UFH in retrofit+extension - sanity check

[Olf]

Context:

- supplied by combi boiler

- extension properly insulated, slab on insulation

- existing ground floor will be taken out, 150mm PIR placed and slab on it

- bamboo flooring throughout

Layout:

Considering that the floor won't like to be too hot, and hopefully my effort with isulation and drought proofing will be good enough, I'm targeting max floor temperature of 25 °C

So here is the plan, help me please and correct if I'm wrong with any of these:

1. Buffer tank needed  -  but how big?

2. Mixer capable of running at 25 °C - Ivar name seems popular, any other alternatives?

3. Salus autobalancing valves

4. 3 zones: kitchen/diner (extension), living room+WC, bedroom with ensuite & walk-in wardrobe.

5. 16mm pipes tied to the mesh in the middle of 100mm thick concrete slab (all the quotes take slab-insulation-screed as default and try to sell me hundreds of clips), laid in serpentine pattern

6. pipes centres? On the forum I've seen numerous references to 100mm ( @Nickfromwales ), but did not manage to track down the whole reasoning of that. Quotes so far suggest 150 or 200 mm.

7. How thick can be perimeter of the slab? I might have plenty of XPS to use somewhere and that looks to me like a good place.

8. Anything special for the run to the extension? One supplier claims it is impossible not to rip the pipes because of differential movement and offered surface retrofit solution only.