3 April 98
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
q = qpipe + ( R c ρ cp ∆Tt )
qpipe = heat loss in the piping between the boiler and the storage tank, kW (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 + R 233.402