Successful design of a thermally active floor system requires an understanding of system physics and the use of sophisticated design tools. The behavior of thermally active slabs is non-intuitive, and the intricacies of complex radiant couplings and stratification do not lend themselves to rules of thumb. The geometric and thermal variations across projects make the design task even more reliant on predictive evaluation of the impact of detailed design alternatives. Fortunately, predictive tools are available to ensure that the design maintains thermal comfort, achieves energy efficiency and avoids condensation.
Design of a thermally active slab system is unlike the design of a conventional HVAC system in that the behavior of the system may not be accessible through simple arithmetic calculations. Two-dimensional heat transfer between fluid in the tubing, through the slab and floor finishes and into the space, must be evaluated, along with the impact of short wave radiant fluxes (solar) on the floor. Analysis of the system should also examine the overall behavior of the space, including the long wave radiant interchange among the room surfaces, thermal stratification of the air in the space, and the distribution of solar fluxes on the active and non-active surfaces of the room. Finally, the psychrometrics of the space must be evaluated to ensure comfort and avoid condensation.
The author accomplishes these goals using five tools; each aimed at a particular aspect of the system performance. The first is a simple calculation tool implemented with Engineering Equation Solver (EES) and using standard ASHRAE heat exchange algorithms. This tool replaces a complex spreadsheet that required iterative manipulation to determine the limiting factor for heat transfer in the system, slab to space coupling, fluid to slab coupling, or fluid flow rate, and then calculates the resulting performance. These procedures were developed before the creation of international standards for these calculations, now documented in ISO-DIS 11855 and EN 15377. EES determines the limiting factor and calculates that performance with a more user-friendly interface. It allows the evaluation of alternatives in floor finish conductance, topping slab depth, tubing loop length and on-center spacing, supply water flow and temperature for different combinations of room temperature and absorbed solar flux on the floor. Based upon these inputs, it calculates the water temperature leaving the floor, the cooling or heating capacity of the floor per unit area and the surface temperature of the floor (Figure 1).
The second tool calculates the pattern of solar irradiation on the building surfaces at various times of the year, including the shading of the building structure and the optical characteristics of the glazing (Figure 2). This tool provides absorbed solar flux input to the floor evaluation tool in EES. This flux, and the solar heat gain that is absorbed by the glass and re-radiated or convected into the space, are used in the computational fluid dynamics (CFD) tool.
The CFD tool evaluates the room level heat transfer for the space, including long wave radiation interchange among surfaces, convective heat transfer and buoyancy induced flow of the room air, cumulative impact of solar flux on both active and non-active surfaces in the space, and impact of ventilation supply air on the air temperature distribution in the space (Figure 3). Heat gains in the space are specified for both radiant and convective components to more accurately characterize convectively generated stratification.
The fourth tool provides validation of the psychrometric balance in the space. This evaluation can be provided in the CFD program by rigorous input of moisture sources and specification of moisture content of ventilation and infiltration air, or it can be provided by a mass balance calculation of moisture gains, airflow and infiltration to calculate a bulk moisture ratio for the space. This latter approach is appropriate for many projects, because condensation avoidance strategies will address location-specific condensation hazards.
Citation: ASHRAE Journal, vol. 55, no. 2, February 2013