On Mon, 6 Nov 2017 17:31:31 +0000, newshound
wrote:

Why a combi? If the present DHW tank has an indirect coil and provided
this and the radiators are not corroded to death, use a system boiler
and make it unvented. This avoids the possible problems of running a
shower off a combi, and keeps the immersion DHW backup. And you are not
seriously proposing using a combi with vented radiators, are you?

Plumbing is not my thing and I thought system boilers had to go with
pressurised tanks, like the megaflow, which I took to be more
expensive.

I'll look at the system boiler options. Combi seems to be the cheapest
deal.

No I was going to make the rads a sealed system, risking that they may
already be corroded inside but they look fine.

AJH