Should I consider a buffer tank?

[jayc89]

I've seen quite a few topics on this, but I'm still struggling to understand when a buffer tank makes sense for a UFH system.

 

Current setup;

- 33kw Baxi Platinum+ system boiler set to 55c

- 250l DHW UVC set to 50c

- Downstairs UFH set to 35c (100mm PIR below 60mm screed, u-value of 0.15 @ a PA of 0.4)

- Upstairs Radiators

 

3x UFH zones (all loops 150mm centres);

Zone 1 - 2x loops @ 61m each

Zone 2 - 3x loops @ 73m each

Zone 3 - 1x loop @ 80m

 

421m of 15mm pipe == approx 50l of water in the UFH system. 

 

Online heat loss calcs suggest around 10kw for the ground floor, although I'm skeptical of that as UFH running at 35c wouldn't be sufficient to heat the place if that was the case. 

 

At what point does it make sense include a buffer tank into a UFH system, and if I was to include one, what size would make the most sense?