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The shaft of a hot water circulating pump (Wilo Star Z25/2) rotates freely when the pump is not under water pressure (i.e. when the isolation valves on the suction and discharge sides are closed). As soon as water pressure is applied, the pump's motor will not rotate any more. Rotating the shaft through the plug with a screwdriver also become more difficult. Why can this happen? Can this problem be fixed?

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I think your pump head isn't enough to win both circuit head and circuit pressure. If the pump was designed to operate at this kind of load, then probably it has a burnt coil.

  • Thank you! The pump is rotating under power when not assembled in the housing, so the coil should be OK. It's also designed for the corresponding load (sanitary hot water circulation). – Diomidis Spinellis May 13 '18 at 10:26
  • Is there an electrical test to prove your theory? Or could it be a bad bearing or shaft seal? – Tyson May 13 '18 at 11:15
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    A motor usually has at least 2 coils (1 main and 1 for start up in case of 120V, two main if 220V), in your case one may be dead. You can try with a multimetre the coils' resistance, if one is extremely low or short-circuited coil has gone. – DDS May 13 '18 at 11:40
  • Based on the comment by @DDS regarding the pump's head, I examined the downstream flow. I found that the discharge side, which is connected to the recirculation input of a vertical water heater, is blocked solid at the point where the pipe reaches the heater. Could the blockage be a stuck non-return valve? The heater's manual does not mention the existence of such a valve. – Diomidis Spinellis May 14 '18 at 15:29

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