How to Plumb a Hot Water Heater for Radiant Heating

Things You'll Need

  • Gas-fired hot water heater

  • Recirculation pump

  • Expansion tank

  • Boiler taps

  • ¾-inch hard copper pipe and fittings

  • 2 di-electric fittings

  • ¾-inch Ball valve(s)

  • Automatic vent valve

  • Pressure relief valve

  • Soldering torch

  • Solder

  • Flux

  • Pipe cutter

  • Tubing cutter

  • Sandpaper

Most radiant heating installations use a boiler to heat the water, but a simple hot water heater can do the job and save you several hundred to a few thousand dollars. You will need to make a few modifications to the heater and have knowledge of basic plumbing techniques. A domestic hot water heater can be sufficient and just as efficient as a boiler for small spaces such as homes 1,200 square feet or smaller. Propane or natural gas-fired water heaters are more cost-effective than electric.

Outlet Side

Step 1

Install the water heater. Leave enough surrounding room for the radiant heat system equipment. Position everything so you can access the equipment during installation and for future servicing.

Step 2

Solder ¾-inch hard copper pipe to a di-electric fitting threaded on one end. Solder the pipe to the unthreaded end. This end will run from the outlet side of the hot water heater to the supply side of the radiant tubing.

Step 3

Thread the threaded side of the di-electric fitting onto the hot water outlet of the hot water heater.

Step 4

Continue soldering ¾-inch hard copper pipe, working from the hot water heater toward the radiant tubing fitting. Install a ¾-inch ball valve for servicing the system at a convenient point in this line.

Inlet Side

Step 1

Solder ¾-inch hard copper pipe to a second di-electric fitting that threaded on one end. Thread the threaded end of the fitting onto the inlet side of the hot water heater.

Step 2

Continue soldering copper pipe from the hot water heater to the recirculation pump. Route ¾-inch hard copper pipe to an expansion tank.

Step 3

Run copper pipe from the expansion tank to the return side fitting of the radiant tubing. When your system is complete, water returning from the radiant tubing will pass through an expansion tank, then a recirculation pump, before returning to the water heater.

Step 4

Install an automatic vent at a high point of the system. Replace the pressure relief valve supplied with the hot water heater tank with a 30 psi relief valve.

Step 5

Install ball valves, where convenient, for future servicing or to repair separate parts of the system. Install two boiler taps with a ball valve inline between them at a convenient point in the system. Use the boiler taps and ball valve to charge the system with a mix of propylene glycol and distilled water.


Plan the layout of your plumbing system before you start cutting and soldering pipe. Determine the highest point of your system, where you will install the automatic vent valve. Decide where you will install inline ball valves and boiler taps, in either the return or supply side, for charging the system.

If you have more than one zone, you will need delivery and return manifolds and zone control valves for each zone.

For most residential applications, you can use the same hot water heater to heat water for the radiant heat system and for domestic (drinking water, showers, cleaning, etc.) use. This is called a dual purpose system.