• In this video, This Old House plumbing and heating expert Richard Trethewey shows how to retrofit an old home with new under-floor heat.

    Steps:
    1. Remove any insulation from between the floor joists located directly below the room you plan to heat.
    2. Use end nippers to snip off any nails protruding from subfloor within the joist bays.
    3. Fasten aluminum transfer plates to underside of subfloor with ¾-inch-long self-tapping screws. Install two plates per joist bay.
    4. Use a right-angle drill to bore two -inch-diameter holes, spaced 3 inches apart, through the floor joists for the radiant-heat tubing.
    5. Connect to the boiler a T-fitting and copper pipe to supply hot water to the radiant-heat system. Also, connect a return pipe back to boiler from the radiant system.
    6. Connect the new supply and return pipes at a mixing station that includes a circulator pump, mixing valve and relay switch.
    7. Run radiant-heat tubing from mixing station to each aluminum transfer plate.
    8. Use rubber mallet to tap tubing into the grooves in the underside of the transfer plates.
    9. Connect the supply and return ends of the tubing to the mixing station.
    10. Cover the transfer plates and tubing with fiberglass-batt insulation.
    11. Fill the radiant-heat system with water and then purge any trapped air.
    12. Hire a licensed electrician to connect a new thermostat to the relay switch at the mixing station.
    • Difficulty: Hard
      This is a job best left to the pros

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      Tools List

      • nippers
        End nippers, to cut nails protruding from subfloor
      • drill
        Drill/driver
      • right-angle drill
        Right-angle drill, to bore holes through floor joists
      • tubing cutter
        Tubing cutter
      • propane torch
        Propane torch
      • mallet
        Rubber mallet

      Shopping List

      Aluminum transfer plate

      ¾-inch-long self-tapping screws

      Radiant-heat tubing

      Mixing valve, circulator pump and assorted plumbing fittings, for connecting new radiant-heat system to existing boiler

      Lead-free solder and flux

      Emery cloth

      Thermostat and relay switch—controls temperature of radiant-heat system