ETL 1110-3-489
3 April 98
Note that there is no diversity in the number of occupants in the building. The peak
volume of domestic hot water is calculated assuming 100% building occupancy. Also
note that equation (1) does not take laundry or dining facilities into account. Add
additional hot water requirements if laundry or dining facilities will be requiring hot water
during the peak demand period.
(2) Tank Size: Once the peak demand is known, the tank capacity and 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:
Vp - ( M St )
R=
(2)
d
where:
R = recovery rate at the required temperature, Lps, (gph).
M = ratio of usable water to storage tank capacity (60 - 80%)
St = storage tank capacity, L (gal).
9.5 min
60 sec
occ
d = duration of the peak, (seconds) =
(SI)
min
occupant
9.5 min
occ
occupant
d = duration of the peak, (hours) =
(IP)
60min
hr
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.
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