Figure 4-70. - One type of automatic air vent.

Figure 4-71. - A typical radiator shutoff valve.

Unit Heaters

Unit heaters are the same as those used in warm-air heating, except hot water is used vice coils for the heating medium. The heater consists of a heating coil supplied with hot water. The coil is usually of the finned type, and an electric fan circulates air over it. A unit heater installed in a distribution main is shown in figure 4-72.

Servicing unit heaters includes a monthly inspection. Each month, check for water leaks, cleanliness of the finned coils, and the operation of the

Figure 4-72. - A typical hot-water unit heater installation.

fan motor. Other accessories which you also should inspect are traps, air vents, fan blades, and valves. Make any needed repairs. Lubricate the electric fan monthly.

Circulating Pumps

A forced hot-water heating system has a water-circulating pump in the return line near the boiler. This pump ensures the positive flow of water regardless of the height of the system or the drop in the water temperature. Greater velocities of water flow are obtainable with forced circulation than with gravity circulation.

Circulating pumps are free of valves and float control elements. They are operated under a sufficiently high water inlet temperature to eliminate the difficulties caused by vapor binding. The pumps are usually operated by electric motors.

During maintenance servicing, check the pump carefully for proper rotation, and lubricate the electric motor and pump according to the manufacturer's instructions. Also, periodically clean the pump of sand, rust, and other foreign matter that has collected in the pump casing. Be sure the pump rotates freely and the shaft packing glands, if there are any, are not drawn up so tight that they score the shaft.

Reducing Valves

A reducing valve is normally installed in the cold-water line going to the boiler. It automatically keeps the closed system supplied with water at a predetermined safe system pressure. These valves are