Verification of the radiant time factor (RTF) and convective time factor (CTF) for radiant systems in the buildings

Abstract

Cooling loads of radiant systems in buildings differ from conventional all-air systems, yet current radiant system standards and guidelines still rely on the principles of all-air systems for their cooling load calculations. We previously developed a Radiant and Convective Time Series (RCTS) method specifically for radiant systems. However, the reliability and accuracy of the RTF and CTF in the method remain to be validated, which hindered the application of the method for radiant systems. This study verifies the RTF and CTF through comparison with two reputable models: the Nigusse model and EnergyPlus model. We started with a detailed case study, followed by parametric studies considering six impact factors. The results show that for all-air systems, the model has negligible differences for three indexes: sum of RTF, root mean square error, and peak load difference. For radiant systems, the RTF and CTF also closely match the other two models, albeit with relatively larger differences. Among the six factors, RTF and CTF are mainly influenced by room characteristics like size, thermal inertia, and exterior wall types, while less correlated to the heat gain schedule, density, or set-point temperatures. The new findings of the study are that: the RTF and CTF of radiant systems adhere to the transfer function principles; the RTF and CTF of RCTS method are rigorous and reliable parameters to calculate the cooling loads for radiant systems. The study offers valuable guidance for generating and validating RTF and CTF, which contributes to the application of radiant systems in buildings.