Have had problems in the past with the glowworm 30cxi combi. when hot water is running (so burner is at full rate) a howling starts from the boiler after about 30 seconds. After checking everything from gas valve to combustion ratios, came to the conclusion that the heat exchanger was blocked. Question is, which one? is it the DHW HE or the main one. only reason I say that is because glowworm thought it was the main one and I thought it was the DHW one. Come on local boys, help me out.
