Hi Julian.....do you use weber set plus on all your heated floors ?Assuming it a screed floor and flat and level to start with with no need for movement or expansion joints.
Prime floor with a primer recommend by your chosen adhesive supplier, not PVA,I us Weber adhesive's and primer.
Stick insulation boards down, (not forgetting to stagger the joints) and fix with Weber set rapid with a 6mm trowel
Test and lay ufh and temp probe, retest.
SLC over the top with a fibre modified product, Weber floor flex for me
Test ufh and probe.
When dry, set out floor and tile using, for me a 10mm trowel, if using a levelling system, and I would be using Weber set rapid plus. If you are happy to use a slow set I would use Weber set plus. Back buttering the tile with the straight side of the trowel.
Allow a expansion joint round perimeter.
Total height would be about 33mm
I've always gone down the weber spf root or added ad250.....Must admit never new it was suitable for heated floors and I've been using it for years... lol ...learn something new every day.
