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. There are many subcategories of radiant heating and cooling, including: "radiant ceiling panels", "embedded surface systems", "thermally active building systems", and infrared heaters. 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. Within this category, it is practical to distinguish between high temperature radiant heating (devices with emitting source temperature >≈300 °F), and radiant heating or cooling with more moderate source temperatures. This article mainly addresses radiant heating and cooling with moderate source temperatures, used to heat or cool indoor environments. Moderate temperature radiant heating and cooling is usually composed of relatively large surfaces that are internally heated or cooled using hydronic or electrical sources. For high temperature indoor or outdoor radiant heating, see: Infrared heater. For snow melt applications see: Snowmelt system.
History Radiant heating and cooling originated as separate systems but now share a similar form. Radiant heating has a long history in Asia and Europe. The earliest systems, from as early as 5000 BC, were found in northern China and Korea. Archaeological findings show kang and dikang, heated beds and floors in ancient Chinese homes. Kang originated in the 11th century BC as “to dry” later evolving into a heated bed, while dikang expanded this concept to a heated floor. In Korea, the ondol system, meaning "warm stone," used flues beneath the floor to channel smoke from a kitchen stove, heating flat stones that radiated heat into the room above. Over time, the ondol system adapted to use coal and later transitioned to water-based systems in the 20th century, remaining a common heating system in Korean buildings. In Europe, the Roman hypocaust system, developed around the 3rd century BC, was an early radiant heating method using a furnace connected to underfloor and wall flues to circulate hot air in public baths and villas. This technology spread across the Roman Empire but declined after its fall, replaced by simpler fireplaces in the Middle Ages. In this period, systems like the Kachelofen from Austria and Germany used thermal masses for efficient heat storage and distribution. During the 18th century, radiant heating gained renewed use in Europe, driven by advancements in thermal storage techniques, such as heated flues for efficient heat distribution and a better understanding of how materials retain and transfer heat. In the early 19th century, developments in water-based systems with embedded hot water pipes paved the way for modern radiant heating, providing indoor comfort through heat transfer. Radiant cooling also has ancient roots. In the 8th century, Mesopotamian builders used snow-packed walls to cool indoor space. The concept resurfaced in the 20th century with hydronic cooling systems in Europe, embedding cool water pipes in structures to absorb and dissipate heat, meeting cooling loads. Radiant cooling became more widely adopted in the 1990s, with the implementation of floor cooling. Today, modern radiant systems typically use water as a thermal medium for efficient heat transfer and are widely adopted in residential, commercial, and industrial buildings. While valued for its potential to enhance energy efficiency, quiet operation, and thermal comfort, their performance varies with design and application, leading to ongoing discussions.
Radiant Heating
Radiant heating is a technology for heating indoor and outdoor areas. Heating by radiant energy is observed every day, the warmth of the sunshine being the most commonly observed example. Radiant heating as a technology is more narrowly defined. It is the method of intentionally using the principles of radiant heat to transfer radiant energy from an emitting heat source to an object. Designs with radiant heating are seen as replacements for conventional convection heating as well as a way of supplying confined outdoor heating.
Indoor The heat energy is emitted from large warm elements, such as a floor, wall or overhead panel, or smaller hot elements, and warms people and other objects in rooms rather than directly heating the air. The internal air temperature for radiant heated buildings may be lower than for a conventionally heated building to achieve the same level of body comfort, when adjusted so the perceived temperature is actually the same. One of the key advantages of radiant heating systems is a much decreased circulation of air inside the room and the corresponding spreading of airborne particles. Radiant heating systems can be divided into:
Underfloor heating systems—electric or hydronic Wall or ceiling panels Higher temperature devices such as electric fireplaces or infrared heaters Underfloor and wall heating systems often are called low-temperature systems. Since their heating surface is much larger than other systems, a much lower temperature is required to achieve the same level of heat transfer, making hydronic ones ideal for use with heat pumps. The maximum temperature of the heating surface can vary from 29–35 °C (84–95 °F) depending on the room type. 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 higher. Medium-size, medium-temperature electric panels are also available for mounting on domestic walls or ceilings. Fireplaces or heaters powered by solid, liquid or gaseous fuels, or electricity, generally glow bright red to yellow. This also applies to portable devices, although some such as catalytic heaters may glow with a dull red not easily seen. Single installations or devices heat persons and animals mainly one-sidedly but to a smaller extent also opposing surfaces. All of these systems may provide a room climate with less circulation, lower air temperature and higher humidity levels than with air heating systems. This saves energy especially with short-term use and in some cases can be healthier.
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