Designing and Installing Radiant Floors with the 2019 NWFA Guidelines

The National Wood Flooring Association (NWFA) put together a Radiant Heat Taskforce, made up of many experts from the radiant heating industry, including members from the Radiant Professionals Alliance (RPA). Together, the Taskforce developed maximum recommended allowable operating temperature guidelines for wood floors. The guidelines were developed in response to recurring failures of hardwood floors placed over radiant floor heating systems.

Failure analysis indicated excessive and extended radiant heating system operating temperatures, in conjunction with poor or no relative humidity control, and in some cases, missing or poor subfloor insulation, leading to longer than normal run times of both hydronic and electric floor heating systems. The net effect resulted in the wood product showing signs of cracking, checking, panelization, warping, and cupping.

If you are in the radiant floor heating business or the hardwood flooring business, it is important that you understand these new guidelines and the limitations they impose on radiant floor heating systems. The new guidelines limit the maximum floor operating surface temperatures of the installed wood floor to 80°F.

When maintaining an indoor air temperature of 70°F, a radiant floor being maintained at a surface temperature of 80°F will deliver approximately 20 BTUs per square foot per hour, which is sufficient for most homes.

As responsible comfort system designers know, the very first task in the design of a radiant heating system is to perform a heat loss calculation for the areas to be served by the system. Beyond the NWFA guidelines, radiant floor heating systems are often limited to a maximum recommended operating temperature of 85°F where prolonged human contact is expected, with the exception of bathing areas, locker rooms, and swimming pool surrounds, where temperatures are allowed to go as high as 90°F.

If the pre-installation heat loss calculation indicates that floor surface temperatures of 85 to 90°F are required to meet the heating load, then hardwood floors are not recommended in those areas.

85°F is not a physical limitation, but rather a human physiology limitation. When a human comes into contact with a surface temperature greater than 85°F, the hypothalamus gland (which regulates body temperature) believes your body is going to overheat, and it goes into cooling mode, which we recognize as sweating. Sweating is not considered comfortable by human comfort standards. According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), a floor surface temperature that creates optimal human comfort for most people lies somewhere between 70-80°F. The 80°F maximum surface temperature limitation was derived through the Radiant Heat Taskforce for wood floors only. (See chart on previous page.)

In newer homes with high R-value wall, floor, and ceiling construction using controlled energy recovery or heat recovery ventilators, the heat load calculations rarely exceed a 15 BTU per square foot per hour floor heat output. In this situation, the floor will only need to be maintained at a maximum operating temperature of around 77°F, posing no risk to wood flooring. It is only in areas with extensive glazing, and older dwellings with lower R-value construction, that the load can be above 20 BTUs per square foot per hour, becoming problematic for a wood floor. In these scenarios, the designer must augment the floor’s maximum threshold of 20 BTUs per square foot per hour with other means of heat delivery, or the warranty on the wood floor may be violated.

For example, depending on the construction methods being employed, the augmentation heat could be accomplished with a conventional hot water baseboard, if both the architect and homeowner are willing to accommodate the convectors and their associated esthetics, and provide a means of mounting the convector to a wall beneath the glazing system.

Generally speaking, for this to work, the designer of a hydronic heating system must provide a high temperature circuit (180°F) to allow for the convectors to perform to their maximum. If the glazing is run from ceiling to floor (which is quite often the case in mountain settings), it may be necessary to incorporate a subfloor convection system, using either a natural or forced-convection design. This may require major modifications to the structure to accommodate the subfloor convector boxes. Forced-convection takes up much less physical space than subfloor convective systems, but they bring the baggage of noise into the comfort equation. Make certain that the consumer is aware of these issues (noise and/ or structural modifications) before signing on the dotted line. If the heat source is limited to an operating temperature of less than 120°F (air or ground-source heat pumps, for example), then the recommendation is to consider the use of a radiant ceiling and/or radiant wall heating system to augment the radiant floor’s limited capacity, and/or the use of forced-air convectors. It is strongly recommended that this second heat source be controlled by a 2-stage thermostatic controller.

The NWFA guidelines recommend the use of a floor temperature-limiting type of thermostat, in addition to the air temperature sensing portion of the typical thermostat. They also recommend the use of a data logger to record the maximum floor operating temperatures for the purpose of assuring compliance with the temperature limitation guidelines, as well as warranty coverage by the wood floor manufacturer. It is also important to note that humidity control is strongly recommended. A humidified home results in a happy floor, and the occupants will be much healthier, too.

Finally, not all wood floor manufacturers limit their products installed surface temperature to 80°F. Check with the hardwood flooring supplier and the flooring manufacturer and make certain that your design is compatible with their recommendations. It is also suggested that you familiarize yourself, and your heating system designer, with the NWFA guidelines, which can be reviewed at nwfa.org/technical-guidelines.aspx.

The bottom line is that details must be recognized, surface temperature limitations followed, and a system developed around the product’s limitations. If you are not responsible for providing the humidity controls or data loggers, make certain that the owners are made aware of this important detail to avoid possible issues with the finished product.

Mark Eatherton is Technical Committee Chair for the Radiant Professionals Alliance. He can be reached at markeatherton@mac.com. For more information about the RPA, visit radiantprofessionalsalliance.org.

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