Vaillant banging noise

23 Sep 2009
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United Kingdom
I recently emptied some water out of the central heating system, to put some inhibitor in. Then I topped up with the filling loop to get to the 1.5 bar pressure. But now when i turn on the hot water tap a banging noise comes from the boiler.
The noise sounds as if there is a metal piece flying inside the boiler and hitting something. It only happens when I turn on the hot water and the boilers ignition turns on. The boiler is a vaillant combi 831 on a sealed central heating system.

This never happened before, and the boilers been working fine for last couple of years.
What could make the noise?
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You've got air trapped in the heat exchanger by the sound of things.
You need to run the test programmes P6 then P0 (page 51 in your manual).
Be careful to do them as described or you may damage the heat exchanger if you haven't already.
Do you do the test P6 first then turn off the boiler and then do the P0 test?
Yes you can do that, or, after P6 has run for a bit just press and hold the plus button and press reset. Then press i, which runs the purging sequence.
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The P0 programme alone takes 15 minutes to run, so you don't seem to have done it properly.
Like I said, be very careful how you do them or you will knacker the very expensive heat exchanger.
Do not run it on hot water demand till you have it running properly on c/heating demand.
the central heating works fine without no noise. Only when i run the hot water. I will do it again and run it for the full 15 minutes. By the way will it be the primary heat exchanger that will be damaged or plate to plate heat exchanger?
I think that has sorted, it is not making the noise for now. Thanks.
By the way if i need to fill my system in the future. Do i put it on p6 , fill it then do p0?

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