Guide to Underfloor Heating
Underfloor heating (UFH), in use since Roman times, is becoming more popular in the UK, particularly for new builds. The advantage of this type of heating is that the whole floor becomes the heat emitter in the room, convection currents do not arise and the room is heated evenly from the floor up.
Conventional wall-mounted radiators set up a convection current and tend to heat up the air closest to the ceiling first, which is less efficient and less comfortable for the room's occupants. UFH has the additional benefits that furniture can be placed anywhere in the room; decorating is simpler; and with no dust traps and low moisture content reducing the number of dust mites, UFH can benefit allergy sufferers too. UFH systems are largely maintenance free with quality systems lasting 25 years or more.
There are basically two types of UFH, electric heating cables and warm water pipes. Both types are set on a well-insulated base to prevent heat loss to the ground and the room temperature is controlled by a room thermostat, as with conventional radiator systems. In practice, users find that the room thermostat can be set lower than with a conventional system because of the extra "comfort factor" of warm feet and cooler head. Electric heating systems have an additional floor thermostat to prevent overheating of the cables. The electric heating elements usually operate at a floor temperature of between 20 and 27 degrees C. A water based UFH system uses water heated to 45 to 65 degrees C, ten to twenty degrees lower than a conventional radiator system, which makes it ideal for use with high efficiency condensing boilers, heat pumps or solar water heaters. This warms the floor to 25 to 28 degrees C, a comfortable temperature to walk on.
Electric heating systems are mainly recommended for use under tiled or other hard floors. It is not recommended that electric heating is installed directly beneath carpet or vinyl flooring without a minimum of 8mm of a flexible levelling compound covering them. The levelling compound will protect the cables, ensure the heat is more evenly distributed and prevent the carpet or vinyl moulding itself to the shapes of the heating cables.
Carpets and tiles act as thermal insulators which means that carpeted floors with UFH take slightly longer to get warm initially but stay warm for longer. With an underfloor system of water pipes buried in screed, the maximum acceptable combined thermal resistance (tog) value of the carpet and underlay recommended by UFH companies in the UK, is 2.5 tog or less.
Electric heating cables buried in screed have a greater tendency to overheat under thicker floor coverings and for this reason manufacturers recommend a combined tog value of 1.5. Rubber crumb-type underlay is recommended and felt underlays should be avoided as they have a greater thermal resistance than crumb rubber. Look out for underlays which have been specially developed for use in conjunction with underfloor heating. They have tog ratings of 0.7 to 0.8.