Principles

An underfloor heating system warms the floor structure, and thus the floor surface, which in turn warms the room or space. Warm air and floor duct systems are not considered to be underfloor heating systems in this context.

The main method of heat transfer from the heated floor surface is by radiation, which amounts to 50% - 60% of output. The balance is made up by convection and conduction.
Because the majority of the heat is radiant it is very effective in creating comfort at slightly lower temperatures than would be expected from a radiator or convector system. A correctly designed and installed system gives rise to only weak air convection currents and temperature stratification, such as is associated with other types of space heating systems.

An underfloor heating system warms the floor structure, and thus the floor surface, which in turn warms the room or space. Warm air and floor duct systems are not considered to be underfloor heating systems in this context.
The main method of heat transfer from the heated floor surface is by radiation, which amounts to 50% - 60% of output. The balance is made up by convection and conduction.

Because the majority of the heat is radiant it is very effective in creating comfort at slightly lower temperatures than would be expected from a radiator or convector system. A correctly designed and installed system gives rise to only weak air convection currents and temperature stratification, such as is associated with other types of space heating systems, and is very significantly reduced.

There is also increase in energy efficiency, as the high temperatures in a room which results from strong convection currents, are absent. Having the feet very slightly warmer than the head is considered to give optimum comfort conditions. If, however, an underfloor heating system is used only for short periods and at infrequent intervals other systems may return lower running costs particularly if a screed or solid floor underfloor heating system is used.

The basic principles of operation of a “wet” underfloor heating system is that heated water is circulated through pipes, which are usually of plastic or plastic metal composite, and are buried into the floor structure. The floor structure must either be conductive itself or conductive elements need to be embedded into the floor to transfer the heat from the pipes to the floor surface. Examples of hoe this is achieved are given in the next page.

Because the floor surface area is large compared to the size of a steel panel radiator, the floor surface temperature required is usually quite low and very close to the actual room temperature. It should, however, be at or below 29°c in all occupied areas so as to achieve an acceptable degree of foot comfort. Lower temperature limits, such as 27°c for timber floors, are sometimes required for delicate structure or surface finishes.

It is essential that floor coverings do not provide too great a degree of insulation or the heat in the underfloor heating system may be able to raise the room temperature to its design level. Care is required, particularly with underlay used with fitted carpets and with laminate floor systems.

Three variables are used to control the underfloor heating system output.

(a) Water temperature
(b) Water flow
(c) Pipe spacing.

Pipe spacing is, of course, fixed once the system is designed and installed so working control is achieved by varying water temperature and flow using either

(a) Room thermostats
(b) Floor thermostats
(c) Weather compensators
(d) A combination of the above

Time control and setback systems are also used, although on//off systems are not favoured because of the slower response of some underfloor heating systems.

Any heat source can be used as many systems are fed through mixing headers. For direct connection condensing boilers and heat pumps can greatly improve the energy efficiency of the underfloor heating system by the lower flow temperatures used.