On 26/08/2020 15:50, Stephen wrote:
Hello,

I want to add an external expansion vessel to my combi boiler as I
think the built-in one has failed. The built-in one is fed off a
15mm port on the DHW exchanger. I think it will be difficult to
run 15mm copper to this, so could I use a short length of 10mm or
will the reduced diameter cause problems?

Thanks in advance!

Shouldn't be a problem, if you can find all the suitable adaptors.

But the expansion vessel can go anywhere on the supply or return
pipework to the radiators. There's a lot to be said for fitting them on
a flexible connection (like the filling loop). One thing I would
recommend is fitting a ball valve for isolation, so that you can test
and when necessary recharge the vessel easily without depressurising the
main system. (You should fit a drain point downstream too, otherwise you
will have to release pressure by cracking a compression fitting). And
obviously, remember to de-isolate afterwards.