There are three basic types of distribution piping arrangements for forced-hot-water systems: series loop, one pipe, and two pipe. The specific distribution system installed usually depends on the size and cost of the house.
This is the simplest and least expensive piping arrangement to install. It is usually used in smaller homes where room-byroom radiator adjustments are not needed for balancing the heat distribution. In a series loop, the radiators, usually baseboard convectors, are an integral part of the supply piping. (See FIG. 14-9.) If a radiator is shut off, the flow throughout the system is stopped. In this arrangement, the temperature of the water entering the last radiator will be considerably less than it was when it entered the first radiator. To minimize the difference in temperature and produce a more uniform heat distribution, the heating system for larger homes is often designed so that the house is divided into two or more heating zones. Each zone has a separate piping configuration and either has a separate circulating pump or shares a common circulating pump with the other zones, but has a separate thermostatically controlled valve in the main supply pipe.
In a one-pipe distribution configuration, as with the series loop, a single pipe makes a complete circuit from the boiler and back again, serving as both the supply and the return. In this case, however, rather than the radiators being integral with the supply pipe, they are attached to it by two risers, one connected to each end of the radiator. (See FIG. 1410.) Each radiator will also have a shutoff valve located at the inlet riser. Each radiator can be shut without affecting the water flow in the supply main. Consequently, this system can provide room-by-room heat control. However, as with the series loop, there is a considerable temperature difference between the water entering the first and last radiators. To compensate for the cooler water entering the radiators downstream, larger radiators are often installed. They emit heat comparable to the smaller ones closer to the boiler.
The two-pipe distribution configuration is the most costly to install, but it overcomes the deficiencies of the other configurations. There are two main pipes, one for the supply and the other for the return. The inlet and outlet ports of the radiators are attached to the mains by risers. (See FIG. 14-11.) The cool water leaving the radiator does not mix with the hot water flowing through the supply main, so the temperature difference between the water entering the first and last radiator is small.
Even though both the one- and two-pipe configurations will provide heat control for individual rooms if the radiator shutoff valve is manually closed or partially closed, the configurations are also often used in homes with zone heating. Zone heating is automatic. All that has to be done is to set the thermostat to the desired temperature for that portion of the house.