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A one-pipe system is one in which a single main pipe is used to carry the hot water throughout a water heating system. In other words, the same pipe that carries the hot water to the heat emitting units (i.e. the radiators, convectors, etc.) in the various rooms and spaces within the structure also returns the cooler water to the boiler for reheating. Each heat emitting unit is connected to the supply main by two separate branch pipes (a separate feed and return line).

The hot water flows from the boiler or heat exchanger (if a steam boiler is the heat source) to the first heat emitting unit, through it to the second unit, and so on through each of the heat emitting units in the one-pipe system until it exits the last one and returns to the boiler or heat exchanger.

One-pipe systems may be operated on either forced or gravity circulation. Special care must be taken to design the system for the temperature drop found in the heat emitting units farthest from the boiler. This is particularly true of one-pipe systems designed for gravity circulation.

A principle advantage of a one-pipe system is that one or more heat emitting units can be shut off without interfering with the flow of water to other units. This is not true of series-loop systems in which the units are connected in series and form a part of the supply line.

In some large one-pipe systems, zoning is possible by providing for more than one piping circuit from the boiler. In such cases, each piping circuit is equipped with its own thermostat and circulating pump. Sometimes these circuits are erroneously referred to as “loops” which results in confusing them with the piping arrangement in a series-loop system.

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