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For both of the HPWHs, their recovery rates were insufficient to return the tanks to their cut-out
temperatures at the end of an hour. Thus, the second draws were all done to the shut-off temperature
from the first draw, and the systems were actively heating throughout the second draw.
For the AirTap unit in the heat pump only mode, the system was unable to recover enough heat in the
tank to produce an outlet temperature in excess of the termination temperature for the first draw. This
means that its first hour rating is only the amount taken in the first draw. As described in the test
procedure, the second draw at the one-hour mark was only 30-seconds to see what temperature was
available. For the first draw, the maximum outlet temperature recorded was 132.5°F resulting in a shutoff
temperature mark of 107.5°F. The maximum outlet temperature observed from the second draw was
only 100.5°F before it began to fall off again, so none of this second draw was included in the rating.
Adding supplemental heat from the upper resistance element did allow the tank temperature to recover
enough to provide additional hot water to the rating for the “Normal” mode. It is also notable in Figure 8
how quickly the two highest tank thermocouples reacted to the operation of the upper heating element.
The Rheem was thought to have been tested in two modes of operation, but the power usage trend for the
Energy Saver mode test suggests otherwise as both the heat pump and resistance elements were allowed
to operate together. In the two tests in Energy Saver mode (it was repeated to confirm the result from the
first), the unit did not switch on the resistive elements as soon as it did in the Normal mode test, so the
test unit may have switched to Normal mode as an override following an especially large draw of hot
water. However, since the manufacturer reports separate ratings for each mode, this seems unlikely that
this is by design, unless the resistance element was locked out in achieving their rating. All three of the
test results are close to the manufacturer’s listing for their Energy Saver mode.
Energy Factor
The simplest interpretation of the Energy Factor is the daily hot water energy output divided by the total
energy consumed (with the electrical consumption converted to a Btu equivalent by multiplying the kWh
by 3,412). The rating is based on a specific volume of hot water removed at a set temperature rise. The
DOE and ASHRAE testing standards include alternative procedures to correct the measured test results to
the standard volume and temperature conditions, and to compensate for changes in the stored energy (as
the result of the average storage tank temperature being different at the beginning and end of the 24-hour
period). Both the measured values adjusted by the two standard methods are included in Table 6 below,
along with a simple calculation method. The “Simple” Energy Factor values use just the sum of the hot
water energy removed and the increase in stored thermal energy divided by the electric energy input.
In many cases, the Energy Factor test for a particular mode of operation was repeated to confirm an
earlier result or when some parameter was changed. The columns marked EF#1 through #4 are the
number of tests run in each mode beginning with the earliest test run. The one major change was with the
Rheem system where the thermostat was adjusted down, and the results are reflected in the EF tests #3
and #4 for the Energy Saver mode tests. The final test in this group (CR Profile) was one done following
a draw pattern developed by Consumer Reports ® for a water heater test program (Reference 5), and used
to represent a more real-world profile. This test should actually produce higher numbers than the
standard Energy Factor test because the total volume was higher (77 gallons versus 64.3), so the tank
standby losses have a lower impact. Only one test was done at this profile, with the systems set to their
Normal operating mode with electrical resistance heat backup. How often the resistance heat is activated
has a large effect on the daily Energy Factor.
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