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Thermally Active Floors--Part 3: Making it Work

©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 3, March 2013.

By Daniel H. Nall, P.E., FAIA, Member ASHRAE

About the Author
Daniel H. Nall, P.E., FAIA, is senior vice president at Thornton Tomasetti Group in New York. He is an ASHRAE certified Building Energy Modeling Professional and High-Performance Building Design Professional.

After the physical design of a radiant slab system has been completed, the project requires the appropriate control strategies and the use of tested construction techniques to be successful. Control strategies for radiant slab systems differ significantly from those of standard HVAC components in that they must not only respond to current space conditions but also recognize the thermal lag of the components they are controlling. The control sequences must avoid either “driving” the slab or causing the slab system to conflict with the ventilation/dehumidification system.


Radiant Heating and Cooling Control Sequences

The entire operating range of temperature setpoints for the radiant floor is limited by condensation and comfort considerations to a minimum of 68°F (20°C) and a maximum of 84°F (28.9°C). Floors colder than the minimum easily can result in temperature contact discomfort or condensation. Temperatures higher than the maximum also result in contact discomfort. Control is not necessarily precise to tenths of a degree, but must logically track the thermal conditioning requirements of the space. Deviation of a setpoint by a degree or two may result in a similar space temperature deviation, but that range is well within the operating behavior of a conventional HVAC system. The fact that the thermally active floor influences comfort both by moderating the air temperature and the mean radiant temperature makes the system very forgiving for minor temperature excursions.

In general, the floor control system is handled by a local panel and individual slab temperatures are not reported back to the building management system (BMS). The BMS typically will recalculate floor temperature setpoints and communicate them to the local floor control panels. The floor control panels will then activate the local two-position valves to maintain the setpoint. Heating and cooling changeover and control of floor loop supply temperature are also typically handled by the BMS.
Control of the thermally active floor is also very different from control of other faster responding systems. A basic rule of controls theory is to avoid using a quickly varying stimulus to control a slowly responding system because doing so can result in “thrashing” the system by sending conflicting inputs within the time frame of the system’s response. The thermally active floor is a relatively slow-acting system, so controlling this system with a thermostat sensing a potentially variable medium, such as the room air, likely will have less than optimal results, especially responding to moving patches of solar radiation. By the time an air thermostat reacts to rising air temperature from a solar heated floor, the floor will have risen to a temperature that may take a significant time to correct. The best control strategy would seek a more stable stimulus and count upon the long time constant of the radiant floor to dampen the oscillations of the more volatile room air temperature. The most important consideration is control of heating/cooling changeover.

Heating cooling changeover should be a rare event and should be controlled to avoid driving the floor from one mode to another. Control sequences that recognize the thermal mass of the floor and exploit reasonable temperature deadbands can avoid such “thermal thrashing” of the floor. Systems designed by the author have used two general control schemes for thermally active floors, both of which use embedded temperature sensors in the floor to enable it to be controlled to a setpoint, but with two different strategies for determining the floor temperature setpoint.

The first of these strategies is applicable to buildings or spaces that incorporate a high thermal mass exterior envelope. In this strategy, the setpoint for the floor temperature is reset based upon the inside surface temperature of an exterior wall. This strategy was pursued in the implementation of a thermally active floor system for the renovation of the Saint Meinrad Archabbey Church in St. Meinrad, Ind., a late 19th century Gothic Revival cathedral church (Photo 1). The church has an air system for ventilation, dehumidification and some sensible cooling and a thermally active floor system. In this project, the floor setpoint temperature is reset based on the temperature of the 2.5 ft (0.75 m) thick sandstone exterior wall, outside of the 0.75 in. (19 mm) insulation and finish drywall applied to the inside of the wall. The actual setpoints were tuned in the field, but the initial ramps were as shown in Table 1.


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