A heat pump water heater uses the same type of refrigeration cycle technology that heat pumps use for space heating. Heat is absorbed from the surrounding air and used to heat water rather than warm the air in the room. This increased efficiency of the heat pump cycle over resistive heating makes these units twice as energy efficient as conventional electric hot water heaters.
There are three different categories of heat pump water heaters. These groupings are referred to as add-ons, drop-ins and desuperheaters.
The add-on unit replaces the function of a conventional hot water heater's lower heating element. It can sit on top of the existing tank or be wall mounted and circulates water from the existing tank through its heating coils. The upper heating element remains in the hot water tank so it remains available to assist as a back-up source of heat during periods of high demand.
Drop-in heat pump water heaters, as shown here, have the heat pump coils built directly into the tank and fit into the same space as a conventional gas or electric hot water heater. Installation is quite similar to a conventional unit except that provisions need to be made to route the condensate line to a drain.
The third type of unit is the desuperheater. These are small auxiliary heat exchangers for central air conditioning systems that use the super heated liquid from the compressor to heat water. The hot water produced is purely a by-product of conventional air conditioning with no additional energy input required.
The heating of this water actually increases air conditioner efficiency because the water can absorb heat more quickly than the condenser fan can expel it. However, these units can only supplement an existing hot water system and must be plumbed accordingly. They can only deliver hot water when the air conditioner is running during the cooling season.
Add-on and drop in units are most effective in warm climates where a long cooling season exists. Efficiency and capacity drop as the temperature drops. Systems my not be able to maintain an adequate hot water temperature or meet demand with the heat pump alone in cold weather. Add-on heat pump water heaters with an electrical back-up are recommended for cooler climates.
These water heaters deliver a refreshing by-product called cool air whenever they are running. It can be used to lower the temperature in the garage or basement or routed through ducting to supplement other cooling within the home. Keep in mind, however, that since the unit is driven by controls that regulate the water temperature it is not suitable for maintaining a constant air temperature in the room like an air conditioner.
The added complexity of the heat pump technology used in these water heaters makes them more expensive than conventional gas or electric hot water heaters. If they can be used to meet all of the hot water needs throughout the year, hot water energy costs can be cut in half yielding a pay back period of 4 to 6 years. If the unit can only provide supplemental heating, investment recovery will be extended accordingly.
If you want to know exactly how long it will take to recover your investment apply load profiling or sub-metering techniques with a multi-point or circuit level energy monitor. Be sure to measure the energy consumption of your old water heater for two weeks to a month before removing it so you will have an accurate baseline. Compare the energy consumption of the new unit to your old baseline to determine the savings.
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