UFC 3-420-01
25 October 2004
(BTUH)
∆Tt = temperature difference between the tank water and the make-up water,
K (F)
Determine actual storage tank and domestic water supply temperatures based
on local requirements. If the water in the tank is assumed to be 60C (140F)
and the make-up water is 4C (40F), equation (5) can be simplified to:
kW ⋅ sec
q = qpipe + Rx233.402
(5-SI)
L
BTUH
q = qpipe + Rx50,000
(5-IP)
gpm
The new value of q can then be inserted into equation (3) to determine the
required flow rate for the pump. Compare this flow rate with the minimum flow
rates required for boiler or water heater operation and require the larger of
the two values. Once the required flow rate is known, the pressure drop for
the circulation pump can be determined. The resources listed in paragraph
6.b(4) (Pipe Sizes) give adequate information on calculating the pressure
drop through the piping. However, several manufacturers should be
contacted to determine the pressure drop through the water heater or boiler.
This value will vary widely between different manufacturers. Therefore, the
circulation pump should be sized to overcome the highest pressure drop.
However, flow rates over or under those required by the boiler or water
heater manufacturer can reduce the efficiency of the unit. As a result, the
drawings should indicate the pump characteristics used for the design. Then
add a note indicating that the pump is to be sized by the boiler or water
heater manufacturer with the horsepower requirements not to exceed those
listed in the schedule.
(4) If a heat exchanger is used, size the circulation pump based on the
flow required for the heat exchanger to meet the recovery calculated in
equation (3) and the heat lost through the piping.
d. Sample Computations:
(1) Given:
h = 36 shower heads
occ= 2 occupants per shower
E-6
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