Similar to conventional air systems, the controls of radiant cooling aim to achieve the desired thermal comfort and energy efficiency. Because of the low-temperature surfaces, condensation control is another major consideration of radiant cooling systems.
Thermal comfort control
Occupants’ thermal comfort is affected by several factors including metabolic rate, clothing insulation, air temperature, mean radiant temperature, air speed and relative humidity. Of these factors, the air temperature and the mean radiant temperature are regarded as the most important ones to determine thermal comfort levels in a space. The mean radiant temperature is defined as the uniform temperature of an enclosure, in which the radiant heat loss from the human body is the same as that would occur in the actual (non-uniform) enclosure. If the difference between surface temperatures is small, the mean radiant temperature can be simplified to the following equation:
Where Tmr is the mean radiant temperature (°C), Ti is the surface temperature (°C) of surface i, Fp-i is the angle factor between a person and surface i.
The combined effects of radiation and convection on occupant’s thermal comfort can be evaluated using operative temperature. For a sedentary person not in direct sunlight and in a space with low air velocities (0.2 m/s), the operative temperature can be approximated as the simple average of air and mean radiant temperature:
According to ASHRAE Standard 55, if the air velocity is lower than 0.2 m/s, the operative temperature should range between 24°C and 28°C to satisfy occupant comfort in the cooling season.
Conventional air systems normally use the air temperature set points to achieve the desired thermal comfort. However, because of the lower surface temperature in radiant systems, the air temperature can be reduced to realize the same comfort level as air systems. As an example, for a person sitting at the center of a 6m by 6m floor, the angle factor is 0.46 for the floor. Thus, decreasing the floor surface temperature by 5°C has the same cooling effect as decreasing the air temperature by 2°C.
For condensation control, the most important strategy is to limit the radiant system’s cold water supply temperature to be higher than the dewpoint temperature of the space air. Because a lower air dewpoint temperature allows a lower cold water supply temperature, the space humidity control is highly important to maximize the cooling potential of radiant systems. Acceptable surface temperatures based on comfort and condensation considerations are 19°C for the floor and 17°C for the wall and the ceiling.
In the case of thermally active building systems, (TABS), they use building structures for energy storage and shift the cooling load to a different time of day. Their control is quite different from other fast-responsive HVAC systems. Using an air-temperature-based thermostat to control TABS likely has less than optimal results, especially if the space is exposed to solar radiation.
Based on real building designs, studies have promoted two general control strategies for radiant slab systems:
1. Floor temperature set point to reset based on the inside surface temperature of an exterior wall. This strategy applies to buildings or spaces that have an exterior envelope with high thermal mass (e.g. concrete block walls). For example, the table below shows the reset strategy for a building that has a 0.75 m thick sandstone exterior wall.
2. Floor temperature set point reset based on the air system operating conditions. This strategy applies to buildings that have the radiant system subordinated to a conventional air system [e.g. a forced air system with an air-handling unit (AHU)]. The control sequence needs to define a dead band of the AHU capacity (e.g. 40% heating and 40% cooling) in which the radiant cooling is inactive. When the cooling output of the AHU varies between 40% and 100% of its cooling capacity, the radiant system is activated and the floor temperature varies between 23.3°C and 20°C. Similar control approaches apply to heating.
Water cooling control
On the water side, the cooling capacity can be controlled by configuring the system as a constant flow, variable temperature system or a variable flow, constant temperature system. Variable flow systems are almost always used in practice because they facilitate individual zone controls.
In parallel to the above controls, model predictive control has been increasingly studied in several recent studies to dynamically optimize the operation of radiant slab systems (i.e. TABS). It is, therefore, important to complement the guidelines explained above with the increased use of design modeling software.