DESCRIPTION

Radiant heating (underfloor) is arguably superior to convection methods (radiators) 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, underfloor 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.)

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

If the boiler water temperature in a hydronic (wet) system is set to the relatively low temperature required by underfloor heat, rather than the higher temperature typically used in other types of radiators, the boiler may have higher efficiency and lower standby losses. However, this is typically only possible in a condensing boiler; 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.

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 heat pump system, electric underfloor heating does not provide for this option.

TECHNOLOGIES

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).
In a hot-water system, warm water is circulated through pipes or tubes that are laid into the floor (usually a solid-screed floor, although joist-based systems also work well).
In 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 thermo siphon 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/air-source 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 low a temperature as 35 °C (95 °F), allowing a heat pump to run at a coefficient 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 mould growth are a concern.

Notes on installation of hot water systems

Thermal Concerns:
Soil conductivities influencing downward heat loss.
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..
Isolating heated/cooled surfaces from ventilation and A/C systems, cold plumbing lines, appliances such as freezers, wine coolers, cold storage areas.
Dew point control for radiant cooling systems

Building Material Concerns:

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.
R values of floor assemblies
Control/expansion joints and crack suppression in concrete and tiled surfaces.
Emissivity of surfaces.
Curing times and temperatures for poured floors (concrete, lightweight toppings).

Control System:

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

COST

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. 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. 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.

ELECRIC UNDERFLOOR 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 £50-£100). 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 or in rooms that require additional heat (such as a cold basement, an addition or a kids' playroom).

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 sub-floor or padding (under carpet or laminate); then the floor covering is placed directly over the heating system.

Electric underfloor heating also benefits from faster installation times, with a typical installation only taking half day to a day to install depending on size. 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 used to be supplied as 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. The main problem with this was that the installation was time consuming, and also the risk of hot and cold spots due to uneven cable spacing; the closer together the cable the more heat was given off, and visa versa. With today’s technology most modern cables have a built in return, meaning that you only have one end to connect instead of having to bring the end of the cable back to the start to make a full circuit. These are excellent and make the installation a lot easier. With the introduction of the built in return came the “cable mat”. These have revolutionised the electric underfloor heating market due to the simplicity of the installation. Cable mats have taken the hard work out of the installation by having the heating cable already pre-spaced on to a nylon mesh. All you have to do is simply start at your thermostat location and roll it out over the floor until it’s all used up. These save time and offer less risk of having hot and cold spots.
One technique is to lay the heating cable directly onto an insulated concrete floor and then apply tile on top of it. Where economy 7 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 build-up, a screen or carbon film heating element is used. These 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. Vinyl, carpet and other "soft" floor finishes can be heated using carbon film 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, the efficiency of generating electricity from fossil fuels is low, so overall system efficiency is significantly lower than combustion/hydronic systems, unless the electricity comes from renewable sources such wind and solar. Electric systems have the advantage of needing no maintenance and can more easily be controlled to run when and where they are needed.

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