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Radiant Design For Performance

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According to the U.S. Department of Energy, heating and cooling account for about 56% of the energy use in a typical U.S. home. Radiant heating can make HVAC systems significantly more efficient. In fact, in a recent study by the New Buildings Institute, nearly half of the zero-energy buildings in the United States incorporate radiant systems to meet their energy goals. Understanding radiant installation methods and their performance attributes is fundamental to achieving energy savings and thermal comfort for the occupants.

There are three practical and proven methods for residential installations of radiant floor heating systems:

Below-floor plates (joist space method)
Thin Slab Overpour (wet installation method)
Above-floor panels (dry installation method)

The scope of each project influences the choice between these installation methods.

Below-floor Plates
Using this method, installers secure PEX pipes with aluminum heat transfer plates to the underside of the subfloor. To drive heat into the room, the joist cavity is filled with insulation, typically 4 to 5 times greater in R-value than the resistance of the flooring above. Achieving optimal space heating comfort may not happen, because it can be difficult to configure perimeter and occupied pipe spacing layouts. It is also difficult to zone rooms, because the joists prevent the installation of the most efficient pipe layouts.

Thin Slab Overpour
hydronicshubimage0514Installers typically fasten PEX pipes to the top of the subfloor, and then pour a thin slab of concrete covering the pipes. Overpour installations allow for the quick and easy placement of complex piping layouts that optimize the occupants’ thermal comfort. The thin slab also has a sound deadening effect. Installers need to make sure they have considered the extra material and labor costs to reinforce the subfloor and double-plate the walls.

Above-floor Panels
Heat transfer panels are an above-floor alternative that fits tighter construction schedules and requires less coordination. Installers fasten panels to the top of the subfloor, and then snap in the PEX pipes. Panels and pipes install quickly using typical framing tools and complex piping layouts are easy to configure with basic carpentry skills. Finished flooring can be installed the same day after pressure testing the PEX pipes.

Evaluating Radiant Performance
The different radiant floor installation methods have significant differences in performance. Pipes underneath the subfloor, either with or without plates, are the least effective radiant systems at transferring heat into the above room. Plate installations require higher water temperatures to heat the room and there are maximum temperature limits that must be considered. Floor coverings, underlayments, adhesives and grouts can be damaged by excessive temperatures leading to discoloration, noise, delamination, warping, cracking and deterioration.

More important than protecting the flooring, is to ensure the occupants’ foot comfort and safety by limiting the surface floor temperatures. Working within these design temperature limits, the designer often selects supplemental heat to achieve the remaining heat requirements. This increases the installation costs and is less efficient, resulting in higher energy usage.

Overpour and high-performance heat transfer panel installations are more efficient at spreading the heat evenly underneath the flooring, allowing the room to be heated with lower water temperatures. High-performance heat transfer panels, with their low thermal mass and high heat conductivity, result in a quicker response time than overpour radiant systems. Quicker response, particularly during the spring and fall “shoulder” months, adds efficiency and increases comfort by reducing the tendency to overheat the room that can occur with slab systems. Also this fast pick-up time can be combined with advanced controls for night time or unoccupied setback, resulting in less demand on the high-performance radiant panel system which translates to less energy consumed.

High-performance panels may provide up to 50 percent higher heat output compared to a thin slab overpour, traditional panel, and plate systems, and more than double the heat output of just pipes installed in the joist cavity. This higher performance results in a lower required heating water temperature, which not only reduces energy consumption, but also aligns with the most efficient operating range of geothermal heat pumps. These radiant designs can be optimized to generate sufficient energy without requiring supplemental heat, positioning radiant as a contributor to zero energy construction.

Jon Bittenbender
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Posted In: ACCA Now, Hydronics

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