Radiant heating and cooling is a category of
HVAC technologies that
exchange heat by both
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...
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",
[ISO. (2012). ''ISO 11855:2012—Building environment design-Design, dimensioning, installation and control of embedded radiant heating and cooling systems''. International Organization for Standardization.] "embedded surface systems",
"thermally active building systems",
and
infrared heater
An infrared heater or heat lamp is a body with a higher temperature which transfers energy to a body with a lower temperature through electromagnetic radiation. Depending on the temperature of the emitting body, the wavelength of the peak of the i ...
s. 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
Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics.
A radiator is always a ...
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
An infrared heater or heat lamp is a body with a higher temperature which transfers energy to a body with a lower temperature through electromagnetic radiation. Depending on the temperature of the emitting body, the wavelength of the peak of the i ...
. For snow melt applications see:
Snowmelt system
A snowmelt system prevents the build-up of snow and ice on cycleways, walkways, patios and roadways, or more economically, only a portion of the area such as a pair of -wide tire tracks on a driveway or a center portion of a sidewalk, etc. It ...
.
Heating
![IH - Frico halogeninfravärmare (värmestrålare)](https://upload.wikimedia.org/wikipedia/commons/7/70/IH_-_Frico_halogeninfrav%C3%A4rmare_%28v%C3%A4rmestr%C3%A5lare%29.jpg)
''Radiant heating'' is a technology for heating indoor and outdoor areas. Heating by
radiant energy
Radiant may refer to:
Computers, software, and video games
* Radiant (software), a content management system
* GtkRadiant, a level editor created by id Software for their games
* Radiant AI, a technology developed by Bethesda Softworks for '' ...
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
Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) is ...
to transfer
radiant energy
Radiant may refer to:
Computers, software, and video games
* Radiant (software), a content management system
* GtkRadiant, a level editor created by id Software for their games
* Radiant AI, a technology developed by Bethesda Softworks for '' ...
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
Radiant heating heats a building through
radiant heat
Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) is ...
, rather than conventional methods such as
radiators
Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics.
A radiator is always a ...
(mostly
convection heating). An example is the Austrian/German
cocklestove
A masonry heater (also called a masonry stove) is a device for warming an interior space through radiant heating, by capturing the heat from periodic burning of fuel (usually wood), and then radiating the heat at a fairly constant temperature ...
(''Kachelofen''), a type of
masonry heater
A masonry heater (also called a masonry stove) is a device for warming an interior space through radiant heating, by capturing the heat from periodic burning of fuel (usually wood), and then radiating the heat at a fairly constant temperature ...
. Mixed radiation, convection and conduction systems have existed since the
Roman
Roman or Romans most often refers to:
*Rome, the capital city of Italy
*Ancient Rome, Roman civilization from 8th century BC to 5th century AD
*Roman people, the people of ancient Rome
*'' Epistle to the Romans'', shortened to ''Romans'', a lette ...
use of
hypocaust
A hypocaust ( la, hypocaustum) is a system of central heating in a building that produces and circulates hot air below the floor of a room, and may also warm the walls with a series of pipes through which the hot air passes. This air can warm th ...
heating. Underfloor radiant heating has long been widespread in
China and
South Korea
South Korea, officially the Republic of Korea (ROK), is a country in East Asia, constituting the southern part of the Korean Peninsula and sharing a land border with North Korea. Its western border is formed by the Yellow Sea, while its eas ...
. The heat energy is emitted from a warm element, such as a floor, wall or overhead panel, and warms people and other objects in rooms rather than directly heating the air. The internal air
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer.
Thermometers are calibrated in various Conversion of units of temperature, temp ...
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/cooling systems can be divided into:
*
Underfloor heating
Underfloor heating and cooling is a form of central heating and cooling that achieves indoor climate control for thermal comfort using hydronic or electrical heating elements embedded in a floor. Heating is achieved by conduction, radiation and ...
systems—
electric
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...
or
hydronic
Hydronics () is the use of liquid water or gaseous water (steam) or a water solution (usually glycol with water) as heat-transfer medium in heating and cooling systems. The name differentiates such systems from oil and refrigerant systems.
H ...
*Wall heating systems
*
Radiant ceiling panels
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
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
. This provides an improved room climate with healthier humidity levels. The maximum temperature of the heating surface can vary from 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 much higher.
Outdoors
In the case of heating outdoor areas, the surrounding air is constantly moving. 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. Even in a no-wind condition, the
buoyancy
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...
effects will carry away the hot air. Outdoor radiant heaters allow specific spaces within an outdoor area to be targeted, warming only the people and objects in their path. Radiant heating systems may be gas-fired or use electric infrared heating elements. An example of the overhead radiant heaters are the
patio heater
A patio heater, also called a mushroom heater or umbrella heater, is a radiant heating appliance for generating thermal radiation for outdoor use.
Types of heater
A burner on top of a pole, it burns natural gas, liquefied petroleum gas (LPG), p ...
s often used with outdoor serving. The top metal disc reflects the radiant heat onto a small area.
Cooling
Radiant cooling is the use of cooled surfaces to remove
sensible heat
Sensible heat is heat exchanged by a body or thermodynamic system in which the exchange of heat changes the temperature of the body or system, and some macroscopic variables of the body or system, but leaves unchanged certain other macroscopic vari ...
primarily by
thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) i ...
and only secondarily by other methods like
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...
.
ASHRAE
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE ) is an American professional association seeking to advance heating, ventilation, air conditioning and refrigeration
The term refrigeration refers to the ...
defines radiant systems as temperature-controlled surfaces where 50% or more of the design heat transfer takes place by thermal radiation.
Radiant systems that use water to cool the radiant surfaces are examples of
hydronic
Hydronics () is the use of liquid water or gaseous water (steam) or a water solution (usually glycol with water) as heat-transfer medium in heating and cooling systems. The name differentiates such systems from oil and refrigerant systems.
H ...
systems. Unlike “all-air” air conditioning systems that circulate cooled air only, hydronic radiant systems circulate cooled water in pipes through specially-mounted panels on a building's
floor or
ceiling to provide comfortable temperatures. There is a separate system to provide air for
ventilation
Ventilation may refer to:
* Ventilation (physiology), the movement of air between the environment and the lungs via inhalation and exhalation
** Mechanical ventilation, in medicine, using artificial methods to assist breathing
*** Ventilator, a m ...
,
dehumidification, and potentially additionally cooling.
Radiant systems are less common than all-air systems for cooling, but can have advantages compared to all-air systems in some applications.
Since the majority of the cooling process results from removing sensible heat through radiant exchange with people and objects and not air, occupant thermal comfort can be achieved with warmer interior air temperatures than with air based cooling systems. Radiant cooling systems potentially offer reductions in cooling energy consumption.
The latent loads (humidity) from occupants, infiltration and processes generally need to be managed by an independent system. Radiant cooling may also be integrated with other energy-efficient strategies such as night time flushing, indirect
evaporative cooling
An evaporative cooler (also known as evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning s ...
, or
ground source heat pumps
A ground source heat pump (also geothermal heat pump) is a heating/cooling system for buildings that uses a type of heat pump to transfer heat to or from the ground, taking advantage of the relative constancy of temperatures of the earth through ...
as it requires a small difference in temperature between desired indoor air temperature and the cooled surface.
Passive daytime radiative cooling
Passive daytime radiative cooling (PDRC) is a renewable cooling method proposed as a solution to global warming of enhancing terrestrial heat flow to outer space through the installation of thermally-emissive surfaces on Earth that require zer ...
uses a material that fluoresces in the
infrared atmospheric window, a frequency range where the atmosphere is unusually transparent, so that the energy goes straight out to space. This can cool the heat-fluorescent object to below ambient air temperature, even in full sun.
History
Early radiant cooling systems were installed in the late 1930s and 1940s in Europe and by the 1950s in the US. They became more common in Europe in the 1990s and continue to be used today.
Advantages
Radiant cooling systems offer lower energy consumption than conventional cooling systems based on research conducted by the
Lawrence Berkeley National Laboratory. Radiant cooling energy savings depend on the climate, but on average across the US savings are in the range of 30% compared to conventional systems. Cool, humid regions might have savings of 17% while hot, arid regions have savings of 42%.
Hot, dry climates offer the greatest advantage for radiant cooling as they have the largest proportion of cooling by way of removing sensible heat. While this research is informative, more research needs to be done to account for the limitations of simulation tools and integrated system approaches. Much of the energy savings is also attributed to the lower amount of energy required to pump water as opposed to distribute air with fans. By coupling the system with building mass, radiant cooling can shift some cooling to off-peak night time hours. Radiant cooling appears to have lower first costs
and lifecycle costs compared to conventional systems. Lower first costs are largely attributed to integration with structure and design elements, while lower life cycle costs result from decreased maintenance. 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
Limiting factors
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.
Condensation caused by
humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity dep ...
is a limiting factor for the cooling capacity of a radiant cooling system. The surface temperature should not be equal or below the
dew point temperature in the space. Some standards suggest a limit for the
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity dep ...
in a space to 60% or 70%. An air temperature of 26 °C (79 °F) would mean a dew point between 17 °C and 20 °C (63 °F and 68 °F).
There is, however, evidence that suggests decreasing the surface temperature to below the dew point temperature for a short period of time may not cause
condensation.
Also, the use of an additional system, such as a
dehumidifier
A dehumidifier is an air conditioning device which reduces and maintains the level of humidity in the air. This is done usually for health or thermal comfort reasons, or to eliminate musty odor and to prevent the growth of mildew by extracting w ...
or
DOAS
doas (“dedicated openbsd application subexecutor”) is a program to execute commands as another user. The system administrator can configure it to give specified users privileges to execute specified commands. It is free and open-source unde ...
, can limit humidity and allow for increased cooling capacity.
System description
While there are a broad range of system technologies, there are two primary types of radiant cooling systems. The first type is systems that deliver cooling through the building structure, usually slabs. These systems are also named thermally activated building systems (TABS). The second type is systems that deliver cooling through specialized panels. Systems using concrete slabs are generally cheaper than panel systems and offer the advantage of thermal mass, while panel systems offer faster temperature control and flexibility.
Chilled slabs
Radiant cooling from a slab can be delivered to a space from the floor or ceiling. Since radiant heating systems tend to be in the floor, the obvious choice would be to use the same circulation system for cooled water. While this makes sense in some cases, delivering cooling from the ceiling has several advantages.
First, it is easier to leave ceilings exposed to a room than floors, increasing the effectiveness of thermal mass. Floors offer the downside of coverings and furnishings that decrease the effectiveness of the system.
Second, greater convective heat exchange occurs through a chilled ceiling as warm air rises, leading to more air coming in contact with the cooled surface.
Cooling delivered through the floor makes the most sense when there is a high amount of solar gain from sun penetration, because the cool floor can more easily remove those loads than the ceiling.
Chilled slabs, compared to panels, offer more significant thermal mass and therefore can take better advantage of outside diurnal temperatures swings. Chilled slabs cost less per unit of surface area, and are more integrated with structure.
Chilled beam/ceiling
Radiant/convective heating/cooling systems are generally integrated into slab or dropped ceilings, or attached to ceilings, but can be attached to walls also. The modular nature of ceiling panels offers increased flexibility in terms of placement and integration with lighting or other electrical systems, but are less efficient than chilled beam systems. Lower thermal mass compared to chilled slabs means they can't easily take advantage of
passive cooling from thermal storage, but controls can more quickly adjust to changes in outdoor temperature. Chilled beams/ceilings are also better suited to buildings with spaces that have a greater variance in cooling loads.
Perforated panels also offer better acoustical dampening than chilled slabs. Ceiling panels are very suitable for retrofits because they can be attached to any ceiling. Chilled ceiling panels can be more easily integrated with ventilation supplied from the ceiling.
Thermal comfort
The
operative temperature
Operative may refer to:
* Operative Media, an advertising company, founded 2000
* ''The Operative'' (film), a 2019 thriller film
* The Operative (Firefly), a character from the ''Firefly'' media franchise
* '' The Operative: No One Lives Forever' ...
is an indicator of
thermal comfort
Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation ( ANSI/ASHRAE Standard 55).ANSI/ASHRAE Standard 55-2017, Thermal Environmental Conditions for Human Occupan ...
which takes into account the effects of both convection and radiation. Operative temperature is defined as a uniform temperature of a radiantly black enclosure in which an occupant would exchange the same amount of heat by radiation plus convection as in the actual nonuniform environment.
With radiant systems, thermal comfort is achieved at warmer interior temp than all-air systems for cooling scenario, and at lower temperature than all-air systems for heating scenario.
Thus, radiant systems can helps to achieve energy savings in building operation while maintaining the wished comfort level.
Thermal comfort in radiant vs. all-air buildings
Based on a large study performed using
Center for the Built Environment's
Indoor environmental quality (IEQ)
Indoor(s) may refer to:
*the interior of a building
*Indoor environment, in building science, traditionally includes the study of indoor thermal environment, indoor acoustic environment, indoor light environment, and indoor air quality
*Built envi ...
occupant survey to compare occupant satisfaction in radiant and all-air conditioned buildings, both systems create equal indoor environmental conditions, including acoustic satisfaction, with a tendency towards improved temperature satisfaction in radiant buildings.
Radiant temperature asymmetry
The radiant temperature asymmetry is defined as the difference between the plane radiant temperature of the two opposite sides of a small plane element. As regards occupants within a building,
thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) i ...
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. In general, people are more sensitive to asymmetric radiation caused by a warm ceiling than that caused by hot and cold vertical surfaces. The detailed calculation method of percentage dissatisfied due to a radiant temperature asymmetry is described in ISO 7730.
Design considerations
While specific design requirements will depend on the type of radiant system, a few issues are common to most radiant systems.
* For cooling application, radiant systems can lead
condensation issues. Local climate needs to be evaluated and taken into account in the design. Air dehumidification can be necessary for humid climate.
* Many types of radiant systems incorporate massive building elements. The
thermal mass
In building design, thermal mass is a property of the mass of a building that enables it to store heat and provide inertia against temperature fluctuations. It is sometimes known as the thermal flywheel effect. The thermal mass of heavy structura ...
involved will have a consequence on the thermal response of the system. The operation schedule of a space and the control strategy of the radiant system play a key role in the proper functioning of the system.
* Many types of radiant systems incorporate hard surfaces which influence indoor acoustics. Additional acoustic solutions may need to be considered.
*A design strategy to reduce acoustical impacts of radiant systems is using free-hanging acoustical clouds. Cooling experiments on free-hanging acoustical clouds for an office room showed that for 47% cloud coverage of the ceiling area, 11% reduction in cooling capacity was caused by the cloud coverage. Good acoustic quality can be achieved with only minor reduction of cooling capacity.
Combining acoustical clouds and ceiling fans can offset the modest reduction in cooling capacity from a radiant cooled ceiling caused by the presence of the clouds, and results in increase in cooling capacity.
Hydronic radiant systems
Radiant cooling systems are usually
hydronic
Hydronics () is the use of liquid water or gaseous water (steam) or a water solution (usually glycol with water) as heat-transfer medium in heating and cooling systems. The name differentiates such systems from oil and refrigerant systems.
H ...
, cooling using circulating water running in pipes in thermal contact with the surface. Typically the circulating water only needs to be 2–4 °C below the desired indoor air temperature.
Once having been absorbed by the actively cooled surface, heat is removed by water flowing through a hydronic circuit, replacing the warmed water with cooler water.
Depending on the position of the pipes in the building construction, hydronic radiant systems can be sorted into 4 main categories:
*Embedded Surface Systems: pipes embedded within the surface layer (not within the structure)
*Thermally Active Building Systems (TABS): the pipes thermally coupled and embedded in the building structure (slabs, walls)
*Capillary Surface Systems: pipes embedded in a layer at the inner ceiling/wall surface
*Radiant Panels: metal pipes integrated into panels (not within the structure); heat carrier close to the surface
Types (ISO 11855)
The norm ISO 11855-2
focuses on embedded water based surface heating and cooling systems and TABS. Depending on construction details, this norm distinguishes 7 different types of those systems (Types A to G)
*Type A with pipes embedded in the screed or concrete (“wet” system)
*Type B with pipes embedded outside the screed (in the thermal insulation layer, “dry” system)
*Type C with pipes embedded in the leveling layer, above which the second screed layer is placed
*Type D include plane section systems (extruded plastic / group of capillary grids)
*Type E with pipes embedded in a massive concrete layer
*Type F with capillary pipes embedded in a layer at the inner ceiling or as a separate layer in gypsum
*Type G with pipes embedded in a wooden floor construction
Energy sources
Radiant systems are associated with low-exergy systems. Low-exergy refers to the possibility to utilize ‘low quality energy’ (i.e. dispersed energy that has little ability to do useful work). Both heating and cooling can in principle be obtained at temperature levels that are close to the ambient environment. The low temperature difference requires that the heat transmission takes place over relative big surfaces as for example applied in ceilings or underfloor heating systems.
Radiant systems using low temperature heating and high temperature cooling are typical example of low-exergy systems.
Energy sources such as geothermal (direct cooling / geothermal heat pump heating) and solar hot water are compatible with radiant systems. These sources can lead to important savings in terms of primary energy use for buildings.
Commercial buildings using radiant cooling
Some well-known buildings using radiant cooling include Bangkok's
Suvarnabhumi Airport, the Infosys Software Development Building 1 in Hyderabad,
IIT Hyderabad
Indian Institute of Technology, Hyderabad (abbreviated IIT Hyderabad or IITH) is a public technical research university located in Sangareddy district in the Indian state of Telanganaa. As with all Indian Institutes of Technology (IITs), IIT ...
,
and the San Francisco
Exploratorium. Radiant cooling is also used in many
zero net energy buildings.
Physics
Heat radiation is the energy in the form of
electromagnetic
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
waves emitted by a solid, liquid, or gas as a result of its temperature.
In buildings, the radiant heat flow between two internal surfaces (or a surface and a person) is influenced by the
emissivity
The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is n ...
of the heat emitting surface and by the
view factor
In radiative heat transfer, a view factor, F_, is the proportion of the radiation which leaves surface A that strikes surface B. In a complex 'scene' there can be any number of different objects, which can be divided in turn into even more surface ...
between this surface and the receptive surface (object or person) in the room.
Thermal (longwave) radiation travels at the speed of light, in straight lines.
It can be reflected. People, equipment, and surfaces in buildings will warm up if they absorb thermal radiation, but the radiation does not noticeably heat up the air it is traveling through.
This means heat will flow from objects, occupants, equipment, and lights in a space to a cooled surface as long as their temperatures are warmer than that of the cooled surface and they are within the direct or indirect line of sight of the cooled surface. Some heat is also removed by
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...
because the air temperature will be lowered when air comes in contact with the cooled surface.
The heat transfer by radiation is proportional to the power of four of the absolute surface temperature.
The
emissivity
The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is n ...
of a material (usually written ε or e) is the relative ability of its surface to emit energy by radiation. A black body has an emissivity of 1 and a perfect reflector has an emissivity of 0.
In radiative heat transfer, a
view factor
In radiative heat transfer, a view factor, F_, is the proportion of the radiation which leaves surface A that strikes surface B. In a complex 'scene' there can be any number of different objects, which can be divided in turn into even more surface ...
quantifies the relative importance of the radiation that leaves an object (person or surface) and strikes another one, considering the other surrounding objects. 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. As a person is often changing position and as a room might be occupied by many persons at the same time, diagrams for omnidirectional person can be used.
Thermal response time
Response time (τ95), aka
time constant In physics and engineering, the time constant, usually denoted by the Greek letter (tau), is the parameter characterizing the response to a step input of a first-order, linear time-invariant (LTI) system.Concretely, a first-order LTI system is a s ...
, is used to analyze the dynamic thermal performance of radiant systems. The response time for a radiant system is defined as the time it takes for the surface temperature of a radiant system to reach 95% of the difference between its final and initial values when a step change in control of the system is applied as input.
It is mainly influenced by concrete thickness, pipe spacing, and to a less degree, concrete type. It is not affected by pipe diameter, room operative temperature, supply water temperature, and water flow regime. By using response time, radiant systems can be classified into fast response (τ95< 10 min, like RCP), medium response (1 h<τ95<9 h, like Type A, B, D, G) and slow response (9 h< τ95<19 h, like Type E and Type F).
Additionally, floor and ceiling radiant systems have different response times due to different heat transfer coefficients with room thermal environment, and the pipe-embedded position.
Other HVAC systems that exchange heat by radiation
Fireplaces and woodstoves
See also
*
Glossary of HVAC
HVAC (heating, ventilation, and air conditioning) is a major sub discipline of mechanical engineering. The goal of HVAC design is to balance indoor environmental comfort with other factors such as installation cost, ease of maintenance, and energy ...
References
Further reading
*ASHRAE Handbook. HVAC Systems and Equipment 2012. Chapter 13. Hydronic Heating and Cooling.
*Kessling, W., Holst, S., Schuler, M. Innovative Design Concept for the New Bangkok International Airport, NBIA.
*Olesen, B.W. Radiant Heating and Cooling by Water-based systems. Technical University of Denmark, International Centre for Indoor Environment and Energy.
External links
Radiant cooling research at the Center for the Built Environment*
ttp://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12590/ US Dept of Energy Guide to Radiant Heatingbr>
Infrared Heater Safety CouncilRadiant Panel AssociationMap of buildings using hydronic radiant heating and cooling systems{{HVAC
Environmental design
de:Strahlungsheizung