Underfloor heating
Underfloor heating is a unique traditional form of central heating gaining newfound popularity. It primarily utilizes radiant heat for indoor climate control, whereas other forms of indoor heating (for example, from a radiator, blower or fireplace) rely mostly on convection.
Underfloor heating can be used with concrete and wooden floors, with all types of floor covering (e.g., stone, tile, wood, vinyl, and carpet) and at ground level or upstairs, although choice of floor finishing requires careful consideration because changes of floor finish may affect performance.
Advantages
Thermal comfort
Radiant heating is arguably superior to convection methods because warm, buoyant air rises wastefully 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.)
Humidification may still be needed for thermal comfort with a radiant system, but, for a given relative humidity, likely less humidification is needed than for forced-air heating due to the lower supply air temperature. Asthma sufferers may benefit from underfloor heating because it reduces the airborne circulation of both dust and dust mites.
Aesthetics
Underfloor heating is invisible from above and does not use valuable wall space with unsightly heating equipment. In a sense, the entire floor is a radiator, although, because of its area, it need not reach the high temperatures of a steam radiator. It has a particular advantage in public areas where exposed hot or sharp surfaces can be dangerous and unsightly.
Energy efficiency
Air-infiltration heat loss is reduced slightly with radiant heat because with radiant systems, the air is only warmed to the temperature of the thermostat setting, so the temperature differential at the outside wall is less, thereby reducing air infiltration due to the stack effect. This is because air infiltration and exfiltration increase as the difference between inside and outside temperature [AT] becomes larger, and so when heated air from a furnace or baseboard heater flows against relatively cold exterior walls, the increased temperature differential results in a stack effect that draws cold air into the house through any cracks.
Also, glass, particularly low-e glass, reflects long-wave radiance produced by residential radiant systems, creating a "greenhouse effect" that serves to contain radiant energy within the heated building cavity, reducing heat loss.
Technologies
Underfloor heating systems are generally either warm water (wet) systems or electric (dry) systems. The former are more common than the latter.
Hot water systems
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, ⅝-inch diameter tubing is popular: ¾-inch and 1-inch tubing are relatively expensive, and ⅜-inch and ½-inch offer too much resistance, which means more energy consumption to pump the liquid through the pipe; and the ⅝-inch tubing is often the minimum size needed for effective thermosiphon.
(In the UK and Europe, 15mm or 16mm pipe is commonly used, with some companies offering 10mm, 12mm and 18mm.)
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, 1/20th of a horsepower (37 watts) 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 operation of underfloor heating systems allows them to operate in their most efficient manner. Underfloor heating can run as at low a temperature as 35 degrees Celsius, allowing a heat pump to run at an coefficient of performance in excess of 4.0, compared to the 3.0 with the temperatures needed for use with wall radiators.
Wet underfloor heating systems can also be used in reverse, where cold water from a chiller 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.
Cost of hot water systems
Although it can be more expensive to install than radiators (although 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. Energy savings of up to 40% can be achieved compared to conventional heating systems if a condensing boiler is installed, but even with a standard boiler up to 15% energy savings are normal [1]. 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 systems
One of the main advantages of electric underfloor heating over a warm-water system is the floor build up/height. Floor build up can be from as little as 2mm. The electric cables are usually installed onto an insulation board, then either tiled directly over using a flexible tile adhesive, or covered using a latex leveling compound if another floor covering is being laid.
Electric underfloor heating also benefits from faster warm up times as the cables are installed directly below the finished flooring.
One technique is to lay the heating cable directly onto an insulated concrete floor and then apply tile on top of it. 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 radiant heat held within the thermal mass of the concrete.
Sometimes to mimimize floor buildup a screen heating element is used for low voltage systems. ZMesh is a bronze screen heating element no thicker than screen door material. ZMesh is a versatile heating element for any floor covering and can be used for retrofit applications.
In comparison to hydronic systems, electric systems are more efficient and cheaper to run. Upfront costs may be more for an electric system but the efficiency rate of an electric system contrasted with no maintenance and the fact that it only runs when it needs to, gives the electric system a more cost effective advantage over traditional forced air and hydronic heating solutions.
Environmental issues
Despite its ease of installation, and in common with other forms of direct electric heating, electric underfloor heating is not considered environmentally friendly. Although—if the floor is properly insulated—most of the heat produced will warm the room, most electricity is generated remotely from fossil fuels. Up to two-thirds of energy in the fuel can be lost at the power station and in transmission losses. In Sweden there are proposals to phase out direct electric heating for this reason—see oil phase-out in Sweden.
In contrast, hot-water systems can use water heated in or close to the building, using high-efficiency condensing boilers, biofuels, or district heating, as well as currently less common technologies such as heat pumps and solar combisystems. Because the source of the heat can be changed relatively easily, and independently of the heating pipes in the floor, wet systems are also relatively future-proof.
Wet underfloor heating is also preferred to conventional radiator heating systems due to similar future-proofing issues. Radiator systems are normally sized to use hot water (in the region of 80 °C or 175 °F), whereas underfloor heating is designed to use much cooler water, making it much more suited for future use with heat pumps and solar heating. The efficiency of heat pump or ground source energy systems is much higher when used in conjunction with underfloor heating because of the lower flow temperature the heat pump has to work less.
Another issue in wet underfloor heating is that embedding heating pipes in concrete requires the concrete to be heated as well as the room space, reducing the responsiveness of the heating system. This is an issue in buildings with varying temperatures during the day, such as homes, requiring the building to be heated for longer and adding to the energy requirements. This is accounted for in the UK's SAP and the Irish DEAP building energy rating assessment criteria. One way to improve heating responsiveness in wet underfloor systems is to lay heating pipes in an insulated wooden floor instead of in screed or concrete slab.
History
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.
Rome
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 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 ft (60 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 (50 °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 workers 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 inches in diameter and up to two feet long which was placed 2-3 feet into the furnace opening. This was allow the air to be drawn in and around the wood and so make 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.
Korea
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 B.C-A.D. 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.
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 underlayed 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 1900's, 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.
External links
- Design details
- Articles on underfloor heating technologies, products, uses and relevance are available here.
- [2] Ireland's Dwellings Energy Assessment Procedure (DEAP)