Radiant heating and cooling

Archaeological findings show kang and dikang, heated beds and floors in ancient Chinese homes.

[3] 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.

[6] 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,[7] their performance varies with design and application, leading to ongoing discussions.

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 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.

[10] The latent loads (humidity) from occupants, infiltration and processes generally need to be managed by an independent system.

However, a recent study on comparison of VAV reheat versus active chilled beams & DOAS challenged the claims of lower first cost due to added cost of piping[18] Because of the potential for condensate formation on the cold radiant surface (resulting in water damage, mold and the like), radiant cooling systems have not been widely applied.

First, it is easier to leave ceilings exposed to a room than floors, increasing the effectiveness of thermal mass.

[13] Chilled slabs, compared to panels, offer more significant thermal mass and therefore can take better advantage of outside diurnal temperatures swings.

While not purely radiant, they are suited for spaces with varying thermal loads and integrate well with ceilings for flexible placement and ventilation.

[9] The operative temperature is an indicator of thermal comfort which takes into account the effects of both convection and radiation.

[21] Thus, radiant systems can helps to achieve energy savings in building operation while maintaining the wished comfort level.

As regards occupants within a building, thermal radiation field around the body may be non-uniform due to hot and cold surfaces and direct sunlight, bringing therefore local discomfort.

The norm ISO 7730 and the ASHRAE 55 standard give the predicted percentage of dissatisfied occupants (PPD) as a function of the radiant temperature asymmetry and specify the acceptable limits.

The detailed calculation method of percentage dissatisfied due to a radiant temperature asymmetry is described in ISO 7730.

This delay can lead to over-adjustments, resulting in increased energy consumption and reduced thermal comfort.

For instance, MPC leverages the thermal mass of radiant systems by storing heat during off-peak times, before it is needed.

Additionally, cooler nighttime air improves the efficiency of cooling equipment, such as air-source heat pumps, further optimizing energy use.

By employing these strategies, radiant systems effectively overcome thermal mass challenges while reducing daytime electricity demand, enhancing grid stability, and lowering operational costs.

[27] Radiant cooling systems can experience condensation when the surface temperature drops below the dew point of the surrounding air.

[28] The risk is particularly high in humid climates, where warm, moist air enters through open windows and contacts cold radiant cooling surfaces.

To prevent this, radiant cooling systems must be paired with effective ventilation strategies to control indoor humidity levels.

[35][36] Heat radiation is the energy in the form of electromagnetic waves emitted by a solid, liquid, or gas as a result of its temperature.

The emissivity of a material (usually written ε or e) is the relative ability of its surface to emit energy by radiation.

In enclosures, radiation leaving a surface is conserved, therefore, the sum of all view factors associated with a given object is equal to 1.

In the case of a room, the view factor of a radiant surface and a person depend on their relative positions.