ASHRAE defines radiant systems as temperature-controlled surfaces where 50% or more of the design heat transfer takes place by thermal radiation. Radiant cooling is the use of cooled surfaces to remove sensible heat primarily by thermal radiation and only secondarily by other methods like convection. The top metal disc reflects the radiant heat onto a small area. An example of the overhead radiant heaters are the patio heaters often used with outdoor serving. Radiant heating systems may be gas-fired or use electric infrared heating elements.
Outdoor radiant heaters allow specific spaces within an outdoor area to be targeted, warming only the people and objects in their path. Even in a no-wind condition, the buoyancy effects will carry away the hot air. Relying on convection heating is in most cases impractical, the reason being that, once you heat the outside air, it will blow away with air movement. In the case of heating outdoor areas, the surrounding air is constantly moving. Radiant overhead panels are mostly used in production and warehousing facilities or sports centers they hang a few meters above the floor and their surface temperatures are much higher. The maximum temperature of the heating surface can vary from 29–35 ☌ (84–95 ☏) depending on the room type. This provides an improved room climate with healthier humidity levels. Since their heating surface is much larger than other systems, a much lower temperature is required to achieve the same level of heat transfer. Underfloor and wall heating systems often are called low-temperature systems.
According to some definitions, a technology is only included in this category if radiation comprises more than 50% of its heat exchange with the environment therefore technologies such as radiators and chilled beams (which may also involve radiation heat transfer) are usually not considered radiant heating or cooling. There are many subcategories of radiant heating and cooling, including: "radiant ceiling panels", "embedded surface systems", "thermally active building systems", and infrared heaters. Radiant heating and cooling is a category of HVAC technologies that exchange heat by both convection and radiation with the environments they are designed to heat or cool. Systems using temperature-controlled surfaces to exchange heat with their surrounding environment through convection and radiation