hot water fault?

I assume you have an instant water heater
Chances are the drop in water pressure from the shower alone is not enough to trigger the boiler
Try dismantling the rose part of the shower head so you have a greater flow (more like a normal tap) and if boiler fires, get a complete new shower head. Water saving heads often restrict flow more and more with age and will stop boilers firing.
Otherwise, you may have a problem with general cold water pressure and depending if you are on tank or mains solution is different....
I'd start with replacing shower head

ricardo
September 2010