Split-System Heat-Pump Water Heaters

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Split-System Heat-Pump Water Heaters

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

Posted on Feb 6 2013 by Martin Holladay

Heat-pump water heaters are a type of air-to-water heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source 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:

  • The units don’t work in very cold climates. Daikin provides capacity ratings for the Altherma down to an outdoor temperature of 5ºF; the manufacturer does not recommend these units for use at lower outdoor temperatures. The rated capacity of a Daikin Altherma unit at 5ºF is about 50% of its capacity at 37ºF.
  • The maximum water temperature produced by the Daikin Altherma is relatively low (131ºF), complicating some hydronic heating applications.
  • The units are expensive; homeowners have been quoted installed prices ranging from $16,000 to $36,000 for a heating system using the Daikin Altherma.

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 is 1.

As a marketing gimmick, Japanese manufacturers invented a generic “pet name” for CO2-based heat-pump water heaters. The appliances are known in Japan as “Eco Cute” heaters — a phrase that resembles other examples of mangled English that have entered the Japanese language. (“Eco” is short for the English word “ecological,” and “cute” refers to both the English word “cute” and the Japanese word kyuto, which means “hot water.”)

So-called Eco Cute water heaters are manufactured by two dozen Japanese companies. One of these manufacturers, a company called Sanden, is now planning to sell its heat-pump water heaters in the U.S.

Sanden’s Eco Cute

Sanden is now contracting for the testing necessary for its water heaters to be listed by Underwriters Laboratory. If the UL approval process goes smoothly, Sanden hopes to begin selling water heaters to Americans in the second half of 2013.

is now being sold and installed in Australia as well as Japan. Here are a few facts about the unit:

  • It uses CO2 as the refrigerant.
  • The water heater has an 83-gallon stainless-steel tank.
  • The appliance works at low outside temperatures; according to Albert Rooks, a Sanden distributor, it can operate at an outdoor temperature of -15°F, at which point its COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415. is between 1 and 1.2.
  • The appliance has a rated heating capacity of 18,500 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /h (presumably at at outdoor temperature of 30°F); when the outdoor temperature drops, so does its heating capacity.
  • The appliance does not include an electric-resistance heating element.
  • The unit produces 149°F water.
  • The outdoor unit has a variable-speed inverterDevice for converting direct-current (DC) electricity into the alternating-current (AC) form required for most home uses; necessary if home-generated electricity is to be fed into the electric grid through net-metering arrangements.-driven compressor.
  • Laboratory tests indicate that the COP ranges from 2.1 at 17°F to 5.0 at 95°F.
  • This water heater is expected to sell for between $3,600 and $3,700 in the U.S.

Field trials

An Australian distributor shipped 15 of these Sanden water heaters (consisting of outdoor unit #GAU-A45HPA and tank #GAU-315EQTA) to the Pacific Northwest, where the units have been installed in homes as part of an ongoing field trial. The performance of the water heaters will be evaluated by researchers from the Northwest Energy Efficiency Alliance (NEEA) and the WSU Energy Program.

The NEEA has also sponsored of the water heater.

Can these water heaters be used for combi-systems?

What are the advantages of split-system heat-pump water heaters?

  • They are energy-efficient.
  • In small, low-load houses with hydronic heat distribution systems, this type of heat pump is able to supply both space heating and domestic hot water — something that can’t be done with a heat-pump water heater with an indoor compressor. Using one heating appliance for two purposes simplifies a home’s mechanical systems.

It should also be pointed out that air-to-water heat pumps are unlikely to be widely used for space heating in the U.S., for the following reasons:

  • Most U.S. homes don’t have hydronic heat distribution.
  • Unlike ductless minisplits, these systems can’t provide space cooling.

What about cost-effectiveness?

One of the first American homeowners to receive a Sanden water heater was Tad Everhart, a Certified Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. Consultant in Portland, Oregon. Although the Sanden water heater is expected to save electricity, the appliance isn’t particularly cost-effective.

, “We hope the total cost to install a Sanden Eco-Cute HPDHW [heat-pump domestic hot water] system providing both water and space heating will cost approximately $5,000 after Oregon Residential Energy Tax Credits and incentives from the Energy Trust of Oregon. For space heating alone, the financial payback period is long (25 years) if electricity prices stay low. At current low natural gas prices, including fuel-switching for our DHW, the financial payback period is even longer. However, if the financial payback analysis includes the cost of global warming (accelerated by conventional refrigerants leaking into the atmosphere), Sanden's Eco-Cute’s benefits outweigh its costs.”

What about service?

Early adopters of Eco Cute water heaters in the U.S. are volunteering to be guinea pigs. The biggest unknown with Eco Cute units concerns maintenance and service issues.

An Australian who goes by the nickname DanRef posted the following comment on an : “I am a refrigeration specialist and heat pumps (air-water) are one of my favorite hobbies. … The lack of agents, repairmen, etc. … not available yet because is too early. CO2 as a refrigerant component requires more specialized and advanced tools which are not quite ready yet. In the event of a failure these units might be fixed only on the manufacturer’s workbench. No one will be able to re-gas or replace any other refrigeration parts at this time. I am sure CO2 will provide high performance, but the practice will show us how reliable they are.”

Author's note: I'd like to thank Dana Dorsett for suggesting the topic of this blog.

Martin Holladay’s previous blog: “Rules of Thumb for Ductless Minisplits.”

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Image Credits:

  1. www.whywait.com.au
  2. www.infinityplumbingsolutions.com

Feb 8, 2013 4:20 PM ET

by Keith Gustafson

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

Feb 10, 2013 5:44 PM ET

I'ts a chicken & egg thing...
by Dana Dorsett

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.

Feb 11, 2013 8:59 AM ET

Sanden's CO2 refrigerant connections
by John Semmelhack

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).

Feb 11, 2013 9:53 AM ET

Response to Keith, Dana, and John
by Martin Holladay

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.

Feb 11, 2013 4:32 PM ET

Response to Martin
by John Semmelhack

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

Feb 12, 2013 2:01 PM ET

Ice maker indeed!
by Dana Dorsett

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):

Apr 30, 2016 6:11 PM ET

by brad h

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

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