Mains pressure is low, which isnt that common, or blockage either in feed pipework or shower filter itself, some showers have a built on strain filter, if this is clogged with debree, it will reduce flow to shower causing a low pressure fault, the heat exchanger on the shower may be furred up, which means a new shower unit or shower head is blocked with calcium, remove head and turn on shower, if okay clean head, if not, then one of other problems i have mntioned. jon
jon wells
August 2009