Underfloor heating warms the floor structure. Heat is transmitted to the floor surface, which in turn warms the room. Heated water circulates through pipes laid in the floor in circuits. The heated water is distributed through under-floor heating manifolds with up to 12 circuits per manifold. 

The two standard flooring systems within which most under-floor systems are installed are:

• screed or solid floor system, which relies on conductivity of the screed to transfer heat from the pipes to the floor surface

•  a timber floor system, which relies on components fitted within the floor, usually aluminium heat transfer sheets, but also sometimes a thin screed or heated air layer, to transfer heat to the floor surface.
   

  

The floor surface temperature is controlled by water flow temperature, mass flow rate through the pipes and pipe spacing. Since the pipe spacing is fixed after installation, room temperature control is achieved by varying flow temperature (common with conventional boiler heating systems), using a thermostatic mixing valve, or mass flow rate of water through the room, by closing individual circuits on the manifolds (common with heat pump heating systems).

Since the floor surface area is large compared to conventional heating systems, a lower floor surface temperature is required to produce the same heat output. Typical floor surface temperatures are around 20 to 25 ºC, whereas the surface temperature of a radiator is 50 to 60 ºC. 50 to 60% of the heat output from an under-floor heating system is radiant, with the balance made up by convection and conduction.

Under-floor heating systems are able to provide comfort at slightly lower air temperatures than conventional heating systems due to the large proportion of the output that is radiant. A 1ºC reduction in air temperature is used to reflect the added value of radiant heat acting on a person. Conventional systems use a design temperature of 21ºC, so with an under-floor system it is necessary to design for a room temperature of only 20ºC. 

Floor structures & finishes

Although it should be possible to fit underfloor to any normal floor, basic requirements are that

• the temperature within the floor structure is not raised to a level at which the strength and characteristics of the floor are adversely affected

• insulation of adequate thickness and value is provided to limit downward heat losses through the floor, so that they are very little more than they would be with a conventional heating system

• there is a continuous conducting path from the underfloor pipes to the floor surface for effective heat transfer

• the floor finish has a sufficiently low resistance to enable the heat to be emitted into the room (most floor finishes are suitable as long as the underside of floor is well insulated)

• the maximum design floor surface temperatures are within the manufacturer’s maximum temperature limits for the floor finish

• consideration is given to any other services contained in the floor that may be affected by the increased temperature, for example, cold water pipes (which may heat and cause a risk of proliferation of the Legionella bacillus) and electrical power cables (which may be at risk of overheating).