i have a potterton 24 he plus system losing pressure. during summer when radiators not in use pressure is fine but as soon as we start using the rads pressure drops and needs topping up daily. pressure only drops when rads have heated up and we turn the thermostat down. if we had heating on all day no pressure would be lost (v expensive). have had new pressure relief valve fitted, no sign of any leaks and this has been happening for a long time any ideas please?