Hot-Water Circulating Methods
If a building circulation loop is employed to maintain circulation of the hot-water supply throughout the building for the purpose of making the hot water more readily available at each fixture, the return line from the circulating loop should be connected to the cold- water supply line of the heater. A swing-check valve must be installed in a horizontal section of the return line at a point as close to its connection to the cold-water supply line as possible to prevent the possibility of backflow of cold water to the hot-water outlets of fixtures that may be connected to the circulation loop at a location that may be closer to the cold-water supply line than to the water heater.
If a check valve were not employed, and the pressure drop in the line between the cold-water supply line and the hot-water outlet were less than the pressure drop in the line between the heater and the hot-water outlet, cold water could flow to the hot-water outlet.
Where a circulating pump is employed to accelerate the circulation in a building hot-water supply loop, it should be installed in the return water line at a point as close to its connection to the cold-water supply line as possible. To eliminate the unnecessary wear and expense incidental to operating a circulating pump continuously, it should be controlled by a direct aquastat installed in the return circulating line at a point conveniently close to the pump.
It is suggested that this aquastat be adjusted to close the pump circuit when the water in the return line drops to (or below) 100° to 110°F and break the pump circuit when the water temperature at that point rises to approximately 120° to 130°F if the desired hot- water temperature at the fixture is approximately 130° to 140°F.
Building and Safety Code Requirements
The building and safety codes of certain states require the use of dip tubes in the hot-water storage tanks of automatic storage water heaters or recovery water heating systems, while other codes prohibit their use.
The apparent purpose of code provisions prohibiting the use of cold-water supply dip tubes in the tanks of underfired storage water heaters is to prevent the possibility of developing dangerously excessive temperatures and pressures in the tank of the gas supply to a manually controlled heater (if not turned off) or the safety control of an automatic water heater that fails to function.
Under such circumstances, the water in the tank would drain to the levels of the holes drilled in the dip tubes close to the top of the dip tube before the siphon action would be broken.
The water remaining in the tank, being practically at zero pressure, would vaporize rapidly if automatic temperature and safety controls failed to function or the gas supply were not shut off. The steam thus generated in the remaining space in the upper portion of the tank would create a personal scalding hazard if communicated to the hot-water supply piping and outlets, or could attain sufficient pressure to rupture the tank or piping.
An equally hazardous condition would be created if the tank of a storage heater not equipped with a cold-water supply dip tube were completely drained.
Under this condition, if the gas supply to a manually controlled heater were not shut off, or the thermostatic and safety controls of an automatic heater failed to function with the gas valve in the ther- mostat in the open position, explosive pressures would be almost instantaneously developed if cold water were introduced into the empty heater tank.
To avoid such hazards, the gas supply to either a manually con- trolled or automatic storage water heater should always be shut off before draining the tank of the heater or the entire hot-water supply system.
Where the use of a cold-water dip tube installed in a tapping in the top and extending to a point close to the bottom of the hot- water storage tank is prohibited, the cold-water supply line should be connected to a tapping as close to the bottom as possible.
If the cold water enters the tank at a point considerably above the bottom, the water below that point will be in a more or less static state, which may be conducive to more rapid deposition of lime or other scale on the tank bottom.
Lime or other scale on the tank bottom retards the transfer of heat to the water if the heater is of the underfired type, which impairs its efficiency.