my boiler will only run with heating and hot water ?

sounds to me like your three port valve is seized or motor heads burned out thats why it will only do two together cos its stuck in mid position. valves the one in your cylinder cuboard RIGHT NEXT TO CYLINDER with three pipes coming off it saying A. B . A/B valve has a lever on back try moving lever with boiler programmer of etc.. youll soon know

tim the boiler man
November 2009