It is a fact that there are key and fundamental differences between in cooling load between radiant and air systems. Markets indicators also show that Interest and growth in radiant cooling and heating systems have increased in recent years because they have been shown to be energy efficient in comparison to all-air distribution system.
Hydronic-based radiant systems have verified advantages over air systems, such as the improved transport efficiency of using water instead of air as the thermal distribution fluid, improved plant side equipment efficiency with warmer cold water temperatures, and, particularly with TABS, the possibility of night pre-cooling using cooling towers.
A radiant system is a sensible cooling and heating system that provides more than 50% of the total heat flux by thermal radiation. There are three primary types of water-based radiant systems: (1) for new construction: plastic tubing (e.g., PEX) which is embedded in the structural slabs, often referred to as thermally activated building system. (2) for retrofit or new construction which is suspended metal ceiling panels with copper tubing attached to the top surface (radiant ceiling panel, RCP); and (3) for retrofit or new construction which is prefabricated or installed-in-place systems consisting of embedded tubing (e.g., PEX, or small, closely spaced plastic tubing “mats”) in thinner layers (e.g., topping slab, gypsum board, or plaster) that are isolated (insulated) from the building structure (embedded surface system, ESS).
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. Simulation results show that (1) zone level 24-h total cooling energy of radiant systems can be 5–15% higher than air systems due to higher conduction load through the envelope; (2) peak cooling rate at the radiant cooled surface can be 7–35% higher than air system for zones without solar load. This difference can increase up to 85% for floor system in zones with solar load; (3) the peak cooling rate differences originate from: (a) radiant cooling surface(s) reduce radiant heat gain accumulation in the building mass; (b) only part of the convective heat gain becomes instantaneous cooling load.
As a result, tools using response factor methods such as radiant time series method for cooling load calculations are not appropriate for radiant system design.