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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 for Installing Radiant Heat

  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 ⅞-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.