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Posted by HeatPro on September 25, 2004 at 16:03:56:
In Reply to: Loud knocking noise in Vaillant combi boiler VCW-sine 18 T3 WF H posted by Raphael Ali on September 25, 2004 at 10:13:18:
Knocking is caused when the water temperature in one spot of the boiler exceeds the temperature at which the water can remain water and turns to steam. Cooler water then collapses the steam bubbles and causes knocking. Verify the water velocity through the boiler, if it is too slow, the boiler will start knocking for the same reasons. Your boiler has been used for 20 years and can accumulate lime with small cave-like spaces in it to produce the noise. Further examination as to liming and water speed through the boiler passages is required.
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