John Aston wrote:
My understanding is that most domestic unvented heating systems use the
boiler's pump to circulate water and on/off valves to divert water to the
hot water cylinder or heating zones.

Under what circumstances should the zone valves be replaced by zone pumps
with a mixing header between the pumps and the boiler? What's the function
of the mixing header, and do you always need it if you have any heating
circuit pumps or DHW pumps external to the boiler?

According to the boiler installation instructions on mhs' web site you
use the mixing header if the resistance of the heating system to be
supplied is more than the boiler's built-in pump can handle.


(I'm currently considering buying a 38kW boiler which offers a vertical low
velocity header as an accessory:
http://www.mhsboilers.com/boilers/premix/strata1.htm)

Why this particular boiler? I haven't looked at the price but I probably
don't need to ;-). Your house must be (a) ginormous (b) draughty and/or
(c) badly insulated to warrant a 38kW non-combi boiler, but if you need
the output for either of (b) or (c) then spending £x on dealing with
these losses is likely to give a better return on investment than
spending the same amount on a more sophisticated boiler. For smaller
and/or more energy-efficient houses the energy savings from installing
even a cheap condensing boiler probably won't actually repay the extra
purchase cost of the beast (though hopefully the prices of condensing
boilers will come down next year when the big builders start buying
1000s of them).