Building Design and Construction Handbook - Merritt - Ventech!

14.8 SECTION FOURTEEN
14.6.1 Temperature Maintenance in Hot-Water Distribution
In large, central, hot-water distribution systems, many fixtures that require hot water
are not located very close to the water-heating equipment. If some means of maintaining
the temperature of the hot water in the piping is not provided, the water
temperature will fall, particularly during periods of low demand. The supply to
remote fixtures would have to run for a long period before hot water would be
available at the outlet thereby wasting precious water. For this reason, designers
should provide a temperature maintenance system whenever a fixture requiring hot
water is over 25 ft away from the source of hot water.
One method of temperature maintenance is to use a hot-water recirculating system,
which consists of a hot-water return piping system, a circulating pump, and a
water-temperature controller to operate the pump. The return piping system starts
at the end of each remote branch main and runs back to the water-heater coldwater-supply
pipe connection. The circulating pump circulates hot water through
the supply piping, return piping, and the water heater whenever the controller senses
that the water temperature has fallen below a preselected set point. To reduce heat
loss, all hot-water supply and return piping should be insulated.
Another method employs self-regulating, electric heat tracing that is applied
directly to the hot-water supply piping prior to the installation of the piping insulation.
The self-regulating heat tracing is made of polymers, which have variable
electric resistances, depending on the surface temperature of the pipe. As the surface
temperature of the pipe falls, the resistance increases and more heat is given off by
the heat tracing. The opposite is true if the surface of the piping is hot. This type
of system requires less maintenance once it is installed and less energy to maintain
the hot-water temperature in the piping.
Horizontal pipe runs should not be truly horizontal. They should have a minimum
slope of about 1 ⁄4 in/ft toward the nearest drain valve when possible. An
adequate number of drain valves should be provided to drain the domestic water
system completely.
14.6.2 Up-Feed Water Distribution
To prevent rapid wear of valves, such as faucets, water should only be supplied to
building distribution systems at pressures not more than about 80 psi. This pressure
is large enough to raise water from 8 to 10 stories upward and still retain desired
pressures at plumbing fixtures (Table 14.1). Hence, in low buildings, cold water
can be distributed by the up-feed method (Fig. 14.1), in which at each story plumbing
fixtures are served by branch pipes connected to risers that carry water upward
under pressure from the water source.
In Fig. 14.1, cold water is distributed under pressure from a public water main.
The hot-water distribution is by a discontinuous system. Hot water rises from the
water heater in the basement to the upper levels under pressure from the cold-water
supply to the water heater.
When an up-feed distribution system is desired, but the city water pressure is
not sufficient to provide adequate water pressure, the water pressure may be boosted
to desired levels by the installation of a packaged, domestic water-booster pump
system. This equipment usually consists of a factory-built system with multiple
pumps, a pressure tank, and all operating controls to maintain the required water
pressure. This type of system may also be used in buildings in excess of 10 stories
by proper zoning and the use of pressure-reducing valves at each zone.