Download the project report.

The manufacturer requires that the bottom element of the existing electric water heater be disabled so as
not to override the heat pump coil. The upper element may be left active to provide quick heat to the
upper part of the tank during large use events, or it may also be disabled as well to ensure high efficiency
operation with just the heat pump. (Electric resistance water heaters typically have upper and lower
heating elements that operate alternately depending on which one’s thermostat is activated; for most draw
events only the lower element will operate.)
There were fewer tests conducted on this unit as the result of a number of errors unrelated to the HPWH
itself. Upon heater startup, the electric resistance element was showing a low power draw even though it
was supposed to be off. Either through a manufacturing defect or because the element may have been
activated without water, the upper heating element had failed and passing current to ground through the
water. While the system could (and would mainly) be tested without supplemental resistance heat, it was
desired to include it as more representative of the typical installation. In case it might have been
damaged, the tank temperature thermocouple array inserted through the anode rod penetration was also
removed for inspection. Unfortunately, the thermostat for the heat pump had become tangled in the
thermocouples, and broke off when they were removed. Thus, the evaluation unintentionally
demonstrated that the system thermostat can be replaced without affecting the other system components.
All of the test results for this unit are from after the thermostat replacement and installation of a new
heating element in the tank.
As an add-on unit rather than a system, the AirTap does not have official performance ratings listed, and
the overall system performance will depend on the efficiency and capacity of the attached water tank. A
test report from GAMA was provided with the unit listing an Energy Factor of 2.11 and a first hour rating
of 42.5, but it does not say how large of a storage tank was used.
Rheem HP50
This is a new product design of an integrated heat pump system with supplemental resistance heat. In this
system, all of the heat pump components including the condenser are located on top of the tank, and water
is circulated by a pump loop that draws water from the bottom of the tank, through the condenser, and
back into the top of the tank. With this circulation, the tank water temperature stays very consistent with
very little stratification. Air to the evaporator is drawn in through a round filter on top of the unit, and
discharged through a coil that forms about 2/3 of a cylinder on the back side of the unit. Because of the
location of the heat pump components, there are no penetrations in the top of the storage tank: the water
inlet and outlet connections are on the sides. The tank also does not have an end-user serviceable
sacrificial anode rod. Because there was no top penetration, the tank temperature thermocouple array had
to be inserted using the pressure relief valve tap, while keeping this safety valve available through a tee
fitting. Because the cool air discharging the evaporator is directed out and down from the top of the tank,
the tank environmental temperature may be depressed, possibly resulting in an increased potential for
standby loss unless the space is adequately ventilated.
This unit also has resistive elements for supplemental heating; however, the element capacity is about half
of that for other similar-sized electric water heater tanks at 2 kW each. The system is enabled with three
distinct modes of operation – Normal, Energy Saver, and resistance heat only - as illustrated in Figure 6.
The chart shows the overlapping trends of power consumption for the three modes following a draw in
the standard Energy Factor test (~10.7 gallons at 3 gpm). In “Normal” mode, the heat pump turns on first
and is allowed to heat the tank up to a certain temperature. As the water temperature rises, the heating
capacity of the heat pump decreases, so one resistance element is brought on some time later to assist the
heat pump. Eventually, once another temperature threshold is reached, the heat pump is turned off in
favor of the resistive element. The “Energy Saver” mode is not all that different, other than allowing the
heat pump to run longer by itself, and then switching over to resistance heat when needed while not
allowing the two components to operate simultaneously. Thus, the Energy Saver mode not only reduces
energy consumption, it also reduces the maximum power demand by about a third. The last stage of both
these modes with the resistance heat operating alone is only needed if the thermostat is set above the
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