glow worm fuel saver mk 11?

sounds like scale in the heat exchanger. you can try to add a descaler but remember to follow directions. generally over the counter descalers are not man enough for the job , but other descalers need neutralizing after use as they are quite strong. another couple of outside possbilities , too high burner pressure , airlock on system but if you have a scale problem already its likely to be that

kieran
August 2008