Aidan wrote in message
om...
"John Aston" wrote in message
...

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 did the Google search, and in doing so realised that the "low velocity
mixing header" actually forms a primary loop, with the secondary pumped
heating loops teed off primary loop. Contrast this with the normal domestic
single pump and multiple zone valve arrangement, which is more like a radial
circuit (sort of).

It seems that primary/secondary pumping is used predominantly in larger
(commercial) heating installations whereas domestic heating systems just
have a primary pump and zone valves. I don't know at what point you should
change from zone valves to p/s pumping. Perhaps, when you want more control
than the 'crude' on/off systems provided by Danfoss Randall.