The depth of hydronic tubing can affect the performance of a heated concrete slab.
Anyone who's installed hydronic floor heating has surely had opportunity to watch the neatly placed tubing circuits get buried in concrete. Sometimes the tubing and reinforcing mesh gets lifted into the slab as the concrete is placed. Other times the masons trample over the tubing as if it's not even there. Any instructions given them by the radiant installer become less imperative as each additional yard of concrete flows down the chute.
More than once I've found myself outnumbered on the issue of lifting the tubing during the pour. Each new crew of masons sets the stage for another "discussion" over this issue. Come to think of it, I'm probably lucky not to have the imprint of a concrete rake across my forehead.