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DHTiling
Less than 10 mtrs Dan with UFH.. Around 7 max i would say. and perimeter joints a must
So.. what is the largest separate area..? and i take it is doorways separating,.
You need expansion joints where different rooms meet, every 10 meters on large floors in length and width, and even if the rooms are knocked through, I'd still have one somewhere for the sake of each end of the room having slightly different heat-up and cool-down times due to all sorts of factors, oven at one end, patio doors at the other for example.
And did you say there's no UFH in the hallway?
That aside, sounds like you've found your problem. Two many rooms with different heat factors to cope with expansion with no expansion joints. Especially if tiles run from a heated floor, to one not heated, and then back to a heated floor the other side of the hallway (if I did read your earlier post correctly).
And you said the tiler said it's okay without strips when you have a de-coupling membrane, which isn't right. So you've got your culprit there too.