ASK THE EXPERTS: Hydronic Heating

I very much enjoyed John Siegenthaler’s article, “Renewable Hydronic Heating” (HP152), and wondered if he might comment on lightweight systems that use aluminum as a heat conductor as opposed to those that use concrete as a thermal mass. I was recently made aware of this type of system and the benefits to the overall flooring process when looking at building a home over a basement foundation. Details of the system can be found at warmboard.com. I hope to hear another opinion on this.

Rick Wimberly • via email

I’m glad you enjoyed the article. There are several radiant panel systems that use aluminum plates or claddings as “wicks” to pull heat away from the tubes and spread it across the panel surface. The product you mentioned—Warmboard—is designed as a complete subfloor/radiant panel system. It is about 1.125 inches thick and, as such, can serve as a structural subfloor and replace conventional plywood or OSB subfloors.

The key to success with any of these products is to keep the water temperature required as low as possible. As I suggest in the article, design the distribution system so that it can supply the maximum required heating load without exceeding a supply water temperature of 120ºF—even lower if possible. The lower the water temperature, the higher the efficiency of heat sources such as solar collectors, heat pumps, and condensing boilers.

Warmboard uses 12-inch tube spacing; other aluminum plate systems use 8-inch tubing spacing. They should not be covered with more than R-1 from flooring materials, and should have at least R-19 underside insulation. This type of system would provide about 19 Btu/hr./ft.² of output, when operated with a supply temperature of 120°F.

I would also highly recommend the use of PEX-AL-PEX tubing (versus standard PEX) for any system involving aluminum heat dissipation plates or claddings. PEX-AL-PEX with aluminum is less likely to make any expansion noise as the system warms up.

John Siegenthaler • Appropriate Designs