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Underfloor heating facts for kids

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Underfloor heating pipes
Underfloor heating pipes, before they are covered by the screed

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 convection. Use of underfloor heating dates back to the Neoglacial and Neolithic periods.


Underfloor heating has a long history 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 and the heat 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.

Time period, c. BC Description
5,000 Evidence of "baked floors" are found foreshadowing early forms of kang and dikang "heated floor" later ondol meaning "warm stone" in Manchuria and Korea respectively.
3,000 Korean fire hearth, was used both as kitchen range and heating stove.
1,000 Ondol type system used in the Aleutian Islands, Alaska and in Unggi, Hamgyeongbuk-do (present-day North Korea).
1,000 More than two hearths were used in one dwelling; one hearth located at the center was used for heating, the other at the perimeter was used for cooking throughout the year. This perimeter hearth is the initial form of the budumak (meaning kitchen range), which composes the combustion section of the traditional ondol in Korea.
500 Romans scale up the use of conditioned surfaces (floors and walls) with the invention of the hypocausts.
200 Central hearth developed into gudeul (meaning heat releasing section of ondol) and perimeter hearth for cooking became more developed and budumak was almost established in Korea.
50 China, Korea and Roman Empire use kang, dikang/ondol and hypocaust respectively.
Time period, c. AD Description
500 Asia continues to use conditioned surfaces but the application is lost in Europe where it is replaced by the open fire or rudimentary forms of the modern fireplace. Anecdotal literary reference to radiant cooling system in the Middle East using snow packed wall cavities.
700 More sophisticated and developed gudeul was found in some palaces and living quarters of upper-class people in Korea. Countries in the Mediterranean Basin (Iran, Algeria, Turkey et al.) use various forms of hypocaust type heating in public baths and homes (ref.: tabakhana, atishkhana, sandali) but also use heat from cooking (see:tandoor, also tanur) to heat the floors.
1000 Ondol continues to evolve in Asia. The most advanced true ondol system was established. The fire furnace was moved outside and the room was entirely floored with ondol in Korea. Europe uses various forms of the fireplace with the evolution of drafting combustion products with chimneys.
1300 Hypocaust type systems used to heat monasteries in Poland and teutonic Malbork Castle.
1400 Hypocaust type systems used to heat Turkish Baths of the Ottoman Empire.
1500 Attention to comfort and architecture in Europe evolves; China and Korea continue to apply floor heating with wide scale adoption.
1600 In France, heated flues in floors and walls are used in greenhouses.
1700 Benjamin Franklin studies the French and Asian cultures and makes note of their respective heating system leading to the development of the Franklin stove. Steam based radiant pipes are used in France. Hypocaust type system used to heat public bath (Hammam) in the citadel town of Erbil located in modern-day Iraq.
1800 Beginnings of the European evolution of the modern water heater/boiler and water based piping systems including studies in thermal conductivities and specific heat of materials and emissivity/reflectivity of surfaces (Watt/Leslie/Rumford). Reference to the use of small bore pipes used in the John Soane house and museum.
1864 Ondol type system used at Civil War hospital sites in America. Reichstag building in Germany uses the thermal mass of the building for cooling and heating.
1899 The earliest beginnings of polyethylene-based pipes occur when German scientist, Hans von Pechmann, discovered a waxy residue at the bottom of a test tube, colleagues Eugen Bamberger and Friedrich Tschirner called it polymethylene but it was discarded as having no commercial use at the time.
1904 Liverpool Cathedral in England is heated with system based on the hypocaust principles.
1905 Frank Lloyd Wright makes his first trip to Japan, later incorporates various early forms of radiant heating in his projects.
1907 England, Prof. Barker granted Patent No. 28477 for panel warming using small pipes. Patents later sold to the Crittal Company who appointed representatives across Europe. A.M. Byers of America promotes radiant heating using small bore water pipes. Asia continues to use traditional ondol and kang—wood is used as the fuel, combustion gases sent under floor.
1930 Oscar Faber in England uses water pipes used to radiant heat and cool several large buildings.
1933 Explosion at England's Imperial Chemical Industries (ICI) laboratory during a high pressure experiment with ethylene gas results in a wax like substance—later to become polyethylene and the re-beginnings of PEX pipe.
1937 Frank Lloyd Wright designs the radiant heated Herbert Jacobs house, the first Usonian home.
1939 First small scale polyethylene plant built in America.
1945 American developer William Levitt builds large scale developments for returning GIs. Water based (copper pipe) radiant heating used throughout thousands of homes. Poor building envelopes on all continents require excessive surface temperatures leading in some cases to health problems. Thermal comfort and health science research (using hot plates, thermal manikins and comfort laboratories) in Europe and America later establishes lower surface temperature limits and development of comfort standards.
1950 Korean War wipes out wood supplies for ondol, population forced to use coal. Developer Joseph Eichler in California begins the construction of thousands of radiant heated homes.
1951 Dr. J. Bjorksten of Bjorksten Research Laboratories in Madison, WI, announces first results of what is believed to be the first instance of testing three types of plastic tubing for radiant floor heating in America. Polyethylene, vinyl chloride copolymer, and vinylidene chloride were tested over three winters.
1953 The first Canadian polyethylene plant is built near Edmonton, Alberta.
1960 NRC researcher from Canada installs underfloor heating in his home and later remarks, "Decades later it would be identified as a passive solar house. It incorporated innovative features such as the radiant heating system supplied with hot water from an automatically stoked anthracite furnace."
1965 Thomas Engel patents method for stabilizing polyethylene by cross linking molecules using peroxide (PEx-A) and in 1967 sells license options to a number of pipe producers.
1970 Evolution of Korean architecture leads to multistory housings, flue gases from coal based ondol results in many deaths leading to the removal of the home based flue gas system to a central water based heating plants. Oxygen permeation becomes corrosion issue in Europe leading to the development of barriered pipe and oxygen permeation standards.
1980 The first standards for floor heating are developed in Europe. Water-based ondol system is applied to almost all of residential buildings in Korea.
1985 Floor heating becomes a traditional heating systems in residential buildings in Middle Europe and Nordic countries and increasing applications in non-residential buildings.
1995 The application of floor cooling and thermal active building systems (TABS) in residential and commercial buildings are widely introduced into the market.
2000 The use of embedded radiant cooling systems in the middle of Europe becomes a standard system with many parts of the world applying radiant based HVAC systems as means of using low temperatures for heating and high temperatures for cooling.
2010 Radiant conditioned Pearl River Tower in Guangzhou, China, topped out at 71-stories.


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. Some systems allow for single rooms to be heated when they are a part of a larger multi-room system, avoiding any wasted heat. Electrical resistance can only be used for heating; when space cooling is also required, hydronic systems must be 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. A range of underfloor heating systems and designs are available to suit different types of flooring.

Electric heating elements or hydronic piping can be cast in a concrete floor slab ("poured floor system" or "wet system"). They can also be placed under the floor covering ("dry system") or attached directly to a wood sub floor ("sub floor system" or "dry system").

Some commercial buildings are designed to take advantage of thermal mass which is heated or cooled during off-peak hours when utility rates are lower. With the heating/cooling system turned off during the day, the concrete mass and room temperature drift up or down within the desired comfort range. Such systems are known as thermally activated building systems or TABS.

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 exchange between the floor and the rest of the space.

Hydronic systems

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.

Various types of pipes are available specifically for hydronic underfloor heating and cooling systems and are generally made from polyethylene including PEX, PEX-Al-PEX and PERT. Older materials such as Polybutylene (PB) and copper or steel pipe are still used in some locales or for specialized applications.

Hydronic systems require skilled designers and tradespeople familiar with boilers, circulators, controls, fluid pressures and temperature. The use of modern factory assembled sub-stations, used primarily in district heating and cooling, can greatly simplify design requirements and reduce the installation and commissioning time of hydronic systems.

Hydronic systems can use a single source or combination of energy sources to help manage energy costs. Hydronic system energy source options are:

  • Boilers (heaters) including Combined heat and power plants heated by:
    • Natural gas or "methane" industry-wide is considered the cleanest and most efficient method of heating water, depending on availability. Costs about $7/million b.t.u.
    • Propane mainly made from oil, less efficient than natural gas by volume, and generally much more expensive on a b.t.u. basis. Produces more carbon dioxide than "methane" on a b.t.u. basis. Costs about $25/million b.t.u.
    • Coal, oil, or waste oil
    • Electricity
    • Solar thermal
    • Wood or other biomass
    • Bio-fuels
  • Heat pumps and chillers powered by:
    • Electricity
    • Natural gas
    • Geothermal heat pump

Electric systems

Electric floor heating installation, cement being applied

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, including under shower floor and seat heating. Large electric systems also require skilled designers and tradespeople but this is less so for small floor warming systems. Electric systems use fewer components and are simpler to install and commission than hydronic systems. Some electric systems use line voltage technology while others use low voltage technology. The power consumption of an electric system is not based on voltage but rather wattage output produced by the heating element.


Airflow from vertical temperature gradients

UFAD Air Stratification Example Diagram
Vertical temperature gradient, caused by stable stratification of air inside a room without underfloor heating. The floor is over three degrees Celsius colder than the ceiling.

Thermal comfort quality

As defined by ANSI/ASHRAE Standard 55 – Thermal Environmental Conditions for Human Occupancy, 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, P. Ole Fanger et al. show that humans at rest with clothing typical of light office and home wear, exchange over 50% of their sensible heat via radiation.

Underfloor heating influences the radiant exchange by warming the interior surfaces. The heating of the surfaces suppresses body heat loss resulting in a perception of heating comfort. This general sensation of comfort is further enhanced through conduction (feet on floor) and through convection by the surface's influence on air density. Underfloor cooling works by absorbing both short wave and long wave radiation resulting in cool interior surfaces. These cool surfaces encourage the loss of body heat resulting in a perception of cooling comfort. Localized discomfort due to cold and warm floors wearing normal footwear and stocking feet is addressed in the ISO 7730 and ASHRAE 55 standards and ASHRAE Fundamentals Handbooks and can be corrected or regulated with floor heating and cooling systems.

Indoor air quality

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, underfloor 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 greenhouse 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 through research funded by the U.S. Department of Energy, Canada Mortgage and Housing Corporation, Fraunhofer Institute ISE as well as ASHRAE.

Safety and health

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 (showering, cleaning, and spills). 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.

There is a likelihood that underfloor heating may add to offgassing and sick building syndrome in an environment, particularly when the carpet is used as flooring.

Electric underfloor heating systems cause low frequency magnetic fields (in the 50–60 Hz range), old 1-wire systems much more so than modern 2-wire systems. The International Agency for Research on Cancer (IARC) has classified static and low-frequency magnetic fields as possibly carcinogenic (Group 2B).

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