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Heat Pump Water Heaters

Heat pump water heaters can provide up to 60% energy savings over conventional water heaters. Heat pumps are a well-established technology for space heating. The same principal of transferring heat is at work in heat pump water heaters (HPWHs) except that they extract heat from air (indoor, exhaust or outdoor air) and deliver it to water. Some models come as a complete package, including tank and back-up resistance heating elements, while others work as an adjunct to a conventional water heater. Because it extracts heat from air, the HPWH delivers about twice the heat for the same electricity cost as a conventional electric resistance water heater.

The simplest HPWH is the ambient air-source unit, which removes heat from surrounding air, providing the additional benefit of space cooling. Exhaust air units extract heat from a continuously exhausted air stream and work better in heating dominated climates because they do not cool ambient air. Some units can even be converted between the two modes of operation for optimum operation in either summer or winter. In mild climates you can locate units in unheated but protected spaces such as garages, essentially using outdoor air as a heat source.

A variation of the stand-alone HPWH is the desuperheater feature available on some central air-conditioners. It provides economical supplemental water heating as a byproduct of air-conditioning. Desuperheater water heating can be part of an integrated package with a heat pump or air-conditioner system. In most such systems, the heat pump water heating only occurs during normal demand for space conditioning, with resistance electric coils providing water heating the rest of the time. During the cooling season, the desuperheater actually improves the efficiency of the air-conditioning system while heating water at no direct cost. In an average climate, a desuperheater might meet 20% to 40% of annual water heating demand.

Applications: Properly applied, HPWHs save energy in almost every situation. Initial investment is recouped fastest if electric rates and hot water usage are high and there is a steady need for the cool air generated as a byproduct. HPWHs have difficulty being cost effective when low-cost natural gas is available. Best energy savings are accomplished when temperatures are mild or warm. Because HPWH efficiency and capacity drop as temperature drops, avoid applications where the ambient air is cold.

The high water heating efficiency combined with the cooling benefits tends to favor applications where there are large hot water demands, which occur for much of the day, and where there is a simultaneous need for spot cooling. Laundries, restaurants and some dormitories are representative of good applications.

  • There are several commercial building types that are typically good candidates for HPWH. They include:
  • Laundries (coin-operated and commercial), where there is a large daily hot water requirement and where space cooling would be useful

  • Restaurants, particularly in the kitchen where large hot water demands for dishwashing coincides with a need for cooling the kitchen and its occupants; locate the HPWH evaporator to take advantage of the heat from the dishwasher

  • Hotels and motels are large users of hot water; locate the evaporator of the HPWH near ice machines to improve their performance

  • Health clubs for spa heating and service water heating

  • Schools, particularly in the kitchen where hot water is used for food preparation and clean-up

  • Multifamily housing and apartments where a single system provides hot water to all residents

  • Finally, in these and other buildings where the cost of energy for conventional water heating is high and where a more efficient water heating option would be attractive.

Costs: Most of the heat delivered to the water comes from the evaporator of the unit, not through the electrical input to the machine. Consequently, the efficiency of the HPWH is much higher than for direct-fired gas or electric storage water heaters. The installed cost of commercial HPWH systems is typically several times that of gas or electric water heaters; yet the low operating costs can often offset the higher total installed cost, making the HPWH the economic choice for water heating. The HPWH becomes increasingly attractive in building applications where energy costs are high, and where there is a steady demand for hot water. This attractiveness is less a function of building type than it is of water demand and utility cost.

Swimming Pool Water Heating: HPWHs are also useful and efficient for pool and spa heating. The water heating principle is identical to that previously described for a HPWH, which uses air as the heat source. In the case of pool heaters, the cooled air can be exhausted outdoors, or can be used to cool and dehumidify the air above the pool. Dehumidification is a particularly attractive feature for indoor pool or spa applications. Systems which provide pool water heating and operate over a number of ventilation air modes are available from a number of vendors in sizes ranging to more than 500,000 Btu/h of water heating.

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