Hydrolic seperation is a great tool to protect boilers and use mulitiple emiters, but can be inefficient.
Why? The distortion factor!
You get flow temprature distortion when you use different flow rates in a system. The different flow rates distort the system mean temprature from the boiler mean temp. This raises flow and return tempratures at the boiler which, although doesn't increase input, could push flow and return out of condensing.
To work out the 'distortion factor' take the dt at the system side away from the dt at the boiler side and divide by 2.
DF=2/(dt1-dt2)
I.e. If you have a 60 40 flow at the boiler dt20 and mean of 50 and your dt is 10 system side 20-10÷2 =5
This is the amount of degrees your boiler is operating at higher then if you had no Hydrolic seperation, and as we all know, low temps is key for high condensing efficiency.
Addd 5 to your flow and return. Your flow will be 65 and return 45 in order to still give you a mean of 50 system side.
The more different the flow rates the more distortion and the more potential the system has to be inefficient.
So when designing consider that if perhaps your flow rates are only a small touch higher then recommendation you could perhaps use a slightly wider dt system side.. some may want to ignore the manufacturer recommendations, as the water ways and components in a 15kw boiler are no different to they're 40kw counterparts, this will save the cost and installation of a llh.. but obveously I wouldn't recommend that!
However in most cases it won't be something you can get round. So my advise would be to set up effectively on install. Buy some decent temp clamps (I haven't found any yet) and see where you can adjust the system to minimise distortion by closing the dt in toward eachother. on the boiler side I would suggest fixing the pump speed at a rate which matches your demand.. However if you have mulitiple circuits or mixers this will be something that constantly changes as demand alters.
This also works you if you have a lower flow rate system side and the same equation applies. 2/(dt1-dt2) dt1 being the higher temp.
This leads quite nicely on to gown low loss headers work. If you have a faster flow system side the return temps match on the primary and secondary side. If you have slower flow system side the flow temps will match.
