"Steve" wrote in message
...
I'm upgrading the plumbing prior to having a Combi installed. I'm going
to
put in a number of (full bore) isolation valves on both the DHW and
heating
circuits in order to make it easier to do additions/changes later, without
having to drain down the whole system.

I don't think that you will find many combi boilers that do not have
isolating valves as part of the boiler installation.

How near the boiler can the valves be? I know the last metre or so of
pipework to the boiler should be in copper (not plastic); since the valves
have a rubber (neoprene?) seal, is there a recommended minimum distance
from
the boiler to a valve?

No.
Valves are usually compression fit so make the solder joint first then fit
valve.
With drain cocks and the like then these are dismantled prior to soldering.

Also, if the engineer wants to make a soldered joint, is there a minimum
distance between the joint and the valve, to avoid heat damage to the
valve
during soldering?

Cheers