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How to Retrofit Radiant Heat From Above

This Old House plumbing and heating expert Richard Trethewey shares tips for installing new radiant-heat tubing on top of an old subfloor when there's no access from below


1. Fasten the plywood-backed radiant-heat panels to the subfloor with 1⅝-in. screws.

2. Stretch a continuous length of PEX tubing back and forth across the floor.

3. Use a rubber mallet to tap the tubing into the grooves of the radiant-heat panels.

4. Run the tubing ends down through the floor to the boiler.

5. Mount the manifold to a wall near the boiler, then connect the tubing ends to the manifold.

6. Run piping from the boiler to the manifold.

7. Install finish flooring over the tubing.

Shopping list:

  • Radiant-heat panels used to hold PEX tubing in place in subfloor
  • 1 5/8-inch drywall screws for fastening down the radiant-heat panels