Ceramic tile over radiant in slab heat problems

[slowzuki]

If the concrete expanded the tiles would not pop up, the grout would break between them.

a few comments,
the American Concrete Institute recomends the maximum temperature for any concrete application is 150 deg.

The radiant panel institute recomends an "anti fracture membrane" between any slab and ceramic tile application. I have used a product called "Ditra", but there are other options including mop on rubber and asphault coatings, and rubber membranes. These products allow the ceramic tile and the concrete slab to move independantly because they have different thermal coefficient of expansions. there is plenty of information on the RPA website on this topic.

120 degree water in a slab sounds high to me for an in-slab application. what were (are) the water supply and return temperatures?

The RPA also recomends constant circulation with reset water temperatures. You mentioned that there is no insulation under this area (really bad).
One possibility is that the slab loop turned on when the floor was cold and 120 deg water went through the tubes. the large delta t caused rapid expansion of the concrete in contact with the tubes, but the tile was still cool and did not expand with the floor it was bonded to, so it had to either crack or pop up.
Another possibility is that the water in the slab tubing went really hot or really cold because of some kind of failure and that is what caused the great differential expansion between the tile and slab.

 

[BrownianHeatingTech]

If the concrete expanded the tiles would not pop up, the grout would break between them.

Yup, but you might not notice that. Then when the concrete shrank, the fractured grout would take up more space, and the tiles would pop.

It's all theoretical, and nearly impossible to diagnose (especially remotely) due to the number of possible causes. The only remedy (short of re-doing the floor) is to do what has already been done.

The only additional thing to do is to re-work the system as necessary, in order to supply the correct water temperature reliably.

Joe