ETL 1110-3-489
3 April 98
q = qpipe + R 50,000 BTUH
(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
Td = 43C (110F)
Ts = 60C (140F)
M = 75% useable tank capacity
∆T = 5K (9F) Maximum temperature drop through distribution system.
∆Tt = 54K (97F)
Piping system consists of:
9 meters of 50 mm pipe
6 meters of 25 mm pipe
15 meters of 20 mm pipe
A separate tank and hot water boiler will be used.
qpipe = 0.10 kW = 341.18 BTUH
(2) Find:
(a)
Peak domestic hot water demand, L (gal).
(b)
Storage tank size, L (gal).
(c)
Recovery rate required given the tank size selected, Lps (gph).
(d)
Flow rate required for domestic hot water circulation pump.
(e)
Flow rate required for boiler circulation pump.
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