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Posted By ericjeeper,
Mar 2, 2008 at 12:35 AM
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.