John Stumbles wrote in message
...
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.


OK, but why not simply put another primary pump in series with the boiler?


(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.

snip

(a) 890 cu.m. and 'yes' to (b) and (c). The property is an old listed
building and I can't change 20 large single-glazed sash windows for
double-glazed ones...but that's a topic for a whole new thread. I'm doing
what the Conservation Officer will allow me in terms of insulation.