Instantaneous Water Heaters are not extensively used for heating large quantities of water, nor for heating water to a high temperature. They are used chiefly for heating water for bathing or other like purposes, and are placed on a shelf near the fixture to be supplied, so the heated water can discharge into it by gravity.

In the type of heaters shown in Fig. 127, cold water enters the coils through the valve, a, and the flow is regulated by the lever, b. The coil, c, grades from the point where the water enters the coil to the discharge tube, d. This causes most of the heat from the burning gas to be absorbed by the water, because the cold water in the top coil is of lower temperature than the hot gases passing over them. If the water enter the coils near the bottom and discharge from the top, the temperature of the water in the top coils would be as hot or hotter than the gases passing over them and therefore could absorb no heat from the gas, hence some heat would be lost. Gas enters through the valve, e, to the burners, f, which are lighted by the pilot, g. Both gas and water are regulated by the lever, b, which controls both valves, so that gas cannot be turned on and lighted without the coils being filled with water. The illustration shows the heater burning ordinary illuminating gas. The better practice, however, is to have an air mixer and burn a blue flame, which gives a hotter fire with a less consumption of fuel.

Fig. 127

Another type of instantaneous water heater, Fig. 128, consists of a sheet metal cylinder in which water passes over the surface of metal plates heated by gas. The water is released from the supply pipe near the top of the heater, falls in a thin sheet and spreads over the entire heating surface to near the bottom of the heater, whence it discharges through a tube into the receptacle where the water is to be used. The gas valve, a, and water valve, b, are so connected that gas cannot be turned on without also turning on the water, thus preventing overheating or burning any of the parts. The amount of water heated per minute is controlled by a water regulator. The gas is ignited by the pilot, c. Gas enters the burners, d, d, from the burner ring, e. f is a drip ring to catch the condensation that gathers on the inside of the heating tube, i.

Either of the above types of water heaters can be changed to a gasoline burner by changing the gas burners for gasoline burners and supplying a storage tank to hold the gasoline.

Instantaneous heaters should be provided with a drip pan or safe to catch the water of condensation, inseparable from instantaneous heaters of all types. A waste pipe to carry off the drip should lead from the waste or drip pan to a trapped and water-supplied sink. It should never discharge into the bath tub, as the water would stain the tub. When installing a heater, the water supply should be connected to a water heater first so the plates or tubes cannot be burned by lighting the gas to test the burners. A stop-cock or valve should be placed on each of the supply pipes to regulate the flow of gas and water to the heater, or cut the supply off if necessary for repairs.

Fig. 128

A separate gas service of sufficient size to furnish an adequate supply of gas, should be run from the gas meter to the heater. When the gas supply is ample and the pressure strong, a one-half inch pipe will be large enough if the distance between the meter and heater is not too great. With a scant supply or weak pressure, the gas pipe should be at least 3/4-inch in diameter. A meter of less capacity than 10 lights should never be used when a water heater using gas for fuel is connected to the supply.

The top of an instantaneous heater should be connected with a vent pipe leading to a flue or the outer atmosphere to carry off the products of combustion. They should never be allowed to discharge into the room where the heater is located. The capacity of instantaneous water heaters range from 1 1/2 to 9 gallons of water per minute, heated from a temperature of 60 to 110 degrees Fahr. They are seldom used to heat water when a greater temperature than 130 degrees is required.