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
- Remove any insulation from between the floor joists located directly below the room you plan to heat.
- Use end nippers to snip off any nails protruding from subfloor within the joist bays.
- Fasten aluminum transfer plates to underside of subfloor with ¾-inch-long self-tapping screws. Install two plates per joist bay.
- 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.
- 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.
- Connect the new supply and return pipes at a mixing station that includes a circulator pump, mixing valve and relay switch.
- Run radiant-heat tubing from mixing station to each aluminum transfer plate.
- Use rubber mallet to tap tubing into the grooves in the underside of the transfer plates.
- Connect the supply and return ends of the tubing to the mixing station.
- Cover the transfer plates and tubing with fiberglass-batt insulation.
- Fill the radiant-heat system with water and then purge any trapped air.
- Hire a licensed electrician to connect a new thermostat to the relay switch at the mixing station.