Cold showers unless "switch" is used

Hi there,
Hope you can help with the following problem.
Central heating works fine but the shower runs cold unles the "switch" is turned on.
Also have low hot water pressure pressure when more than one thing is turned on.

Any advice greatly appreciated.

petew

Hi Pete,

Probably sensor failure, but could be a scaled up heat exchanger if flow is poor.

I'd need to see it to say which though.

Cheers, Mike

Hi Mike,
I've replaced the pump speed sensor and the plate heat exchanger and everything now working fine.

Many thanks.