Probably because they are thermostatically controlled and do not draw a constant water pressure through the boiler. The 240 relies on water pressure differential in the diverter valve to activate the water on hot water draw off. Check the shower you have got is a Combi-flow, or is compatable with combi boilers. if not, the diaphragm is up the spout and needs replacing.
craig, plymouth also! well saltash!
May 2006
howtomendit.com for free repair help, information and advice.
