Its to do with a sticking Diverter Valve, if this valve gets stuck, the hot water from the main heat exchanger cannot flow through tyhe DHW heat exchanger and the taps will run cold after a while, and instead the central heating radiators will start becoming warm, as the heated water in the main heat exchanger is flowing straight across to the radiator circuit. Most diverter valves can be repaired simply by cleaning them of residual deposits and then reassembeled using silicon grease, some diverter valves operate by a differential pressure and requires a rubber diaphram that can be obtained from most boiler spares by providing correct boiler details and model number and replacing the worn or torn diaphram with a new one. its definitely not a rocket science. Just watch out for dodgy Corgi registerd plumbers. Saw one on telly the other day using a stolen ID from a corgi registerd plumber, and got himself electrocuted!!
Mike, December 2008
