Figure 4-70.One type of automatic air vent.
Figure 4-71.A typical radiator shutoff valve.
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
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
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
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.
up so tight that they score the shaft.
A reducing valve is normally installed in the
cold-water line going to the boiler. It automatically
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
keeps the closed system supplied with water at a
predetermined safe system pressure. These valves are