CH plumbing

Its not critical.

Indeed. The only thing that matters is that the returns from all the rads in each zone combine with each other *before* the zone combines with the other zones - otherwise all sorts of 'reverse flow' type nasties can occur!

One thing you should aim to do is to bring all the returns from each individual zone together before (or at the same point) as the zone returns join together. Otherwise, you get a pressure difference between the returns within an individual zone, this will cause flow through that zone when it's not calling for heat (mixture of backflow and forward flow through different radiators). This may not be enough to worry about though - depends on pipe layout.

The DHW and CH flow and returns go to separate ports on my conventional boiler. I suppose each side form a manifold. Would it make any difference if one port at each side was plugged and the two circuits teed together to the remaining ones?

That was usually where you had gravity circulation to the hot water primary. Fully pumped it's more usual to have just the one set.

My recently bought system boiler is set out for separate flows and returns

- but gives instructions for using a common return if you wish to.

There are some system boilers with dual outputs which can be set to different temperatures, e.g. for UFH, radiators, HW, which you are likely to want at different temperatures.

Thank you. I partially remembered reading an old post of yours talking about returns; I knew it was something about position of returns but I couldn't remember what.

Thanks again, Stephen.

My old boiler was exactly like that: a 28mm side for HW and a 22mm side for CH. When I converted that to fully pumped, I did exactly as you described and used only the 22mm side and capped the 28mm off. My understanding was that both pipes are connected internally so it makes no difference which you use.

On most old boilers with cast iron heat exchangers they are simply extra ports into the same chamber. Might make some difference with gravity circulation but probably not when pumped.