Underfloor Heating: Getting the Basics Right | Peter Arrow


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There is no better way to increase the thermal comfort of a living space than by introducing an underfloor heating system. Here, Peter Arrow EngTech MIET provides guidance on best practices to ensure a safe and long-lasting installation. The Romans invented underfloor heating systems, by heating water pipes that traveled through the ground. With advances in technology, we are now able to provide electrical solutions that meet this need. However, there is a little more to the installation process than simply sticking a mat on the floor. It is important to remember that there are two main types of underfloor heating: a mat, where the cable is fixed with a grid, and with double-sided tape attached to the floor. Single cable The cable is fed from a drum and fixed to the floor (usually by hot glue). Both methods provide the same overall effect of fast-acting radiant heat in the room. Radiant heat is very similar to the way we feel from the heat of the sun. It does not actively heat the room, but instead the objects inside it when it intercepts the radiant waves. We also use electric floor heating to achieve conductive heat. This is best described when we feel the warmth of a heat source on our feet through the floor covering. This method can be ideal when removing the cold from tiled surfaces in the colder months. Check the capacity of local circuits When designing an underfloor heating system, it is a good idea to check the capacity of local circuits and whether you need to run in a dedicated source (Annex 15 of BS7671 gives instructions). In extreme cases, you may even need to contact the DNO to upgrade the incoming supply. One detail missing while scanning is the height up to the final floor height. An existing subfloor to be covered with a vinyl tile floor covering will need an average build-up of about 24mm, including thermal shingle, cable, self-leveling compound and floor finish, which increases the height of the floor much higher than any subfloor. Different flooring finishes can vary a lot, with tiles coming in a number of thicknesses, as well as adhesive beds to level your existing floor. There should be contact with the flooring contractor to see if the final product will work with underfloor heating, and if so, what depth of coverage they need. There have been several cases where the floor covering was not deep enough and the cables started burning in the floor. It is a good idea to get a layout of the room before designing the system. Heat blocking (laying carpets, bean bags, etc.) on the floor can reduce the cable’s ability to dissipate heat and may even cause the system to fail. Units where food is stored should not have underfloor heating installed as it can cause reduced shelf life and unwanted odors. After checking the availability of the supply, not allowing diversification of the electric heat source, it is worth noting the current RCD protection of the system. It is recommended to install a Type A RCD. Further safety considerations When designing a system, it should be noted that a 30 mA RCD should not protect more than 7.5 kW / 230 V or 13 kW / 400 V. Further safety considerations can be found in BS7671 Chapter 753 Heating cables and built-in heating systems. The system itself will consist of: a spot insulator, a suitable thermostat for underfloor heating (usually rated at 16 amps max), an underfloor heating cable and a floor sensor. The underfloor heating cable has a cold tail (part of the cable does not heat up), the heating cable itself, and a resistance connection at the end. It is good practice to install a secondary ground sensor to allow for potential malfunctions in the future. The floor sensor should also not be located near other heat sources that could affect its readings. The cold tail connection, the cable resistor end, and the floor sensor must be completely incorporated into an adhesive or compound to prevent overheating. Neither cable must intersect with the other and the cables must be at least 40 mm and not more than 110 mm apart (see manufacturer’s instructions). The surface temperature should not be more than 35°C (according to CENELEC Guide 29) and this can be achieved by following the manufacturers’ installation instructions. The thermostat is best installed in a 47mm deep pad or flush box to allow for proper connection distance and additional floor sensor. Cables shall be terminated with good practice, bearing in mind that according to IEC 60228 Class 5 and 6, the conductors shall be bent. The additional floor sensor can be left isolated in the box for future use. The heating cable usually has a protective earth shield surrounding the line and the neutral conductors. To test the cable, a continuity test must be performed and recorded for maintenance and future fault finding. The measured test results should simulate the readings provided by the manufacturer and should be tested: a) before removing the cable from the box (to check that it arrived in good condition), b) after installation, and c) after the flooring contractor has done their work. This will identify any errors and if so, what stage of the installation may have been corrupted. The floor sensor must also be tested and a measurement recorded to demonstrate its functionality. Insulation resistance tests between live conductors and ground conductors must be performed at 500 volts, unless otherwise specified by manufacturers. A drawing “as installed” showing room dimensions and hot areas is good practice to avoid future damage, eg where people might drill into the floor to install door bumpers. When the system is turned on, it can push for a thermal camera or thermal pad to speed up the process. These devices will determine the changing temperatures of the surface and even display the cable locations underneath with a reasonable degree of accuracy. By following best practices, you can proactively ensure that your installed system will last for many years, providing your customer with an efficient underfloor heating solution. In his spare time, Pete co-produces the Hit The Lights podcast, providing electricians with a chance to chew fat about the latest issues and share stories from the trade. Follow the podcast on Twitter @HittheLightsPod


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