Underfloor Heating

Underfloor Heating System (UFH) is generally defined as that which warms the floor structure, including the floor surface, in order to heat the room or space above.

The method to heat transfer from the heated floor surface is circa 55% by radiation plus 45% by convection depending on the surface to air temperature differential. Because the majority of the heat is radiant, it creates occupancy comfort at lower air temperatures than would be expected from radiators, convectors and warm air systems.

Properly designed and installed underfloor heating systems produce a uniform room temperature with almost no stratification. The absence of heat stratification benefits fuel economy appreciably and, in addition, years of experience has proved that when an occupant´s feet are slightly warmer than the head, optimum human comfort conditions are assured.

Warm water underfloor heating systems operate by circulating a fluid through pipes of mostly plastic or metal/plastic composite materials embedded in floor structures. Therefore flooring components need to be conductive or have conductive metal elements provided in the floor to transfer the heat from the pipes to the floor surface.

Because heated floor surfaces are unusually large when compared to the size of a steel panel radiator, the floor surface temperature required is very low, indeed only a few degrees more than room temperature. However, it should be at or below 29ºC in all occupied areas so as to achieve an acceptable degree of foot comfort. Lower temperature limits, such as 27ºC for timber floors, are primarily required for protecting delicate structures or surfaces finishes.

Any boiler or heat generator or heat source can be used with underfloor heating but direct connection to Condensing Boilers and Heat Pumps improve efficiency resulting from the lower flow and return water temperatures normally used in underfloor heating systems.

Most rooms in modern well-insulated properties have a required design heat flow density in the range of 50-70 W/m2, well within the limitations of UFH systems.

Floor surface temperature is critical to comfort, as well as to heat output. New well insulated houses very often require only 26ºC floor surface temperature from UFH systems; this equates to a UFH output of around 66 W/m2 (based on 10.8 W/m2 per degree of temperature difference between the mean floor surface temperature and test standard room air temperature of 20ºC). BS EN 1264 Floor Heating, Systems and Components, Part 2. Determination of Thermal Output gives guidelines on the maximum values for floor surface temperatures.

It is normally recommended that the floor finish covering resistance R does not exceed 0.15 m2 K/W.

Distribution manifolds or headers are normally constructed from a non-ferrous metal, but engineering plastic versions are also available. All are configured with a flow barrel, and a return barrel, with individual circuit isolating valves, circuit regulating valves, drain cocks, air-vents and wall brackets.

Manifold positions need to be located strategically to minimise the amount of uncontrolled energy and the length of the circuits.

Pipe materials should be restricted to those specified within BS EN 1264 and BS 7291, namely Polybutylene (PB), Cross-linked Polyethylene (PE-X), PE-RT Polypropylene (PPC-2), Multilayer Plastic/Aluminium composite or Soft Annealed Copper Tube (BS EN 1057).

It is recommended the addition of a suitable corrosion inhibitor to the water in circulation to protect the appliance and other fittings during use.