Technology is rapidly changing the face of domestic water heating. Federal appliance efficiency standards, passed in 2010 and effective in 2015, require that electric tank water heaters must use less energy. For electric-powered tanks 55 gallons or smaller, like those used most often in smaller homes, efficiency standards inched higher from 92% to 96%. Larger tanks, on the other hand, took a significant leap from 90% to almost 200% thanks to heat pump technology.

Electric heat pump water heaters (HPWHs) are now available from all major water heater manufacturers. HPWHs use a compressor similar to the one in your refrigerator. Instead of pulling heat from inside the refrigerator’s insulated box and pumping it into the surrounding air, a HPWH pulls heat from the surrounding air and pumps it into a tank full of water. SomeThe better models already exceed the new appliance standard – reaching between 200% and 350% efficiency.

Today’s HPWHs contain a compressor housed on top of the tank, so the unit stands about 20-inches taller than a standard electric tank heater. They are often marketed as “hybrid” water heaters, because they contain electric resistance heating elements that kick in when the compressor alone can’t keep up.

CO2 and Comfort Issues

Today’s typical HPWHs currently use approved hydrofluorocarbon refrigerants just like the compressors in refrigerators, air-conditioners, and space-heating heat pumps. These refrigerants are known as R-134a or R-410a. Despite being ozone friendly, these newer chemicals are powerful greenhouse gases. Both are greater than 1,000 times more potent than carbon dioxide (CO2). Standard heat pump water heaters have additional limitations, especially in colder climates. Because they pull heat from the surrounding air, they cool the space where they are located, so they need to be placed in areas that aren’t heated directly. These so-called buffered spaces include unheated basements or garages. The side-effect is free air conditioning, which is useful in warm climates. But cooling a building in winter is problematic in colder climates, as it can drive up space heating costs and make these spaces less comfortable.

New CO2 Split-System Heat Pumps Address These Issues

The limitations of today’s typical HPWHs are addressed by the next big advance in technology. The most visible improvement is that the system comes in two parts. Called split-system heat pumps, the variable-speed compressor sits outdoors so it doesn’t rob heat from the building. And for once, there is good news about carbon dioxide. Using CO2 as the refrigerant leads to efficiency as high as 400% and effective cold temperature operation as low as -20°F. This technology also generates water temperatures up to 175°F, significantly higher than the HFC-based heat pumps. To reach these new heights of performance requires more robust equipment so that it can cope with significantly higher operating pressure in the refrigerant loop. Finally, the global warming potential of CO2 is 1,000 times lower than the HFC refrigerants in more conventional heat pumps. That’s good news all around.


Robust performance means that split-system, CO2-based HPWHs will work in just about any North American climate. Short periods when the outdoor temperature drops below -20°F are handled with electric resistance backup heat elements, like those found in a standard electric tank. Better performance also opens the possibility that these units could supply hot water for hydronic space heating applications. Current residential models can serve as “combo” heaters in homes with very low heating requirements, such as Passive House or others striving for zero energy level of performance. In the future more robust models may be developed for heating larger or less energy-efficient homes.

Available CO2 Split System HPWH Products

Current products all originate from Japan, where they are known as “eco-cute” devices. Currently, only the SANCO2 split-system HPWH system from Sanden is available for residential use in North America. Similar products, such as the Sanyo unit, are already available in Europe. Mitsubishi is rumored to have a residential product under development. More choices exist for commercial and industrial scale equipment from major companies such as Mayekawa, and Panasonic.

Technology is moving fast in the water heater industry; therefore CO2 Split System HPWHs should leap over current limitations. Soon, Eexpect to see many new products based on CO2 refrigerants in buildings of all sizes soon. While the cost is higher than older, less efficient, more polluting technology, there is great promise for systems combining space heating, space cooling and water heating into a single system. This spreads the added cost across all three functions. And as their popularity increases, the prices will come down, just as they did for the current generation of HPHWs.