and moved production to the U.S. The AirTap units are still a question mark in my opinion).

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Keith, Dana, and John
    Thanks to all three of you for your comments.

    The fact that the connections between the outdoor unit and the insulated water tank are all water connections is reassuring. There is a good possibility that these Eco Cute water heaters will prove to be as dependable as refrigerators.

  5. User avater
    John Semmelhack | | #5

    Response to Martin
    Yes, there's a good chance they'll be as dependable as long as they don't add on an ice-maker to the outdoor unit.

  6. User avater
    Dana Dorsett | | #6

    Ice maker indeed!
    As I understand it, it's the ice-up potential of the water connections between the outdoor unit and tank that limits the operating temp of the Sanden version of EcoCute to +14F, a limitation of their anti-freezing design. If used for space heating only it's likely that it would still do OK at sub-zero with a bit o' glycol in the water (albeit at somewhat reduced heat transfer efficiency on the water side of the heat exchanger in the outdoor unit.)

    Sanyo's (discontinued) heating/hot-water combi EcoCute w/CO2 refrigerant had a specifed output & efficiency at -20C/-4F (see the second page):

  7. Brad H | | #7

    It looks like the sanden unit is available in the US now.
    I haven't found a price.

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