I guess that's more suited to where there's multiple boilers?

and the LLH isn't plumb as well as the flue looks as though it has been "forced" over as it certainly is not in line with the boiler.
I'm not being picky either though

What's the black box below the boiler?

Ok , i'm gonna start being picky , no more mr nice guy......................watch this space.

the black box (Low Loss Header) does various things. The pump on the boiler inst big enough to supply the whole system under full load - so the boiler pump ( now called a shunt pump) only needs to pump the hot water from the boiler to the LLH. It is sized up for the boiler (presumably this is a 60Kw LLH) It also helps the boiler keep its designed delta T across the outputs to reduce thermal shock across the heat exchanger.
It also means the the boiler services the LLH rather than all the various demands the other 4 zones have. It can also be used to 'even out' using 2 boilers. So if 2 x 30Kw were used on a cascade then their joint inputs would go into this LLH to be distributed to the various zones. This is done for various reasons - break downs being the obvious one , as at least one boiler will still b running but if a very small load is at times required then the ratio of max to min modulation on the boiler ( lets say its 6:1 means max mods on a 60 is 60 but the min 10.) if the min loading is 5 Kw then the boiler will cycle uneciserily. So using two boilers @30 with 6:1 means with only one boiler running the lowest o/po is only 5Kw.

This is not my area of expertise & GW & LeeC do these every day. I do see where GW is going with his 28mm pipe though,. I am sure more knowledgeable people than me on these things will add or correct.
A not so clued up plumber ( or one just putting in a cheaper quote to get the job) would just bang on a slightly larger head pump in the boiler and pipe it straight up to zones via valves. It would still all work to a fashion and everything would probably still get hot - but it would not be ' due dilation'
This is a good example of a big house installation. Added to this there could be W/C on each (or just some) circuit and even different flow temperature requirements on differing zones.

Thats a pretty good discription eco, I will just add in case it wasn't clear that any system demands ie valves closing ,pumps turning on off or ramping up/down etc have no effect on the flow through the boiler
Eco's mention of the boiler shunt pump is what me and gaswizzard were getting at when we commented on ebees earlier post on the first page
"the big loop" ebee refered to is also a low loss header and the pump he thought "should be bigger" would have been specified to match the boiler req flowrates etc
around the header, the size of the system punps have no baring
Matt

Just been reading thru this again- have to have a chuckle at the sparkies & their 'ELV' & the plumbs with their 'LV'

I gave up with 'ELV' ages ago or no plumbum has a clue what u r referring to. Its like u have to hang up the phrase when u purchase a set of bending bars!
Even the boiler MU's call it 'low voltage side' and '230 volts side'

As usual the sparkies are right though

Nah, just as equally, selectively pedantict

forgive my ignorance on low loss headers but how does the system expansion get taken? doesn't the low loss header keep heating circuits seperate from the boiler circuit? is it not like a huge plate to plate?

can i have that in english now please!! does or does it not keep the two halves seperate? i:e like a plate to plate or is it just like a 4 pipe boiler where the 2 mix together inside?
never got involved in big stuff hence i know nought about these things.