Hot Water Distribution
Hot water distribution, known as hydronic systems, use hot water to carry heat from a boiler where it is heated to roughly 200°F, through pipes to the rooms where it is needed. An advantage of hydronic systems is that there are no drafts or chilling effects from air moving over the body, as are characteristic with forced air systems. Another advantage is controllability. By using zoned valves or circulating pumps, different temperatures can be maintained in each area of the home, thereby improving comfort.
Hydronic systems distribute water by either forced circulation or by gravity. The two systems are easy to distinguish by following the flow of hot water from the boiler through the distribution system. In a gravity system, the pipe will slop downward toward the boiler. In a forced circulation system, a circulating pump forces water into the boiler and then through the distribution system. Also, pipes used in forced systems are smaller than those used in gravity systems.
In a one-pipe hot water system, both the supply pipe and the return pipe of each radiator are connected to a single main line running from the boiler. As the water flows in a one-pipe system from radiator to radiator down the line away from the boiler, the temperature of the water supplied is successively lower.
The size of the radiators must therefore be increased successively, in order to produce a balanced system. This is not necessary in two-pipe systems where the supply pipe and return pipe are separate.
Because they both use pipes to distribute heat, hot water and steam systems may be confused. You can distinguish between them by looking for the air relief valve and traps on the radiator, which will be present in steam, but not in a hot water distribution system.
Hot water systems need some way of adjusting to the expansion of the water when it is heated, and of protecting the system if steam is accidentally produced. An expansion tank partially filled with water is installed in the system to meet this need.