usually set at the factory, but you may adjust them in the shop to a desired pressure. You should install this valve at approximately the same level as the top of the boiler.

Flow-Control Valves

Forced hot-water circulating systems use the flow-control valve shown in figure 4-73. It is normally installed in the distribution main. This valve prevents gravitational flow of water through the system. The valve does not offer any serious resistance to the flow of water when the circulating pump is in operation. However, when the pump is not operating, the small gravitational head of water cannot open the valve. Each week you should check the flow-control valve for proper operational down-free movement. Examine the valve for water leaks and repair it when necessary.

Special Flow Fittings

Various types of special tees designed to deflect main-line water into the radiator branches are used in one-pipe and two-pipe forced-circulation systems. These fittings are designed and calibrated to the size of the radiator and system-operating temperature. Fittings of this type are required with one-pipe, forced-flow systems, and they do equally well for radiators above and below the distribution mains.

MAINTAINING AND TROUBLESHOOTING HOT-WATER HEATING SYSTEMS

Hot-water heating systems require little maintenance other than periodic checks to make certain that all air is out of the system and all radiators are full of water. The circulating pumps should be oiled regularly according to the manufacturer's instructions, and the pressure-relief valves should be checked periodically.

Figure 4-73. - One type of flow-control valve.

Some of the common discrepancies encountered when troubleshooting hot-water heating systems are contained in appendix II, table O.

Operator maintenance on the electrically driven feed pump consists mostly of cleaning the pump and motor. However, the pump motor is lubricated according to the manufacturer's specifications. Remember that not using enough lubricant can result in the bearings running dry or seizing on the motor shaft. But, too much lubricant causes the motor to become dirty, and it can result in the motor windings becoming saturated with oil and burning out.

When a water leak develops around the pump shaft, tighten the packing-gland nuts or repack the stuffing box as necessary. The strainer, installed between the pump and the condensate receiver, should be kept clean to avoid any restriction of the flow of water to the pump.

The maintenance of feed-water heaters and economizers normally includes removing solid matter that accumulates in the unit; stopping steam and water leaks; and repairing inoperative traps, floats, valves, pumps, and other such associated equipment.

Q27. What is the main reason to install a two-pipe, open tank, gravity distribution system over a one-pipe, open-tank, gravity distribution system?

Q28. The operations of what component of the one-pipe, closed-tank distribution system results in higher heat emission from the radiators?

Q29. Air vents release trapped air in the system. If the air is not released, how would this affect the system?

HIGH-TEMPERATURE HOT-WATER SYSTEMS

Learning Objective: Recognize different types of high-temperature hot-water systems, their components, and understand their application and installation.

High-temperature hot-water (HTHW) systems operate at high pressure to maintain water temperature that exceeds the normal boiling temperature of 212F (at atmospheric pressure) used in other types of heating systems.

High-temperature hot-water systems consist of standard and heavy-duty equipment, including boilers (sometimes referred to as generators), expansion drums, system circulator pumps, distribution piping, and heat-consuming equipment.