Hi , Was the floor spread with a notched trowel to get a good even cover of adhesive of we blobs of adhesive used this would lead to the tiles sounding hollow
Is the adhesive direct onto the wires.? usually would bring both floors
up to the same level then ufh fitted then a thin layer of levelling compound to cover wires then tile
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An even floor would usually be levelled with levelling compound before electr underfloor heating going down.