National Oilheat Research Alliance 17 Figure 5 In the two pipe direct return systems, one pipe carries water from the boiler to the heat emitters and another pipe brings water back to the boiler. Heat emitters on these systems are connected to the supply and return piping like the rungs on a ladder. As you can see in Figure 6 below, the heat emitter closest to the boiler has the shortest run of piping while the heat emitter farthest away from the boiler has the longest piping run. In this example, the water must flow a total of: 38' to travel from the boiler, through the first heat emitter and back to the boiler. 58' to travel from the boiler, through the second heat emitter and back to the boiler. 78' to travel from the boiler, through the third heat emitter and back to the boiler. Less water will flow through the heat emitters that are further away from the boiler and they will generate less heat. On these systems, balancing valves must be installed on each heat emitter’s piping to increase the resistance of the shorter piping circuits to even out the flow of water through each circuit. The terminal units in a two pipe reverse return system are piped in “parallel” to separate supply and return pipes. For proper operation, these systems rely on a “first supplied, last returned” piping arrangement. Figure 7, on following page, illustrates that the supply connection of the first terminal unit is the closest one to the boiler and the return connection is the furthest one from the boiler. The first unit supplied Figure 6
NORA Advanced Hydronics
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