Twin Outlet Shower Pump - Problem Only on One Side? Or Blockage Elsewhere?

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Having problems (possibly) with a Stuart Turner dual shower pump (old Aura model, looks like the current Showermate) - it supplies a single shower and also (I know it probably shouldn’t) the hot water to three mixer taps. The cold feed to all three taps is from the mains supply and they all work.

The shower is fed from a CWST and the hot water cylinder - the hot side works fine (if you like 65 degree showers), but there is virtually no flow at all when set to cold (pump powered on or off), and the lack of flow means the pump either pulses or does not stay on at all when trying to turn down the temperature on the shower thermostat. Shower head is clean and descaled, no difference in flow with it on or off.

The pipe from CWST to pump is clear, and I can blow through the cold outlet pipe from pump to shower when the shower is turned on. I can also blow through the cold side pump inlet to outlet with seemingly little restriction with both pipes off.

Is it possible that one pump impeller can have failed, or would both sides be affected if one was blocked or seized? There is some corrosion to the cold side of the pump, possibly because one of the end cap screws was loose.
 

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Suggest removing pump cold pipe outlet and fit a a gate or ball valve on the end, ensure one of the mixer taps is open and using some hot water, start the pump and gradually open isol valve and see what kind of flowrates, you should get a fairly high flowrate even with valve well throttled in.
 
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Thanks. There‘s virtually no cold flow from the CWST with pump either on or off, which is why I thought it was either a cold-side pump blockage or possibly cold-side pump outlet to shower blockage (not sure if I can reverse-flush that pipe via the shower hose without damaging anything in the shower unit).

What would flow rate with a tap running a mix of mains cold/tank hot diagnose?
 
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Only suggested running a mix of hot/cold to avoid damage to hot side pump as both sides should be flowing.

Have you checked the cold side pump filter?, you can't reverse flush due to non return valve, might also worth checking this out for damage/seizure, if you are getting a flow through the cold water hose when removed at the pump inlet then problem points to either filter or NRV?, you may get a poor gravity flow as the NRV even if OK still requires maybe 0.5bar to open it. Impeller failure is the other reason for non performance.
 
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Unfortunately there aren’t any in-line filters in the pipe from either tank, and it doesn’t look like the pump has any. I’m wondering if the issue may be in the plastic pipe to the shower post-pump, as hot flow with the pump off is adequate. With the cold hoses removed, there’s no obvious foreign material in the inlet or outlet side - I’ll have a look for a big enough screwdriver to remover the end cover plate tomorrow.
 
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These pumps don't have a bypass therefore if one side is running against a closed head (hot running and the cold isn't) it is eventually going to damage itself. Those bends in the outlet flexi's wouldn't help either. Is that water staining on the cold side of the pump?

There usually would have been filters in the flexi's at the connectors to the pump, time to strip it down and see methinks and start to budget for a new pump.
 
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Thanks - yes, one of the end cap cover screws was loose and it’s been leaking, albeit very slightly.

Am I likely to cause a problem back-flushing the attached thermostatic mixer shower unit with mains pressure?
 
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These pumps don't have a bypass therefore if one side is running against a closed head (hot running and the cold isn't) it is eventually going to damage itself. Those bends in the outlet flexi's wouldn't help either. Is that water staining on the cold side of the pump?

There usually would have been filters in the flexi's at the connectors to the pump, time to strip it down and see methinks and start to budget for a new pump.
What does "regenerative" refer to in a Stuart Turner Universal Twin??. I know universal refers to it being both a positive & negative head pump.
 
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New pump fitted, some improvement (especially in noise levels), but the problem was a completely clogged shower valve. Will re-plumb the outlet side to sort the flexi angles another day.
 
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Glad you are sorted, post #2 test would have shown no pump problem but you have got a good run from it and you now have a spare.
Running constantly against a closed head on one side generally results in a pump seal failure as the water gets very hot.
 
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Limescale covering the cold entry gauze, hot with bits of (I think) wasp bodies possibly via the previously-partially uncovered CWST.
 

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