We've got a Halstead boiler that feeds both hot water & central heating system, fully pumped with a 3 way valve. A couple of months ago had problems with lack of hot water & have since replaced programmer with a Horstmann Centaurplus C27 & cylinder stat with a Grasslin clamp-on stat. Everything was fine til this evening when I noticed it was getting really hot in the house. Appears that the boiler is not shutting down when the room gets to set temp (using a room thermostat). When I switched both hot water and CH controls to "off" position, boiler and pump continued to run. Only way to stop them was to turn off main switch. Is this likely to be a room stat, valve or boiler problem?!!