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I believe it's a 3-port valve situated inside the boiler (it's a system boiler). I'm afraid I have no idea about the by-pass. Chris

For some time now my Baxi boiler has made a loud thumping noise when switching from hot water to heating. It sounds like a valve shutting off too quickly or hitting something hard and the thump echoes around the house. I've had various suggestions as to what might be causing it - does anyone...