Shower pump issue

[Welshie947]

Am after some advice regarding an existing shower pump that worked ok before fitting new shower valve to an on suite.
When turn shower on no water flow, as soon as I touch the hot water pipes to pump the flow starts straight away, then is ok if the shower turned off and on again within an hour or so, all works fine, if left overnight, need to wiggle pipes again for flow . Only difference is new shower mixer valve is approx 300mm higher than before and shower head higher. Pump height has not changed relative to hot water tank. Pump at waist height.top of tank head height , approx 6ft.
Help would be much appreciated.

 

[stem]

You don't say what sort of pump you have, so I'm assuming that it's the most common type needing a positive head and having a dual impeller for hot and cold supplies. If so, here are some possible scenarios.

In order for the pump to start there must be some water flow by gravity when the shower valve is opened. This gravity water flow operates the electrical switch(es) that then starts the pump motor running. It may be that by either because of raising the shower head, or an increased resistance with the new mixer, that there is not quite enough pressure from the stored water to initiate flow, so the switch doesn't operate. Check the pump manual to ensure that your pipe installation is correct, especially regarding the height of the shower head in relation to the tank, pipe diameter, and the type of manual isolation valves installed.

Try putting the shower head in the bath / shower tray before switching it on, what happens then?

Have you changed anything else at the same time? for example, have you installed small bore isolation valves whilst modifying the pipework, these may restrict the flow and stop the pump operating. Most pump makers specify that isolation valves should be a full bore type, so as not to impose a restriction to the flow.

Another possibility, if you have changed a 'manual mixer' for a 'thermostatic mixer', could be that because of sensing a low temperature, when operating from cold, it may only start the flow of hot water. Sometimes because of the additional pipework and cylinder in the hot system, the hot water flow may be less than in the cold supply which is direct from the tank, and not operate the switch. If you did previously have a 'manual mixer' maybe the switch in the cold flow was the one triggering the pump. This could explain why it now works when the shower is still warm as the cold valve may be opened from the start.

Try starting the shower on the lowest (cold) setting, so that it starts the cold water flow. What happens then?