ETL 1110-3-489
3 April 98
(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 Standard Plumbing Code, Chapter
45 of the 1995 ASHRAE Applications Handbook and Chapter 33 of the 1997 ASHRAE
Fundamentals Handbook provide an ample set of resources on this topic. Therefore, this
letter will not discuss this process.
(5) Domestic Hot Water Circulation Pump: After the domestic hot water distribution
piping has been sized, the domestic hot water circulation pump can be sized. This pump is
used to circulate the domestic hot water through the distribution piping system. Size the
pump using equation (3) below:
q
Qp =
(3)
c ρ cp ∆T
where:
Qp = pump capacity, Lps (gpm)
q = heat loss in the piping system, kW (BTUH)
kW ⋅ sec
( 60 min )
c = constant, 1
kJ
hr
kg
( 8.33 lb )
ρ = density of water, 0.9971
gal
L
cp = specific heat of water, 4.18 kJ ( 1 BTU )
kg ⋅K
lb ⋅F
∆T = allowable temperature drop through the system, K (F)
Equation (3) can be simplified to :
q
Qp =
(4 - SI)
kW ⋅ sec
∆T
4.1679
L⋅K
q
Qp =
(4-IP)
500 BTUH ∆T
gpm ⋅ F
Heat loss in the piping system (q) can be calculated using Table 2 in Chapter 45 of the
1995 ASHRAE Applications Handbook. However, a common rule of thumb is
0.032 kW/m (30 BTUH/ft.).
The allowable temperature drop through the piping system (∆T) is usually
2 to 5K (5 to 10F). It is recommended to use 2K (5F) to assure that sufficient hot water
is provided for all occupants under peak conditions. If the temperature drop through the
piping system will result in a temperature less than 40C (105F) at the most remote
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