Underfloor heating: Wikis


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Electric floor heating installation, cement being applied

Underfloor heating and cooling is a form of central heating and cooling which achieves indoor climate control primarily through thermal conduction and thermal radiation rather than convection (forced or natural air movement). Heat can be provided by circulating heated water or by electric cable, mesh, or film heaters.


History of underfloor heating

In pre-Roman times underfloor heating was a rare and somewhat radical technology in a world that typically relied on open fireplaces. But not only were fireplaces inefficient in warming an entire room, they were dangerous as well from the risk of fire and smoke inhalation.



Roman world

Caldarium from the Roman Baths at Bath, England. The floor has been removed to reveal the pillars of the hypocaust.

Underfloor heating was first used by the Romans. Initially the preserve of the rich, underfloor heating became increasingly commonplace in public buildings and villas, particularly in the colder regions of the Roman Empire.

The Roman system was based on hypocausts, comprising ducts that underlay the floor (itself built on raised brick piles), and flues that were built into walls. Hot air or steam from fires circulated up through this system, warming the floor and walls, with heat passing into the rooms.

More specifically, the floor was laid out as series of concrete slabs supported by columns of layered tiles, with a furnace at the bottom of one exterior wall. By placing the fire here, the draught (draft) would take the heat under the floor, and up through the walls to chimneys located in the corners of the room. The height of the stack of tiles was about 2 feet (61 cm) as this was found to be the most efficient height for the air to travel through.

Once the air had passed under the floor, the air was drawn into the walls and up the flues by the action of the hot air already rising in the flues creating a partial vacuum and so pulling the air below into the walls. The walls were very often made of bricks with two holes horizontally through them. This had the effect of passing the air through the walls and into the flues, thereby warming the walls also.

In the Roman baths, the furnace was placed next to the hottest room (caldarium) in which three walls of this room were heated so that the room reached a temperature of up to 120 °F (49 °C). The warm room (tepidarium) only had one wall heated which made this room cooler than the caldarium.

The furnace was the heating source of the system and this was placed on the outside of the house, below the floor that ran under the room that was to be the hottest room in the house. One room was always hotter than the rest, as the air flowing under the floor would naturally lose some of its heat as it was traveling under the floor.

The Roman underfloor heating system was a labor intensive device that required constant attention to feed the fire and remove the ashes. (Again, it was originally only the wealthy that could afford to have it.) The fire would need regular attention from a household worker who would have to rake out the ashes with a long handled tool and, using the same tool, push new fuel into the fire.

The fuel was mainly small branches and twigs (up to about 3 in/76 mm in diameter and up to 2 ft/610 mm long), which were placed 2–3 feet (610–910 mm) into the furnace opening. This would allow air to be drawn in and around the wood and so made sure the air flowed freely. Logs were not used as these burned too slowly to be effective, and too many would block the passage of air. The height of the fire was restricted to around half the height of the opening so that air could flow through the flames and so accelerate circulation and increase heat output. This was essential in the baths, where the maximum amount of heat had to be generated.

The hypocaust was recently voted the most important heating invention ever by the British HVAC industry.[1]

Islamic world

The hypocaust continued to be used in the Mediterranean region during late Antiquity and by the Umayyad caliphate. By the 12th century, Muslim engineers in Syria introduced an improved central heating system, where heat travelled through underfloor pipes from the furnace room, rather than through a hypocaust. This central heating system was widely used in bath-houses throughout the medieval Islamic world.[2]

Castile in the Middle Ages

The gloria was inspired by the hypocaust and continued to be used in Castile until recent times.


In contrast to the eventual disappearance of the Roman underfloor hypocausts, underfloor heating has remained in use for millennia in Korea, where it is known as ondol. It is thought that the ondol system dates back to the Koguryo or Three Kingdoms (37 BC-AD 668) period when excess heat from stoves were used to warm homes.

Ondol continues to be a typical feature of the South Korean home, and is widely credited with making possible distinctively Korean customs such as removing one's shoes upon entering a home and sitting on its floor. (The "sitting culture" brought about by ondol influenced the design of hanbok, the traditional Korean outfit; hanbok trousers are loose and have enough room for people to easily bend their knees and sit for long periods of time, and traditional shoes were also made to be easy to take off and put on compared to Western shoes.)

In fact, when Western forms of heating, such as blowers venting hot air, became more widely used in Korea, many families began to miss the ondol system that had long been an integral part of Korean life. As a result, developers in Korea during the 1990s began to discard Western forms of heating, and started to incorporate ondol in new housing developments. Even the most modern Korean hotels offer guests the option of selecting a traditional ondol room with no beds.[citation needed]

Korean ondol technology

Ondol, literally meaning "warm stone", comprised three main components: a fireplace or stove, which is also used for cooking and located below floor level; a heated floor underlaid by horizontal smoke passages; and a vertical chimney, located lower than the roofline, to provide a draft.

The heated floor comprised a network of underground flues that transported heat from the kitchen to each room. These flues were covered by thin, flat, wide stones two or three inches thick called kudul that lay underneath the floor. Kudul, literally meaning "fired stone", was covered with yellow earth, and the floor was leveled. To top it off, several layers of yellow paper sheets were pasted on the floor. This process was efficient since the heat and smoke generated during cooking would be transported automatically to each room in the house. Usually the kitchen would be built at a lower level (about one m), and the heated rooms would be in an elevated position to allow the flues to run underneath. Notably, with just one heating the floors would retain their warmth for extended periods, ranging from more than 30 days to three months depending on the design of the flue structure.

The traditional ondol rooms found in the northern part of the Korean Peninsula differed somewhat from those in the south. In the north the ondol-heated room and the kitchen were not separated by a wall. Heat from both the fireplace and the ondol floor kept the room warm. In the south, a wall separated the kitchen from the living room, preventing the smoke from disturbing people sitting there. Also, in a room heated by ondol, the floor at the far end of the room tended to be cool. (Elders such as grandparents or parents as well as guests were invited to sit in the warmer area as an expression of respect.)

The continuing legacy of ondol

In the early 1900s, when the American architect Frank Lloyd Wright was building the Imperial Hotel in Japan, he was invited to the home of a Japanese nobleman. There Wright found a room that was different from typical Japanese rooms, with a warm floor covered with yellow paper—a Korean ondol room. The Japanese gentleman had experienced ondol in Korea and, once back in Japan, had an ondol room built in his house. "The indescribable comfort of being warmed from below" impressed Wright.

Wright decided then and there that ondol was the ideal heating system and began incorporating it in his buildings. Wright invented radiant floor heating, using hot water running through pipes instead of hot air through flues. In Korea, ondol has likewise been adapted to modern technologies and changes in fuel. Modern Korean homes and apartments are built with heating pipes embedded in floors that are typically concrete covered with vinyl or oiled papers. Heated water circulating through the pipes, warmed by a gas or oil boiler, has replaced heated air, minimizing the danger of carbon monoxide poisoning or burns.

With its modernization, ondol has received international recognition and has become increasingly popular abroad, particularly throughout Asia. Moreover, a new type of ondol product, to which Western living patterns have given birth, is increasing in sales outside of Korea. Tolchimdae, meaning stone bed, has emerged as a hot seller in the furniture market beginning in 1998. Tolchimdae, which first appeared in the market in the early 1990s, is a stone bed filled with either carbon film or copper coils that are electrically heated. Its development was based on the concept of the ondol system. The new product is especially popular among older customers who want to enjoy ondol, but on a Western-style bed.


Modern underfloor heating systems are generally either warm water systems or electric systems. Systems can be poured into a masonry mix (called a poured floor system or a wet system) or fastened directly to the sub floor (called a sub floor system or dry system). The Roman hypocausts (see below) used heated air.

Hot water systems

Underfloor heating pipes, before they are covered by the screed

In a hot-water system, warm water is circulated through pipes or tubes that are laid into the floor (usually a solid-screeded floor, although joist-based systems also work well). Various types of pipe are used including PEX, multi-layer (a composite of PEX, aluminium and PEX) which is also known as Alupex (there is also a version using PERT instead of the PEX) and polybutylene (PB): copper pipes are not normally used.

Because it offers a good balance between cost and pressure drop, 5/8-inch (16 mm) diameter tubing is popular, as 11/16" (19 mm) and 1" (25 mm) tubing are relatively expensive while 3/8" (10 mm) and 1/2" (13 mm) offer too much resistance, requiring more energy to pump the liquid through the pipe. In addition, 5/8" tubing is often the minimum size needed for effective thermosiphon.

In Europe, including the United Kingdom, 15 mm or 16 mm pipe is commonly used, with some companies offering 10 mm, 12 mm and 18 mm.

However, a system designed to use solar-heated water that circulates by thermosiphon is susceptible to blockage by air bubbles. They are hard to avoid where the tubing lies so flat or may have high spots. Bubbles in the water accumulate in the smallest high spots, finally blocking the flow. A small in-line centrifugal pump, 0.05 horsepower (37 W) in rating, can be used for purging. It will circulate water through the tubing fast enough to dislodge an air bubble. The purge pump only activates when the system stagnates and the solar collectors near overheating. When circulation is restored, the pump shuts off.

Gas, oil, solid fuel, or electric-resistance hot-water boilers can be used as the source of heat for any underfloor heating system, as can a number of other technologies. Condensing boilers and ground-coupled heat pumps are particularly well-suited as the large radiating area of underfloor heating systems allows them to operate in their most efficient manner. Underfloor heating can be run at as low a temperature as 30 °C (86 °F), allowing a heat pump to run at a coefficient of performance in excess of 4.0, compared to less than 3.0 with the high temperatures needed 75 °C (167 °F) for use with small wall-mounted radiators. The critical factor for achieving a high coefficient of performance is to reduce the "uplift" or delta-Temperature needed between the heat source and the heat sink [3].

Wet underfloor heating systems can also be used in reverse, where cold water from a chiller or solar cooler is placed in the system taking heat energy out of the building. However, care is needed to ensure that surfaces' temperatures remain above the air's dew point temperature. Otherwise, slipping hazards or mold growth are a concern.

Notes on installation of hot water systems

Architectural/Mechanical Interfaces for Radiant Based HVAC Systems - Click Image for Larger View

Thermal Concerns:

  1. Soil conductivities influencing downward heat loss.
  2. Insulation and vapour barrier details under slabs, cantilevered sections, under heated sub floors, above heated ceilings, behind heated walls and at header and trimmer joists..
  3. Isolating heated/cooled surfaces from ventilation and A/C systems, cold plumbing lines, appliances such as freezers, wine coolers, cold storage areas.
  4. Dew point control for radiant cooling systems

Building Material Concerns:

  1. Selection of wood flooring species, milling (quarter sawn or plane sawn) acclimation period, regulation of relative humidity for dimensional stability and surface temperatures for comfort.
  2. R values of floor assemblies
  3. Control/expansion joints and crack suppression in concrete and tiled surfaces.
  4. Emissivity of surfaces.
  5. Curing times and temperatures for poured floors (concrete, lightweight toppings).

Control System (see Hydronic heating systems):

  1. Fluid temperature in heating and cooling plant.
  2. Fluid temperature in distribution network.
  3. Fluid temperature in the pex piping systems. A function of the spacing, load (Btuh/sf), upward and downward losses and floor r value.
  4. Operative temperature (average of mrt and dry bulb).
  5. Surface temperatures for comfort, safety, and material integrity.

Cost of hot water systems

Although it can be more expensive to install than radiators (it can be comparable due to the increasingly competitive market), wet underfloor heating often proves more economical in the long run, particularly in well-insulated larger properties[citation needed]. Energy savings of up to 40% can be achieved compared to conventional heating systems if a condensing boiler or solar thermal system is installed, but even with a standard boiler up to 15% energy savings are normal.[citation needed] The efficiency of condensing boilers is enhanced thanks to water returning at a lower temperature.

By employing full lengths of piping without any joints, wet underfloor heating loops are practically maintenance free. The piping used can have a lifespan of up to 100 years. Reliable materials are critical because repair is difficult. The central heating equipment, pumps, and controls, like others, requires periodic maintenance and replacement.

Electric floor heating systems

Electric floor heating systems have very low installation cost for smaller spaces (1-5 rooms) because they are easy to install and have a very low start-up cost (a thermostat is all that is required and costs only about $100–$200). Although electric floor heating systems work well as a primary heat source, most systems are installed in the bathroom to add comfort and warmth to cold tile.

Electric floor heating systems are also typically installed in kitchens, bathrooms or in rooms that require additional heat (such as a cold basement, an addition or a kids' playroom). Rooms can be zoned with their own thermostat and programmed around the schedule of the inhabitant, this can add efficiency to an entire homes energy consumption.

Another advantage of electric underfloor heating over a warm-water system is the floor build up/height. Floor build up can be as little as 1 mm. The electric cables are usually installed onto an insulation board or directly onto the subfloor or padding (under carpet or laminate); then the floor covering is placed directly over the heating system or thinset.

Electric underfloor heating also benefits from faster installation times, with a typical installation only taking half day to a day depending on size to install. Also warm up times are generally a lot quicker than "wet" systems because the cables are installed directly below the finished flooring making it a direct acting heat source rather than a storage heater.

Electric system are supplied in several different forms. They can be one long continuous length of cable with the consumer having to weave the cable up and down the floor at a pre-determined spacing and making a return loop to complete the circuit, or they can be provided in mat form, where the heating element is pre-formed in a mat that rolls out onto the floor. Most high voltage cables have a built in return, meaning that you have one end to connect. Most technologies consist of a loop with a start and end. With the introduction of the built in return came the “cable mat”. Cable mats have added to the ease of installing a floor heating system by having the heating cable already pre-spaced on to a nylon mesh. Cable mats however can cause issues for odd shaped spaces where a free rolled cable may be able to cover in a more custom pattern.

One technique is to lay the heating cable directly onto an insulated concrete floor and then apply tile on top of it. Where time-of-use electricity metering is available, this type of system can be turned on at night when electricity rates are low, and then allowed to warm the house during the day by relying on the heat energy held within the thermal mass of the concrete.

Sometimes, in order to minimize floor buildup, a bronze screen or a carbon film heating element is used. Carbon film systems are normally installed onto a thin insulation underlay (approx 6mm) to reduce thermal loss to the sub-floor. Carbon film is used under various floor finishes, traditionally laminate flooring or engineered wood. Vinyls, carpets and other "soft" floor finishes can be heated using carbon film elements or bronze screen elements, provided a suitable overboarding system is used.

In comparison to combustion/hydronic systems, electric systems can be more efficient, if only the efficiency of the equipment in the building is considered. However, as discussed in the article on electric heating, the efficiency of generating electricity from fossil fuels is low. Electric systems however are either on or off and do not require idling times. Electric systems also have the advantage of needing no maintenance and can more easily be controlled to run when and where they are needed. However, electric underfloor heating systems cannot provide cooling in summer.


Thermal comfort

Radiant heating is considered superior to convection methods because warm, buoyant air rises to the ceiling in convection-heated rooms, warming the upper body (often with some discomfort, particularly to the head) but leaving the lower body cooler.

In contrast, in-floor radiant heating warms the lower part of both the room and the body because when warm air convects from the radiant floor surface, it loses approximately two degrees Celsius at two meters above the floor. This imparts a feeling of natural warmth, since the limbs should ideally be warmer than the head. (The most acceptable indoor climate is one in which the floor temperature ranges between 19 and 29 °C and the air temperature at head level ranges between 20 and 24 °C.) The increased mean radiant temperature provided by in-floor radiant heating also contributes to thermal comfort.

Humidification may still be needed for thermal comfort with a radiant system, but, for a given relative humidity, less humidification is needed than for forced-air heating since forced-air systems, when improperly balanced (e.g., because of leaky ducts), can create an exchange of moist inside air with dry outside air.[4]

Dust control

Asthma sufferers may benefit from underfloor heating because it reduces the airborne circulation of both dust and dust mites.[citation needed]


Underfloor heating is invisible from above and does not use valuable wall space with unsightly heating equipment. It has a particular advantage in public areas where exposed hot or sharp surfaces can be dangerous[citation needed] and unsightly.

Low temperature and energy efficiency

Air-infiltration heat loss is reduced significantly compared to forced-air systems in which imbalance due to duct leakage can drive infiltration of outside air into the building. Heating cold air from the outside requires extra energy and decreases humidity, as discussed above.

The system is set to the relatively low temperature required by underfloor heat (30 °C (86 °F)-50 °C (122 °F)), rather than the higher temperature typically used in other types of radiators. In a sense, the entire floor is a radiator, although, because of its area, it need not reach the high temperatures of a conventional steam radiator.

However, this is only possible in a boiler designed for condensing operation; in many boilers, the water temperature in the boiler must be set higher, and reduced by a mixing valve. Depending on the piping layout and insulation, there may also be lower heat losses in the water distribution system because of the lower temperature. Water heated in solar thermal panels can be directly used in underfloor heating.


Electric heat inefficiency

Although electric underfloor heating can deliver almost 100% of the electric energy coming into the building to the heated space, overall system efficiency of electric heating is low because generating electricity from heat in a power plant is less efficient than using the heat directly. In addition, whereas hydronic underfloor systems (and even forced-air systems) can be incorporated as part of an ultra-efficient geo-exchange system, electric underfloor heating does not provide for this option.

Maintenance and repair

Underfloor heating may be difficult to repair and maintain. When placed within screeds the floor will have to be removed in order to reach a collapsed or burst pipe.


Although underfloor heating is generally less expensive to run than radiator heating and more comfortable, the floor covering needs to be compatible: a highly insulating floor covering (such as thick carpets or cork) can reduce the efficiency of underfloor heating to the point where a conversion to underfloor heating may not be practical. Tests carried out by UHMA and the Carpet Federation indicate that a maximum tog value of 2.5 should not be exceeded.

See also

External links



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