"John Aston" wrote in message ...
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.

The main advantage of this is cheapness. One pump costs less than
several pumps and the associated controls. One pump is generally
adequate for most UK domestic installations. Many UK 'heating
engineers' cannot comprehend anything more complex than one pump and a
3-port mid-position valve.

Shutting a zone-valve will cause an increase in the flow rates to all
the other open zones. This may cause problems with intermittent noise,
especially from TRVs. It will mess up any intelligent control systems,
anything with sensors and modulating valves.



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?

Does a zone pump arrangement confer any control advantages over using a
boiler pump + zone valves?

Yes. You can get a more consistent flow rate to each zone. Shutting
off any one zone won't affect the others. It costs more. I'm not sure
why they call it a mixing header, I don't have time to look at the
link. It's more usually a low-loss header, with no mixing of the water
taking place before the returns. A thermal store/buffer vessel would
also perform this function, as mentioned in the post about the heating
controls.

Try also a Google search for primary/secondary pumping and "closely
spaced tees".


(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)