mite sound silly but have you checked the basics? gas burner presures? is it modulating down 2 quickly? same as below if hot water probs with sludge usually caused by the plate/domestic heat exchanger fins becoming blocked check the temp of the flo going into the plate heat exchanger and the temp at the hw outlet of it usually if its blocked near the inlet gets piping hot but the water cant flo through so the boiler cant get rid of the heat and modulates down because it thinks its up to temp
toke
September 2009