Radiant heating and cooling system
A radiant heating and cooling system refers to temperature-controlled surfaces that exchange heat with their surrounding environment through convection and radiation. By definition, in radiant heating and cooling systems, thermal radiation covers more than 50% of heat exchange within the space. Hydronic radiant heating and cooling systems are water-based. It refers to panels or embedded building components (floors, ceilings or walls). Other types include air-based and electrical systems (which use electrical resistance for heating purpose mainly). Important portions of building surfaces are usually required for the radiant exchange.
- 1 Applications
- 2 System description
- 3 Hydronic radiant systems
- 4 Non-hydronic radiant systems
- 5 References
- 6 External links
Radiant heating and cooling systems can be used in commercial, residential, education, and recreational buildings, museums, hospitals, and other type of buildings. The application depends on the type of radiant system (see below types of radiant systems), on climate conditions and on ventilation system used.
Radiative heat exchange
Heat radiation is the energy in the form of electromagnetic 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 of the heat emitting surface and by the view factor between this surface and the receptive surface (object or person) in the room. The heat transfer by radiation is proportional to the power of four of the absolute surface temperature.
The emissivity 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 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, 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.
Operative temperature and thermal comfort
The operative temperature is an indicator of thermal comfort 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) 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 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.
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 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
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
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.
Notable buildings using radiant systems
|Building||Year||Country||City||Architect||Radiant consultant||Radiant system category|
|Kunsthaus Bregenz||1997||Austria||Bregenz||Peter Zumthor||Meierhans+Partner||Thermally activated building system|
|Suvarnabhumi Airport||2005||Thailand||Bangkok||Murphy Jahn||Transsolar and IBE||Embedded surface systems|
|Zollverein School||2006||Germany||Essen||SANAA||Transsolar||Thermally activated building system|
|Klarchek Information Commons, Loyola University Chicago||2007||United States||Chicago, IL||Solomon Cordwell Buenz||Transsolar||Thermally activated building system|
|Lavin-Bernick Center, Tulane University||2007||United States||New Orleans, LA||VAJJ||Transsolar||Radiant panels|
|David Brower Center||2009||United States||Berkeley, CA||Daniel Solomon Design Partners||Integral Group||Thermally activated building system|
|Manitoba Hydro||2009||Canada||Winnipeg, MB||KPMB Architects||Transsolar||Thermally activated building system|
|Cooper Union||2009||United States||New York, NY||Morphosis Architects||IBE / Syska Hennessy Group||Radiant panels|
|Exploratorium (Pier 15-17)||2013||United States||San Francisco, CA||EHDD||Integral Group||Embedded surface systems|
Non-hydronic radiant systems
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- ASHRAE Handbook. HVAC Systems and Equipment. Chapter 6. Panel Heating and Cooling, American Society of Heating and Cooling, 2012
- Oxford Reference, Oxford University
- Babiak, Jan (2007), PhD Thesis, Low Temperature Heating and High Temperature Cooling. Thermally activated building system, Department of Building Services, Technical University of Denmark
- ISO, EN. 7726. Ergonomics of the thermal environments-Instruments for measuring physical quantities, ISO, Geneva, International Organisation for Standardisation, 1998
- Ning, Baisong; Schiavon, Stefano; Bauman, Fred S. (2017). "A novel classification scheme for design and control of radiant system based on thermal response time". Energy and Buildings. 137: 38–45. doi:10.1016/j.enbuild.2016.12.013. ISSN 0378-7788.
- ISO 11855-1. Building Environment Design - Design, Construction and Operation of Radiant Heating and Cooling Systems - Part 1, ISO, 2012
- Karmann, Caroline; Schiavon, Stefano; Graham, Lindsay T.; Raftery, Paul; Bauman, Fred (December 2017). "Comparing temperature and acoustic satisfaction in 60 radiant and all-air buildings". Building and Environment. 126: 431–441. doi:10.1016/j.buildenv.2017.10.024. ISSN 0360-1323.
- Karmann, Caroline; Bauman, Fred S.; Raftery, Paul; Schiavon, Stefano; Frantz, William H.; Roy, Kenneth P. (March 2017). "Cooling capacity and acoustic performance of radiant slab systems with free-hanging acoustical clouds". Energy and Buildings. 138: 676–686. doi:10.1016/j.enbuild.2017.01.002. ISSN 0378-7788.
- Karmann, Caroline; Bauman, Fred; Raftery, Paul; Schiavon, Stefano; Koupriyanov, Mike (January 2018). "Effect of acoustical clouds coverage and air movement on radiant chilled ceiling cooling capacity". Energy and Buildings. 158: 939–949. doi:10.1016/j.enbuild.2017.10.046. ISSN 0378-7788.
- Babiak, Jan; Olesen, Bjarne W.; Petras, Dusan (2007), Low temperature heating and high temperature cooling: REHVA GUIDEBOOK No 7, REHVA
- ISO 11855-2. Building Environment Design - Design, Construction and Operation of Radiant Heating and Cooling Systems - Part 2, ISO, 2012
- Nielsen, Lars (2012), "Building Integrated System Design for Sustainable Heating and Cooling" (PDF), REHVA journal: 24–27