When planning a primary, electric underfloor heating installation, where the floor heating system is providing the sole source of room heating, it is important to take into consideration some basic principals.
All structures lose and retain heat according to their U-values. The U-value is a measure of how much thermal energy will pass through a square metre of structure when air temperatures on either side differ by 1 degree Celsius. The fabric of any building will typically consist of a number of components, each with their characteristic resistance values; the U-value is a combination of all these values.
In designing a primary under floor heating system, greater consideration of these factors of heat loss and heat gain need to be taken into account. For a typical convection heating system, on cold days, where the temperature on the outside of the structure is low, the room heating can be increased by turning up the boiler and opening up the radiators to improve the heat output.
On the other hand, an underfloor heating system acts as a low-level heat source, radiating thermal energy into the room as radiant heat. Unlike a traditional convection heating system, where a wall radiator or freestanding heater is used as sole source of room heating, the under floor heating cannot be turned up to produce a higher heat output, due to the comfort factor on the feet. Effectively, because we are converting the floor into a low level radiator and walking over it, the human foot factor limits the maximum floor operating temperature to a maximum of 27-28 degrees Celsius.
Electric underfloor heating systems are controlled using a digital thermostat. The controller can be programmed with 4 on/off settings over a 24-hour period. Once the floor heating reaches the programmed floor and room temperature settings, the thermostat will cycle on and off continuously (within 1 degree C) to maintain these settings.
The floor heating will radiate heat energy uniformly, warming the room evenly to your preset ambient thermostat settings, maintaining a comfortable room temperature; providing heat is not being lost to the outside by poor insulation (U Value) and frequent changes of air (drafts).
With a high performance insulation layer between the heating cable and the sub-floor below (concrete or wood), the underfloor heating will operate efficiently and very effectively. For cement sub-floors where no insulation can be fitted, a high-output (200W/m2) heating mat is normally recommended to compensate for the heat loss into the concrete slab below. With no insulation, the floor heating would otherwise be very slow to reach operating temperature; the extra output capacity of a 200Wm2 heating mat is used to good effect to bring the floor up to operating temperature quickly.
