PM wrote:
How long can I safely leave a CH system drained down? I need to replace the
boiler and two or three rads, most drain cocks, probably the CH pump and
nearby pipework, the header tank, add a 3-way valve.... I'm not going to get
it done in a weekend!

If I leave the system drained down, what happens? Is the copper pipework OK
but the rads will start to rust? Anything else I should be aware of?

You can leave it drained for a long long time. Of course the insides
of radiators will begin to rust, but that won't be significant - if
you leave a piece of steel outside in a pail of water, how long will
it take to rust away? Ages. Make sure you're finished by October(ish).