Looking for some advice - planning a self build - but at the early stages such that a lot of this hypothetical at the moment (we don't have exact floor plans) and therefore exact heat calcs. 
However I am looking more for advice on the layout of the system.
It will be a house to PassivHaus standards, we will have a ASHP.
I will be running KNX throughout.

Looking at UFH with 10 loops in the concrete slab, and for the purpose of this lets assume all 10 loops are done in ridgeline eco max 22mm pipework, and have a length of 50m each (so 500m total, with a total water volume of 108L.)
The loops will all be controlled with Salus THB23031 auto balancing valves, and a UPM3 circulator pump. 
The plan is to run this as an open loop, on 24/7, on WC. The auto balancing valves will throttle (or not) to maintain a deltaT across each individual loop (will not be zoning with them - all open all of the time). 

ON top of this I will have a series of 7 fan coil units - for arguments sake lets say Dakin - FWF03DT.
My plan at the moment is to use these as trim heating or trim cooling. The FCUs will tee off the ASHP flow, have a zone valve, a magna1 circulator pump, split into 22mm pipe work for the runs, each FCU will have a PICV valve.

The exact strategy I have not yet worked out but anticipating something like this:
During heating season - UFH does the bulk of work on the WC, if the temp drops below setpoint -2, modulate PICV and FCU fan in that room to give some top up heating. I am anticipating this being unlikely during the winter in a passivhaus.
During the cooling season, as the heatpump hits the low end of the weather curve, switch into cooling mode. Run cooling at worst case dew point + 2. Apply a large moving average to this calc and only update every 5 mins or so (using KNX bridge to talk to ASHP) to avoid hunting and drastic changes.
Use UFH to cool the slab. if room with FCU is setpoint +2 modulate PICV valve and FCU fan. I anticipate this to happen more often, due to solar gain and the lack of efficiency cooling with a UFH slab.

I have been trying to do due diligence with the plumbing layout. I have had one ASHP with a buffer, and my most recent without. 
I have read/watched plenty of information on the pro/cons of buffers volumisers etc. But have not been able to get my head around whether this is a case where I should be looking at them or not. 

So assume worse case flow rate/volume for ASHP, as the system is above, plumbed direct. 
This is FCU zone shut - all auto balance valves fully throttled (but importantly that is always CP -0.3mm on the stroke of the actuator). I have estimated this drops the flow rate per loop to around 0.6L/min [but this pure guessing?]. Giving a total flow of 6L/min in the system, and a volume of +108Ls. 

Looking at valiant arotherm plus 5kW requirements (min flow 400L/h, min volume no back up heater 40l) I easily exceed the volume requirement, but I am below the 6.67L/min requirement (albeit only just). 
I have ballparked guessed the 5kW for the passivhaus as I am not expecting big heating loads, this becomes even worse if I upsized to the 7kW (9l/min) or the 10kW (16.5L/min).

The other end of the spectrum, lets assume we have fully opened UFH valves, (relatively short loops, with a large, for UFH, diameter) we may achieve 30L/min on that loop. Lets assume that the 7 FCUs all turn on, so zone valve opens and PICVs start to modulate - for the FWF03DT, highest flow rate is  568L/h (heating) or 515L/h (cooling).
which gives 7 x 9.5L/min or another 66.5L/min.

Now I know the chance of all UFH zones being fully open, and the FCUs running full whack simultaneously is extremely unlikely - however I want to discuss the case as it is possible - the house has been empty for a while, we came back when it was truly cold and the heating had previously been off?? I don't know, but I want to understand what would happen hydraulically. 

So this gives a total flow rate of 96L/min and a system volume in excess of 108L + FCU pipework/coils etc.

The volume is not an issue, but on the 5kW the max flow rate is 14.3L/min. on the 7Kw it is still only 20L/min.

Now even with more sensible assumptions I think the UFH could exceed or approach that on its own, and even if the FCUs were ticking over slowly, I think I would be way over the 20L/min.

Which brings me to the crux of my question - would a buffer be required? I appreciate distortion/mixing makes it normally a no/no. But hydraulically I think I am likely to be at the bottom end of the heat pump flow rate for the majority of the time (ie UFH is stable, nicely at temp, the balance valves will be mostly throttled, and assuming I will be in the region of around 6-10L/min) the FCUs however can demand a significant flow rate if they want. especially given that I am running them both cooler (for heating) and warmer (for cooling) than their design point.
Do I need the hydraulic separation that a 4P buffer provides, and if so is it worth going large, like say 200L to provide the ability for it to stratify? 
Or another option I have looked at is including a 0-10V belimo ball valve as a bypass valve. Have a flow meter on the ASHP return with 0-10V output, and using KNX to proportionally open the bypass as the flow rate approaches the minimum ASHP requirement, which fixes the low flow rate issue, if I opened it at the other end of the scale, would it also fix the high flow rate issue. Obviously at the expense of efficiency, but only in the shoulder cases as it opens, and not permanently like a 4P buffer (with mixing/standing losses etc) would.

Another question I had, is there any disadvantages to having a 50L or so volumiser on the flow line, despite being above min volume, if I could absorb some of the cold water prior to the slab on defrost cycles, that seems like  good thing.
Yes it would give me slightly more standing losses, and it would add a lag into the system, but for an open loop 24/7 WC plan, I don't see this being an issue in a passivhaus?

Sorry for the long post, if you have any more questions just ask, I know some of this is over engineered, but I do enjoy that side of it as well!