We are frequently asked by customers why, in relation to circulating domestic hot water systems, we show the circulator on the hot water flow rather than on the return on all our typical application schematics.
We always show the circulator in a by-pass arrangement on the flow, with a check valve to prevent simple recirculation around the bypass loop. That enables a circulator of any size to be put in the pipework, and does not restrict the circulator size to that of the pipework diameter. Thus it is perfectly acceptable to put a DN25 circulator on DN50 pipework if that is required. More importantly, it ensures that the circulator always has water flowing through it. During periods of no draw-off, the circulator works and moves the water around the circuit. During periods of heavy draw-off, the circulator “idles” but still has water flowing through it; the main flow bypasses the circulator and flows through the check valve.
Now let us consider the circulator on the return line, just after the last draw-off. During periods of draw-off, the circulator is starved of water by virtue of the water being drawn off upstream of the circulator. Most customer installations that feature a circulator on the return line show it in-line, restricting the circulator size to that of the return pipework diameter. Locating it in the return line can lead to the circulator cavitating, where it can become noisy and introduce vapour pockets into the system, generating noise and encouraging airlocks. The starved circulator can also overheat, un-cooled by the required throughput of water.
Operators of Horne installations can verify the starvation principal described above by observing the ball flow indicator on the return line. During periods of substantial draw-off, the ball in the ball flow indicator drops, indicating that the recirculation flow reduces. Also, on gravity-fed installations, it is not unknown for a circulator on the return line to draw air into the system from an open outlet during periods of heavy draw-off. This is often accompanied by a complaint of low or intermittent flow at the outlet. Usually, switching off the circulator proves the source of the problem.
So in summary, a correctly sized circulator will always work when located in the flow, but it may not work and may cause problems when located in the return line.
A schematic for each scenario with associated pressure calculations further explains the potential problems with siting the circulator on the return.
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