Q: A client wants a portion of a basement slab to have radiant heat to take the chill off the floor of a planned playroom (there will be supplemental heat). The hvac contractor wants the radiant tubing, which will be attached to wire mesh, to be lifted into the middle of the slab during the pour to put the heat closer to the surface. The concrete contractor doesn’t want to do this because he insists that cracks will show up along the tubing. He wants to leave the tubing at the bottom, and says the insulation board will drive the heat up anyway. He recommends at least 3 inches of concrete-above the tubing. Which is correct?
A: Hydronic heating contractor Bill Clinton responds: We typically try to keep at least an inch of concrete above our tube. If the slab is 4 inches thick and the mesh is in the middle, a 5/8-inch o.d. tube would have about 1 1/4 inches above it. Pulling mesh up during the pour is by nature somewhat inaccurate, so it’s probably best to err on the side of keeping the mesh low in the slab. I don’t believe the height of the tube in the slab is going to make a significant difference in system performance even if it’s on the bottom — the entire slab will be heated in any case. My advice is to let the concrete contractor have it his way.
One point about laying out the tube: Tie it perpendicular to the wire as much as possible, away from parallel strands of wire. The reinforcing value of the wire depends on concrete bonding all the way around it, so it’s best to keep the tube out of the way.