Is it possible to change most of the water in a CH system by opening
drain cock and letting the system top up from the header tank at the
Forgive me if this is silly, I'm a total newbie with DIY.
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
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.
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