Radiant Cooling Theory and Fundamentals

Radiant cooling utilizes ceiling panels to absorb radiant energy from warmer room surfaces and people.  The same principles that apply to radiant heating also apply to radiant cooling.  In addition, part of the overall cooling mechanism is the convective cooling of warmer stratified air at the ceiling.  Water circulating through the radiant panels, carries off the sensible heat absorbed by the panels. 

ASHRAE has recognized that excellent comfort levels can be achieved by radiant cooling systems at higher room air temperatures than convective systems due to the reduced surface temperatures and occupant heat transfer to the cooled radiant ceiling.  The chilled water temperature circulating through the tubing of the radiant panels must be 1-2 degrees F higher than the dew point of the entire space.  Normal design water temperature rises are usually 4-5 degrees F. The cooling capacity of the radiant ceiling is dependent upon the difference between room air temperature and the mean cooling water temperature.  Higher room air temperature design and lower mean water temperatures based upon decreased summer design relative humidity levels allow for lower water supply temperatures which in turn result in greater radiant cooling capacity.

Air requirements are reduced because radiant panels are doing the majority of the sensible cooling.  The required ventilation air picks up the entire latent cooling load to maintain the design dew point.  The remaining portion of the sensible cooling is done by the ventilation air. 

Systems have been successfully employed in high humidity southern states.  By maintaining design water temperature, conditioned ventilation air quality and capacity, with tight building construction (minimum outside air infiltration) have resulted in many years of successful radiant cooling without condensation.  Standard modulating hydronic control valves can be used with radiant systems providing excellent system response due to the low thermal mass of the ceiling.

Radiant cooling and heating provide one of the best balances between the human body and its surroundings.  In winter, the radiant ceiling transfers heat to the outer walls, furniture, and floors which reduce heat loss from the occupants to these surfaces.  In summer, a cooled radiant ceiling absorbs sensible heat from these same surfaces and the human body dissipates heat to the cooler radiant ceiling.   Typical forced air convective systems require large quantities of heated or cooled air which may not be as comfortable.