Hiya guys, I went to one of these last friday (second man on job, first one said there was nothing wrong) anyway the boiler runs fine on DHW. When you switch over to Central Heating the demand light comes on, the boiler fires and runs for about 20 secs then goes off along with the demand light, then the demand light comes back on and the process is repeated. There are no fault lights at all on the boiler either, I had a CH sensor with me so I checked it and changed it anyway, flue looks perfect, customer rang me tuesday to say boiler was fine and now tonight they've rang to say its started again...any ideas before I traipse back there !! Thanks.