Problems with shower pump

Hi,

I recently had a plumber fit a pump in the hotpress which was supposed to be isolated just for the en-suite mixer shower. It turned out afterwards that the pump comes on for all hot water taps in the house (including dishwasher!) but 'pulses' when the mixer shower is used.
The plumber has said over the phone that the problem is with the mixer shower & not the pump. Has anyone else come across this before? basically as the hot water is turned on in the en-suite shower, the pump starts cutting in & out & the water pulses from the shower head.
This is the only instance in the house where both hot & cold are pumped so I'm wondering if it's the pump or a problem with the mixer as the plumber has said. The pump is fine for the cold water but starts pulsing when the hot water is added.

Any experience/knowledge??

Thanks!

I can't think of anything that the mixer could be doing to cause this.
Sounds like the pump could be 'hunting' which could be down to the way it has been connected to the hot pipe.
Have a read of this (scroll down) and a look at this which may help.
You may have to have a good look at your cylinder to establish what way the pump has been connected.

Thanks for that Hoagy, I'll have a look through those links.