HW only when CH is on

Hi,
I have a Baxi Bermuda back boiler which is gas fired. I can noly seem to get HW when the CH is also on. Setting HW on it's own seems to fire the boiler but the water in the tank never seems to heat. I believe that it is a gravity fed system. Any ideas would be greatly appreciated. Thanks

[ I SUSPECT YOU HAVE A "GRAVITY" SYSTEM WHICH DOES NOT GIVE YOU MUCH IN THE WAY OF CONTROLLING YOUR HEATING AND HOT WATER. YOU COULD HAVE A ROOM STAT FITTED SO THAT THE RADIATORS WILL ONLY COME ON IF THERE IS A DEMAND FOR HEAT FROM THE ROOM STAT.THE ROOM STAT TURNS THE PUMP ON WHICH MOVES THE HEAT ROUND THE SYSTEM. MY ADVICE IS TO CHANGE IT INTO A FULLY PUMPED SYSTEM.THIS INVOLVES INSTALLING A THREE PORT VALVE. A ROOM STAT AND ALSO A CYLINDER STAT. YOU WILL THEN HAVE FULL CONTROL OF WHAT COMES ON AND WHEN. DIFFERENT PROGRAMMERS ARE AVAILABLE GIVING DIFFERENT OPTIONS. THIS IS A MUCH MORE ECONOMICAL WAY OF RUNNING A BAXI AND YOU SHOULD FIND THAT THE INSTALLATION OF MORE CONTROLS WILL SAVE YOU THE COST OF UPGRADING IN NO TIME! HEATING CONTROLS CAN SAVE YOU AROUND 30% ON YOUR FUEL COSTS!!!!!! HOPE THIS HELPS!!!!!!

very sorry for "shouting" i did not realise capital letters were shouting till my son just told me! new to using computer

Check the small header tank in the loft as the ball valve is probably stuck and the water level in the system has fallen below a level where the gravity circulation will work .