What's the worse that can happen?Ever have a bad stink that just wouldn't go away,(Someone I know did once)if whoevers paying the bill(the customer not the contractor)isn't happy with the job,it'll be your *** they're dogging no one elses,tell the contractor that you seriously doubt the UFH will work through three layers of
tile,I personally have never heard of anyone tiling a third set ,it definatly has "bad dreams"written all over it,has the contractor even considered the excessive weight ,if ther's a floor underneath you could be crossing some kind of weight per sq ft code that might bring legal problems should the floor cave in,I don't know,just throwing some things for thought out there,good luck with that one,I'd give that job to my competition,cheers,Mike