radiators and the relatively cooler, heavier column of water returning from the radiators. The second type is the forced-circulation system in which water is circulated by a power-driven pump.

GRAVITY SYSTEMS

The distribution systems and piping for hot-water heating systems and for domestic hot-water supply systems are simpler in design than those for steam because there are no traps, drips, or reducing valves. Several items, such as supports, insulation, and some valves and fittings, are the same for steam and hot-water distribution.

Gravity hot-water distribution systems operate because of the gravitational pull on the heavier cool water, which sinks as the heated water becomes lighter and rises. At this point, some of the types of gravity systems that are currently used are discussed.

One-Pipe, Open-Tank System

The one-pipe, open-tank gravity distribution system shown in figure 4-63 consists of a single distribution pipe that carries the hot water to all of the convectors or radiators and returns it to the boiler. This system is easy to install and moderate in cost.

The water that flows into the radiators at the end of the system has a lower temperature than the water entering the first radiators. A system of this type should be designed so the water reaching the last convector is not too much cooler than the water reaching the first convector. Because of this progressive temperature drop in the distribution system, convector radiators should be installed at the end of the system to equalize the amount of heat radiation per radiator. It is difficult to get enough circulation by gravity to give the system small convector temperature drops; consequently, we do not recommend the one-pipe, open-tank gravity system.

Two-Pipe, Open-Tank System

Many hot-water gravity distribution systems are two-pipe, open-tank systems, as shown in figure 4-64. This heating system is constructed with separate water mains for supplying hot water and returning cold water. The radiators are connected in parallel between the the two mains. In the two-pipe, open-tank gravity system, the distributing supply mains are either in the basement with upfeed to the radiators or in the attic. When the system is in the attic, it has overhead downfeed supply risers. The return mains are in the basement. Return connections for the two-pipe system

Figure 4-63. - A one-pipe, open-tank gravity hot-water distribution system.

Figure 4-64.vA two-pipe, open-tank gravity hot-water distribution system.

are usually made into a gravity return, which pitches downward to the return opening in the heating boiler. The water temperature is practically the same in all radiators, except for the allowance to be made for temperature drop in the distribution supply mains occurring between the boiler and the end of the circuit. Water temperatures are the lowest at the end of the circuit. The amount of temperature drop between the beginning and the end of the line depends upon the length of the main and upon the heating load.