Changing CH water


Is it possible to change most of the water in a CH system by opening the drain cock and letting the system top up from the header tank at the same time? Forgive me if this is silly, I'm a total newbie with DIY. Thanks.
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I would expect this to be a hit or miss affair. It's quite possible that after a period of time, after some dirty water has run out, that the replacing clean water starts to come out and there will be left a rump of dirty water which won't shift without you stopping the clean water coming in. Whether this happens or not will depend on the unique finer detail of your system.
Rob Graham
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But I can't harm anything by trying it? Will I need to open any other bleeds or anything? Thanks for your help
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No, you won't harm anything. But would you change your engine oil by letting it out slowly from the bottom and replacing it from the top? Not quite the same thing, I agree. But similar.
The more bleeds you open the more likely your plan is to work.
Rob
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Thank you both for your help and advice. Much appreciated. Jerry
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