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Thank you for all the kind answers. I forgot to mention that BG engineer did some adjustment with the Overhead tank's ball valve and overflow has stopped. . But is not a permanent solution. From his equipment he could pinpoint the pipe measuring about a meter near the boiler that has affected due to sludge.My radiators are working, so I was wondering why bother about power flush. Like someone said in earlier posts, there was no power flush system some 10-15 years ago. In fact my house not very old, built in early 2000.