UFC 3-420-01
25 October 2004
corresponding recovery rate can be determined. Since space in the
mechanical room is frequently limited, select the desired tank capacity first.
An initial estimate of tank size can be determined by using 50 L per
occupant (12.5 gal per occupant). Normally a selection of 50 L (12.5 gal)
will provide acceptable operation at a reasonable cost but it should be
noted that other factors including larger tank sizes and higher storage
temperatures will reduce amount of recovery required. Larger tank sizes
and increased storage temperatures will also result in greater heat loss
from the storage tank. Compare selected tank size to standard tank
capacities available commercially and with the space available in the
mechanical room.
(3) Recovery Rate: Once the tank capacity is known, the recovery
rate can be calculated. The recovery rate is the quantity of
water to be heated from the inlet temperature to the desired
storage temperature. The difference between the inlet water
temperature and the water storage temperature is often
assumed to be 55C (100F). However, this temperature
difference should be coordinated with local conditions and
revised as necessary. Equation (2) below is used to
determine the required recovery rate:
- (M
V
S)
p
t
R=
(2)
d
where:
R = recovery rate at the required temperature, Lps, (gph).
M = ratio of usable water to storage tank capacity (60 - 80%)
50L
St = storage tank capacity (initial estimate), L (gal) = (h)(occ)
occupant
9.5 min
60 sec
d = duration of the peak, (seconds) =
occ
occupant
min
The duration is calculated assuming that the peak usage period will be 9.5
minutes per occupant. Therefore, if two occupants share a bathroom the
duration is 19 minutes, 3 occupants would be 28.5 minutes, etc.
Note that the recovery rate is an output condition. Insure that manufacturer's data,
submitted for the hot water generation unit, indicates that sufficient input capacity to
satisfy the recovery rate with actual inlet water and storage temperatures will be provided.
(4) Pipe Sizes: The next component to be sized in the system is the
domestic hot water distribution piping. Appendix A of the 1997 National
E-3
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