Underfloor heating and cooling is a form of central heating and cooling which achieves indoor climate control for thermal comfort using conduction, radiation and convection. The terms radiant heating and radiant cooling are commonly used to describe this approach because radiation is responsible for a significant portion of the resulting thermal comfort but this usage is technically correct only when radiation composes more than 50% of the heat exchanged between the floor and the rest of the space.
Underfloor heating has a long history extending back into the Neoglacial and Neolithic periods. Archeological digs in Asia and the Aleutian islands of Alaska reveal how the inhabitants drafted smoke from fires through stone covered trenches which were excavated in the floors of their subterranean dwellings. The hot smoke heated the floor stones which then radiated into the living spaces. These early forms have evolved into modern systems using fluid filled pipes or electrical cables and mats. Below is a chronological overview of under floor heating from around the world.
Modern underfloor heating systems use either electrical resistance elements ("electric systems") or fluid flowing in pipes ("hydronic systems”) to heat the floor. Either type can be installed as the primary, whole-building heating system or as localized floor heating for thermal comfort. Electrical resistance can only be used for heating so when space cooling is also required, hydronic systems are used. Other applications for which either electric or hydronic systems are suited include snow/ice melting for walks, driveways and landing pads, turf conditioning of football and soccer fields and frost prevention in freezers and skating rinks.
Hydronic systems use water or a mix of water and anti-freeze such as propylene glycol as the heat transfer fluid in a "closed loop" that is recirculated between the floor and the boiler.
Electric systems are used only for heating and employ non-corrosive, flexible heating elements including cables, pre-formed cable mats, bronze mesh, and carbon films. Due to their low profile they can be installed in a thermal mass or directly under floor finishes. Electric systems can also take advantage of time-of-use electricity metering and are frequently used as carpet heaters, portable under area rug heaters, under laminate floor heaters, under tile heating, under wood floor heating, and floor warming systems thermal comfort is, “that condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation.” Relating specifically to underfloor heating, thermal comfort is influenced by floor surface temperature and associated elements such as radiant asymmetry, mean radiant temperature and operative temperature. Research by Nevins, Rohles, Gagge exchange over 50% of their sensible heat via radiation.
Underfloor heating can have a positive effect on the quality of indoor air by facilitating the choice of otherwise perceived cold flooring materials such as tile, slate, terrazzo and concrete. These masonry surfaces typically have very low VOC emissions (volatile organic compounds) in comparison to other flooring options. In conjunction with moisture control, floor heating also establishes temperature conditions that are less favorable in supporting mold, bacteria, viruses and dust mites. By removing the sensible heating load from the total HVAC (Heating, Ventilating, and Air Conditioning) load, ventilation, filtration and dehumidification of incoming air can be accomplished with dedicated outdoor air systems having less volumetric turnover to mitigate distribution of airborne contaminates. There is recognition from the medical community relating to the benefits of floor heating especially as it relates to allergens.
Under floor radiant systems are evaluated for sustainability through the principles of efficiency, entropy, exergy and efficacy. When combined with high performance buildings, under floor systems operate with low temperatures in heating and high temperatures in cooling in the ranges found typically in geothermal and solar thermal systems. When coupled with these non combustible, renewable energy sources the sustainability benefits include reduction or elimination of combustion and green house gases produced by boilers and power generation for heat pumps and chillers, as well as reduced demands for non renewables and greater inventories for future generations. This has been supported through simulation evaluations and though research funded by the U.S. Department of Energy, Canada Mortgage and Housing Corporation, Fraunhofer Institute as well as ASHRAE.
Low temperature underfloor heating is embedded in the floor or placed under the floor covering as such it occupies no wall space and creates no burn hazards nor is it a hazard for physical injuries due to accidental contact leading to tripping and falling. This has been referenced as a positive feature in healthcare facilities including those serving elderly clients and those with dementia. Anecdotally, under similar environmental conditions, heated floors will speed evaporation of wetted floors . Additionally, underfloor heating with fluid filled pipes is useful in heating and cooling explosion proof environments where combustion and electrical equipment can be located remotely from the explosive environment.
Equipment maintenance and repair is the same as for other water or electrical based HVAC systems except when pipes, cables or mats are embedded in the floor. Early trials (for example homes built by Levitt and Eichler, c. 1940-70’s) experienced failures in embedded copper and steel piping systems as well as failures assigned by the courts to Shell, Goodyear and others for polybutylene and EPDM materials. There also have been a few publicized claims of failed electric heated gypsum panels from the mid 90’s.
The engineering of underfloor cooling and heating systems is governed by industry standards and guidelines.
The amount of heat exchanged from or to an underfloor system is based on the combined radiant and convective heat transfer coefficients.
- Radiant heat transfer is constant based on the Stefan–Boltzmann constant.
- Convective heat transfer changes over time depending on
- the air's density and thus its buoyancy. Air buoyancy changes according to surface temperatures and
- forced air movement due to fans and the motion of people and objects in the space.
When heated and cooled pipes or heating cables share the same spaces as other building components, parasitic heat transfer can occur between refrigeration appliances, cold storage areas, domestic cold water lines, air conditioning and ventilation ducts. To control this, the pipes, cables and other building components must all be well insulated.
Underfloor heating and cooling systems can have several control points including the management of:
- Fluid temperatures in the heating and cooling plant (e.g. boilers, chillers, heat pumps).
- Influences the efficiency
Illustrated is a simplified mechanical schematic of an underfloor heating and cooling system for thermal comfort quality with a separate air handling system for indoor air quality. In high performance residential homes of moderate size (e.g. under 3000 ft (278 m) total conditioned floor area), this system using manufactured hydronic control appliances would take up about the same space as a three or four piece bathroom.
Modeling radiant piping (also tube or loop) patterns with finite element analysis(FEA) predicts the thermal diffusions and surface temperature quality or efficacy of various loop layouts. The performance of the model and image to the right are useful to gain an understanding in relationships between flooring resistances, conductivities of surrounding mass, tube spacing’s, depths and fluid temperatures. As with all FEA simulations, they depict a snap shot in time for a specific assembly and may not be representative of all floor assemblies nor for system that have been operative for considerable time in a steady state condition. The practical application of FEA for the engineer is being able to assess each design for fluid temperature, back losses and surface temperature quality. Through several iterations it is possible to optimize the design for the lowest fluid temperature in heating and the highest fluid temperature in cooling which enables combustion and compression equipment to achieve its maximum rated efficiency performance.
Thermography is a useful tool to see the actual thermal efficacy of an underfloor system from its start up (as shown) to its operating conditions. In a startup it is easy to identify the tube location but less so as the system moves into a steady state condition. It is important to interpret thermographic images correctly. As is the case with finite element analysis (FEA), what is seen, reflects the conditions at the time of the image and may not represent the steady conditions. Thermography can also point out flaws in the building enclosures (left image, corner intersection detail), thermal bridging (right image, studs) and the heat losses associated with exterior doors (center image).
There is a wide range of pricing for underfloor systems based on regional differences, application and project complexity. It is widely adopted in the Nordic, Asian and European communities consequently the market is more mature and systems relatively more affordable than North America where market share for fluid based systems remains between 3% to 7% of HVAC systems (ref. Statistics Canada and U.S. Census Bureau).
System efficiency and energy use analysis takes into account building enclosure performance, efficiency of the heating and cooling plant, system controls and the conductivities, surface characteristics, tube/element spacing and depth of the radiant panel, operating fluid temperatures and wire to water efficiency of the circulators. The efficiency in electric systems is analyzed by similar processes and includes the efficiency of electricity generation.
System efficiency is also affected by the floor covering serving as the radiational boundary layer between the floor mass and occupants and other contents of the conditioned space. For example, carpeting has a greater resistance or lower conductance than tile. Thus carpeted floors need to operate at higher internal temperatures than tile which can create lower efficiencies for boilers and heat pumps. However, when the floor covering is known at the time the system is installed then the internal floor temperature required for a given covering can be achieved through proper tube spacing without sacrificing plant efficiency (though the higher internal floor temperatures may result in increased heat loss from the non-room surfaces of the floor).
Relevant Industry Organizations, Institutes and Associations (based on contributions to scientific research, standards development and professional education for engineers, architects, interior designers and trades)
- American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)
- ASHRAE Technical Committee TC 6.5, Radiant and Convective Space Heating and Cooling (ASHRAE T.C. 6.5)
- ASHRAE Technical Committee TC 6.1, Hydronic & Steam Heating Equipment & Systems (ASHRAE T.C. 6.1)
- American Institute of Architects (AIA)
- American Society of Interior Designers (ASID)
- Canada Mortgage and Housing Corporation (CMHC)
- Canadian Institute of Plumbing & Heating (CIPH)
- Dutch Building Services Knowledge Centre (ISSO)
- Dwellings Energy Assessment Procedure - Ireland (DEAP)
- Federation of European Heating and Air-Conditioning Associations (REHVA)
- Hydronics Industry Alliance (HIA)
- Heating, Refrigeration and Air Conditioning Institute of Canada (HRAI)
- International Energy Agency, Energy Conservation in Buildings and Community Systems (IEA/ECBCS)
- International Organization for Standardization, TC 205/WG 8, Radiant heating and cooling systems (ISO TC205/WG8)
- National Research Council Canada / NRC Institute for Research in Construction, Hydronic Radiant Floor Heating (NRC/IRC)
- Radiant Panel Association (RPA)
- Thermal Environmental Comfort Association (TECA)
