A single pipe is used to carry the heated water from the boiler to radiator to radiator. Normally, the single pipe runs above the floor and the radiators are fitted standing on it with pipe connections are at the top (flow) end and at the opposite bottom (return) end of a radiator.
The main disadvantage of this system is that each radiator receives water that
has been mixed with the cooled water from previous radiator. This causes the
water temperature drop towards the end of the circuit. In the diagram
A will be
at boiler temperature 80șC, after losing heat on the first radiator
B will be at
70șC, this water will mix with that flows in the pipe causing
C less than 80șC.
Say 76șC. At D the second radiator add some cooled water into the pipe work.
This will cause E at even low temperature than 70șC, may be about 68șC. This
water will be used by the third radiator. Notice that the radiators have to be
large as we progress from the boiler to get the same output as the previous one.
The radiators at the end will be at lower temperature than the radiators
near the boiler. To obtain reasonable heat output, the sizes of the radiators
have to be larger at the end. The main single pipe carries the full load, its
size is determined at design stage, varying or adding extra radiators at later
stage can over load the system. Nowadays this system is not recommended by
boiler manufactures, seldom installed in houses.
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