Cooling load differences between radiant and air systems

Abstract

Unlike the case of air systems where the cooling load is purely convective, the cooling load for radiant systems consists of both convective and radiant components. The main objectives of this energy simulation study were to investigate whether the same design cooling load calculation methods can be used for radiant and air systems by studying the magnitude of the cooling load differences between radiant and air systems over a range of configurations and to suggest potential improvements in current design guidelines. Simulation results show that 1) zone level 24-hour total cooling energy of radiant systems can be 5-15% higher than air systems due to differences in conduction load through the building envelope; 2) peak cooling load at the radiant system hydronic level can be 7-31% higher than air system for zones without solar load. The differences can increase up to 93% at the hydronic level for floor system in zones with solar load; 3) the cooling load differences between the two systems originate from: a) radiant cooling surface(s) directly remove part of the radiant heat gain and reduce heat accumulation in the building mass; b) only part of the convective heat gain becomes instantaneous cooling load. This indicates that simplified methods such as Radiant Time Series Method is not appropriate for cooling load calculation in radiant system design. Radiant systems should be modeled using a dynamic simulation tool that is capable of capturing radiant heat transfer for cooling load calculation.

Experimental comparison of zone cooling load between radiant and air systems

“…These numbers agree well with previous simulation results, which showed differences from 6-15% for interior zones depending on the radiation factor of the heat source [8].…”

“…This means that radiant cooling systems may impact zone cooling loads in several ways: (1) heat is removed from the zone through an additional heat transfer pathway (radiant heat transfer) compared to air systems, which rely on convective heat transfer only; (2) by cooling the inside surface temperatures of non-active exterior building walls, higher heat gain through the building envelope may result; and (3) radiant heat exchange with non-active surfaces also reduces heat accumulation in building mass, thereby affecting peak cooling loads. Using simulations we previously demonstrated that dynamic responses of rooms when conditioned by radiant cooled surface(s) are significantly different from the case of air systems and consequently the cooling loads for system sizing are also drastically different (in fact, often higher for the studied cases) [8]. Thus, current cooling load calculation and modeling methods may not be applicable for radiant systems.…”

Thermal comfort in buildings using radiant vs. all-air systems: A critical literature review

“…Building energy simulation offers researchers an effective method to simultaneously investigate thermal comfort and energy consumption in buildings. In many cases, the papers compared energy use under equivalent comfort conditions between the multiple radiant and allair variants [57]- [61]. These studies were not retained as they focused on energy savings given thermal comfort constraints.…”

Cooling capacity and acoustic performance of radiant slab systems with free-hanging acoustical clouds

“…For both locations and with lower coverage, the air temperature is slightly higher than the mean radiant temperature. We note that in the case of air systems, this difference is commonly greater [25]. As the coverage increases, the difference between both temperatures diminishes and the mean radiant temperature eventually gets slightly higher than the air temperature.…”