UFC 3-420-01
25 October 2004
Heat loss in the piping system (q) can be calculated using Table 2 in
Chapter 45 of the ASHRAE Handbook - Applications 1995. 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 fixture, require the supply water
temperature leaving the mixing valve to be above 43C (110F) and revise
the storage tank and recovery capacities accordingly.
c. Systems using a separate hot water generation unit and storage tank:
The following information applies to systems using a separate storage tank
and forced circulation type water heater, boiler or heat exchanger to generate
and store domestic hot water. A forced circulation type water heater is similar
to a boiler in that it is designed to heat domestic water as it passes through a
series of coils rather than heating water in a storage tank but is designed for
generating domestic hot water only.
(1) Locate the storage tank and hot water generation unit in the same
mechanical room whenever possible. This keeps the head requirements at a
minimum for the pump circulating water between the hot water generation
unit and the storage tank.
(2) Require the domestic water supply be connected in the line supplying
hot water to the storage tank. This allows the cold water to mix with the
warmer water in the storage tank before entering the boiler, minimizing
problems associated with condensation and thermal stress and improving
overall system efficiency. Require a submittal from the manufacturer
addressing whether a thermostatically controlled bypass line between the
boiler supply and return lines or other means are needed to preclude the
possibility of thermal shock to the boiler.
(3) If a forced circulation type water heater or boiler is used, equation (3)
again can be used to size the circulation pump between the heater and the
storage tank. In this case limit the temperature differential to no greater than
16K (30F) to minimize problems with condensation and thermal stress and
improve overall system efficiency. Also note that the sizing of the circulation
pump must account for the heating of the domestic cold water being
provided. Therefore, the value of q required to use equation (3) is found with
equation (5):
q = qpipe + (R c ρ cp ∆Tt)
(5)
where:
qpipe = heat loss in the piping between the boiler and the storage tank, kW
E-5
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