Plate 7 depicts the hot/cold water mixture temperature versus
percent of hot water as a function of hot water temperature supply and cold water
temperature supply. For example, for a comfortable 100 F shower with a 115 F hot water
supply, the percent of hot water out of the mixture valve will be 68% when the cold water
supply is 70 F. Percent hot water in the mixture valve will be 77% when the cold water
supply is 50 F, and 82% with 30 F cold water.
Proceeding a step further, the maximum shower time, the recovery time, and the cycle
time are shown in Plate 8. This uses as an example a 50 F cold
water supply, 40 gal tank, 40,000 Btu/hour recovery, a 100 F mixed water shower
temperature, and 4 GPM mixed water out of the shower, or into the bath.
With a 115 F maximum hot water supply, the mixture initially has 76.9 percent hot
water, and the percent hot water will have to be steadily increased to maintain a 100 F
mixed water from the shower. With these conditions note that the allowable shower time is
4 minutes but only 7.5 minutes for recovery, or 11.5 minutes cycle time. So that means 4
people can take a comfortable 4 minute shower at a constant 100 F by gradually increasing
the percent of hot water from 76.9 to 100 over the 4 minutes, with a cycle time for 4
people of 46 minutes.
Next, consider the hot water supply to initially be at 160 F. This extends the
allowable shower time at a comfortable 100 F mix to 10.5 minutes, but the recovery time is
extended to 30 minutes, or 40.5 minutes cycle time per person. This now then requires 162
minutes or 2 hours 42 minutes total for 4 people to shower.
Net effect of all of these numbers is that turning a hot water heater up from 115 F to
the extremely dangerous temperature of 160 F, the allowable time for shower increases by
10.5/4 = 2.62, but the cycle time increases more, by 162/46 = 3.52. So for how long a
shower should the installation be designed?
Substantially increasing the allowable shower time is just like asking for a cup of
coffee from a fast food restaurant at the standard 180 F holding temperature so that you
will be able to drive down the highway for another 100 miles with hot coffee. Why not just
buy a thermos bottle? Or stop again for a refill of coffee that you can drink without
burning your mouth or suffering 3rd degree burns when you spill excessively hot coffee on
yourself?
If the shower time simply must be increased, then the solution is to buy a larger
capacity tank, and not to increase the hot water supply temperature to extremely dangerous
temperatures. And the lower the heater temperature, the greater the energy savings.
Burns
