Cold and hot water may be conveyed to plumbing fixtures under the pressure of a water source, such as a public water main, by pumps, or by gravity flow from elevated storage tanks.
The water-distribution system should be so laid out that, at each plumbing fixture requiring both hot and cold water, the pressures at the outlets for both supplies should be nearly equal. This is especially desirable where mixing valves may be installed, to prevent the supply at a higher pressure from forcing its way into the lower-pressure supply when the valves are opened to mix hot and cold water. Pipe sizes and types should be selected to balance loss of pressure head due to friction in the hot and cold-water pipes, despite differences in pipe lengths and sudden large demands for water from either supply.
Care should be taken to assure that domestic water piping is not installed in a location subject to freezing temperatures. When piping is installed in exterior walls in cold climate areas, the piping should be insulated and should be installed on the building side of the building wall insulation. Piping installed in exterior cavity walls or chases may require heat tracing, although the installation of high and low wallmounted grilles, which allow heated air from the building to naturally flow through the cavity, will usually prevent the temperature in the cavity from falling below a temperature where water in the piping will freeze. Designers should thoroughly investigate local climatic conditions and building methods to assure proper installation.
Designers should also specify freeze-proof-type hydrants (hose bibs) for exterior applications.
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 1â„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.