With energy-consuming equipment, such as water heaters and refrigerators, we have good data on energy consumption and can set clear standards accordingly. In some product categories—clothes washers, for example—Energy Star standards were adopted because those standards provide a high enough threshold to represent just the very top segment of the product market (less than 10%). In other product categories—e.g., refrigerators and dishwashers—we set a higher threshold than ENERGY STAR: for example, exceeding those standards by 10% or 20%. With lighting and lighting control equipment, certain generic products qualify, such as compact fluorescent lamps and occupancy/daylighting controls, while in other categories only a subset of products qualify. In some cases, products that meet the energy efficiency requirements are excluded, because of evidence of poor performance or durability. Microturbines are included here because of the potential for cogeneration (combined heat and power) that they offer.
The AirTap A7 is an inexpensive air-source heat pump module that can be retrofit onto a standard electric or gas-fired storage-type water heater. The unit installs easily using the standard inlet and outlet fittings on the water heater; as part of the conversion, the gas or electric-resistance heating elements are either turned off or set at a lower temperature to function as a back-up. The A7 has an output of 7,720 Btu/hour and efficiency of 258%, more than double that of an electric-resistance water heater. The first-hour rating is 44 gallons, the maximum water temperature is 130°F, and the energy factor is 2.2 (as certified by the Gas Appliance Manufacturers Association). These units are 18" wide x 14" deep x 13" high, and weigh 48 pounds. The Air Tap uses the non-ozone-depleting R-417a refrigerant.
Electric heat-pump water heaters use compressors to evaporate and condense refrigerant in a closed loop, pulling heat from the air—similar to an air conditioner—and transferring that heat into water stored in an insulated tank. They are available as retrofits that connect to standard storage-type water heaters, or as stand-alone products with integral tanks.
A heat-pump water heater can achieve an energy factor of 2.0 or higher, while a storage-type electric water heater relying on electric-resistance heating elements has a theoretical maximum energy factor of 1.0. The performance of some of these products is measured using “coefficient of performance” or COP, but there is no standard for COP, so manufacturer claims may not be comparable.
GreenSpec lists products certified by Energy Star, which certifies heat pump water heaters with an Energy Factor of 2.0 or greater and a first-hour rating of 50 gallons per hour or greater. (First-hour rating is the amount of hot water in gallons a storage water heater can supply per hour.)
Heat pump water heaters dehumidify and cool the air where installed, so when located in conditioned space they help reduce summertime air-conditioning costs and improve comfort. But in the winter, such a heat-pump water heater increases heating loads by cooling the surrounding air. Depending on the climate and the building, heat-pump water heaters may be installed in the living space, garage, basement, or attic. They should not be installed in a space that will fall below 40°F (4°C). Be aware that heat pump water heaters contain fans and compressors, so they produce noise similar to that of a refrigerator.
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