Pump delivers hot, but turns off on cold

Paul D 2 Posts 2 Karma
Paul D posted this 22 December 2017

I have a Monsoon S2 (1422065-076) feeding two showers. The current situation is that the pump starts, but pumps hot water only. If I switch the shower to cold, the pump stops. Then if I select hot, it runs. I've looked at the filters. They are fine, and there's water in the pump, i.e. it's primed. Nothing's changed with the pipework, so the problem must lie with the pump.

It's possible (and I'm fuzzy on the details) that an extended showering session by my wife may have drained the cold water tank, and consequently run the cold side dry. I am aware that this is a bad thing and can damage the seal. What are the likely consequences? Would this have the effect of actiing like a closed valve?

PumpAssist 228 Posts 159 Karma
PumpAssist posted this 26 December 2017

The pump has a single shaft so both the hot and the cold ends spin at the same time. If you have pressurised hot water coming out but no cold then either the cold end of the pump is airlocked, you have a blockage or both shower valves are faulty.

The most likely scenario is an airlock due to running out of water. This can be cured by turning off the pump, isolating the water on the cold side and then disconnecting the cold outlet pipe and running the water through the pump and into a bucket under gravity. Then when you connect up the pipe again and run the pump, as the housing is now full of water, it can pressurise it.

Paul D 2 Posts 2 Karma
Paul D posted this 28 December 2017

Thank you for this - it seems to have done the trick. I was a bit sceptical at first. When I took the inlet off to check the filter the other day the pump looked full of water; then this morning when I took the outlet off, it again looked full. But it seems that an air pocket can lurk unseen in the body of the pump.


A couple of points for clarification, if you don't mind:

  • The trouble-shooting section of my handbook under probable cause 'air in system', it says Run system on full hot with pump switched off (i.e. gravity only) for several minutes. I'd previously tried this with the hot-side when I'd checked the filter, and it works. But it doesn't seem to work with the cold-side; hence the need to take the outlet connection off. Is there a reason for this? 
  • Just in case of future issues: what are the symptoms of a failed seal? Low flow rate? A puddle in the airing cupboard?
PumpAssist 228 Posts 159 Karma
PumpAssist posted this 02 January 2018

It can be difficult to bleed air out sometimes if you are trying to do it through a shower valve or at the end of a long run.

If you do it at the pump, you will get an unrestricted flow so the water is flowing better and it has more chance of dragging the air out. Usually have more problems with hot water than cold though.

If the seal fails the pump will leak, it won't reduce performance as such.

thomaswhite07@blueyonder.co.uk 1 Posts 1 Karma
thomaswhite07@blueyonder.co.uk posted this 18 January 2018

I had a whole house twin 1.5 bar pump fitted ( bungalow ) a short while ago (6 months ) and all was fine untill now. Pump works fine on all taps both hot and cold but fails to start up when the tap for drinking water is turned on in kitchen.

PumpAssist 228 Posts 159 Karma
PumpAssist posted this 18 January 2018

If the pump works on some outlets then the pump is not faulty.

Usually the drinking water wouldn't be from the pump but would come from the mains. If you are sure that it is from the pump, turn on another hot or cold tap to get the pump running and then open the drinking water tap. The water will be pressurised if it is from the pump. If the pump is making no difference to the flow or pressure then it's not connected to it.

If the pressure is good from the tap once you have the pump running but it won't activate the pump when used on its own, then how much water comes out of the tap under gravity when the pump doesn't start? If the water is just dribbling then the lack of flow is the issue as you have a positive pump, it requires a fairly good flow to activate the flowswitch.