Hi, I have a conventional Worcester Bosch condensing Boiler on ground floor- no problems. Hot water tank (with tank stat) in bathroom where the pump and 3 port valve are situated (Danfoss Hsa3) I have a Danfoss fp715 programmer which I have set to come on early morning (water and heating) and evening (water and heating). All good, room stat and tank stat working fine and switch on and off as desired temp is reached. In summer I just turn down the room stat and that works fine, no heating just hot water, perfect. My problem is that occasionally I want heating only, I hit the '+1hr' button on the controller for heating only, the boiler kicks into action for about 4 seconds then shuts down ! If I hit both +1hr buttons I get heating and hot water so in every other aspect my system is running like a dream, just cant have heating on its own. Any ideas.