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For the AirTap A7, the test results agree very well with the test result provided with the unit. However,
the results are suspect because the recovery rate of the system was insufficient to return the tank to its
initial temperature in the time following each draw, so the average tank temperature was less than what
the system should have had. With a lower average tank temperature, the efficiency of the system is
higher than it would be if the temperature stayed close to the tank temperature setpoint. The CR Profile
test allowed more time for recovery between draws, and included two draw events that caused the upper
tank heating element to activate, and thus produced a lower EF.
The lower than rated Energy Factor numbers seen for the Rheem in the first tests is directly a result of the
system switching over to electric resistance heat once the tank temperature reached about 130°F. As seen
in the later test results, the unit certainly can meet and exceed their published rating so long as the system
only uses the heat pump. Most consumers will likely leave the temperature setpoint at “Normal” where
the electric resistance elements will likely not be activated in the Energy Saver mode. Consumers should
also be actively advised to operate the system in the Energy Saver mode over the Normal mode to achieve
consistently high performance. Another concerning result from these tests are the low numbers for the
operation with electric resistance heat only, with numbers significantly lower than what would be
expected from a typical electric water heater. Some of this can be attributed to a standby power draw of
at around 5½-Watts that is present even when the system is not actively heating, which may primarily be
due to the indicator lights. (The AirTap indicated no measurable standby power.) Another factor is the
external pipe for circulating water through the heat pump, which even though insulated still creates
additional surface area for heat loss.
Several sample charts of the start of an Energy Factor test are included in the Appendix. Figure 12 shows
the general DOE standard draw profile in terms of when the flow draws occur and the quantity of each.
While this chart shows the full 24-hours of the test window, the subsequent figures in this group only
show the first 8-hours or the first third of the test. Figure 13 shows the test for the AirTap unit in heat
pump only mode, and it can be seen that the heat pump was on continuously throughout the test and
continued to run for more than two hours following the last draw until the thermostat was satisfied. It
also indicates a decreasing trend in the maximum water outlet temperature for each draw. In contrast, the
charts for the Rheem HP50 unit in Energy Saver (Figure 15) or Normal mode (Figure 14) show that in
both modes the system was able to heat the water back up to the setpoint following the standard draw, and
the outlet water temperature was consistent.
Temperature Sensitivity
Determining the sensitivity of the system performance (in particular power consumption) as a function of
the water temperature and the ambient air temperature is subjective to the ability to control and measure
these parameters and the ability of the test unit to react consistently to them. The results obtained from
this test program are not completely clear and are open for interpretation, and could use a more rigorous
examination than was included in this program.
The sensitivity of power consumption to the average tank temperature was apparent in the test results
shown in the charts for Energy Factor and especially First Hour Rating (because of the larger draw
quantity and the resultant large range in tank temperature). As these tests were all done while keeping the
room temperature relatively constant, the ambient conditions have little impact. Data were consolidated
from the various standard tests and extracted from the periods when only the heat pump was operating (no
electric resistance heat) and no draw was occurring. Curves were fitted to the consolidated data set, and
these are shown in Figure 16 without the underlying data. Bounding parallel curves indicating one
standard error of the estimate (SEE) are drawn with dashed lines to signify the tightness of the data set to
the curve. The curves for the Rheem are cut off below 105°F because no data were collected below this
temperature. The important point to gather from these curves is the rise in power consumption as a
function of the tank temperature, which is set by the system thermostat. Lowering the thermostat thus has
the effect of capping the maximum amount of power that the heat pump will draw when heating water.
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