Shower runs cold when kitchen sink hot is used

[Johnyc86]

Came across a tricky job last Friday.gravity fed vented hot water cylinder that is pumped via twin impeller pump.also cold from tank is pumped with same pump.pump is upstairs.feed for bar mixer shower pipes go up into loft and drop down into the shower area.cylinder up stairs.would shower run cold when hot is used downstairs as it would take path of less restriction and reduce flow rate of hot to shower??or is the bar mixer shower the fault??hope this makes sense.

 

[Ric2013]

Obviously if you allow water to flow through one outlet there is likely to be less pressure and therefore a reduced flow to the other outlet. Depends on the head of pressure developed by the pump at the flow rates required and the resistance in the pipework as to how serious the effect will be.

Is the pump designed to be installed as a 'whole house' pump, or has someone fitted a (cheaper) pump designed to be installed as a dedicated shower pump and tried to feed the whole house with it anyway?