In a series-loop system, the heat emitting units form a part of the piping circuit (i.e. loop) which carries the hot water from the boiler around the rooms and spaces within the structure, and back to the boiler again for reheating. In other words, there are no branch pipes connecting the main supply pipe to the heat emitting units as in the one-pipe system. In the series-loop system, the hot water flows from the boiler through a length of the main supply pipe to the first heat emitting unit in the circuit (loop). It then flows through the unit to a length of the main supply pipe connected at the opposite end; flows through this pipe to the second heat emitting unit in the circuit, and so until then entire circuit is completed.

The series loop system is cheaper and easier to install than other piping arrangements, because it eliminates the need for branch pipes and reduces the amount of pipe used in the main circuit to the comparatively short lengths connecting the heat emitting units. There is no need for one continuous length of main pipe.

Because the heat emitting units are connected in series and constitute a part of the main supply line, the same hot water supply passes through each unit in succession. As a result, the heat emitting unit closet to the boiler receives the hottest water, whereas those farther away receive water several degrees cooler. Furthermore, individual units cannot be shut off (unless there is a special bypass piping arrangement) without obstructing the flow of water to units further along the line.

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