Underfloor heating: getting the basics right | Peter Arrow

Underfloor heating: getting the basics right | Peter Arrow

There’s no better way to increase the thermal comfort of a living space than through the introduction of an underfloor heating system. Here, Peter Arrow EngTech MIET, offers best practice guidance to ensure a safe, long-lasting install.

Underfloor heating systems were invented by the Romans, by heating water pipes that travelled through the ground. With advances in technology, we’re now able to provide electrical solutions that solve this need. However, there’s a bit more to the installation process than just sticking a mat to the floor.

It’s important to remember that there are two main types of underfloor heating:


Where the cable is fixed to a mesh, with a double-sided tape fixing to the floor.

Single cable

The cable is fed from a drum and is fixed to the floor (usually by means of hot glue).

Both methods provide the same overall effect of a fast-acting radiant heat in the room. Radiant heat is much like the way we feel the heat from the sun. It doesn’t actively heat the room, but instead the bodies within it when they intercept the radiated waves.

We also use electrical underfloor heating to achieve conducted heat. This is best described when we feel the warmth of the heat source on our feet through the floor covering. This method can be ideal when taking the chill off a tiled surface in the colder months.

Check the capacity of local circuits

When designing an underfloor heating system, it’s a good idea to check the capacity of local circuits and whether you need to run in a dedicated supply (Appendix 15 of BS7671 gives guidance). In extreme cases you may even need to contact the DNO to upgrade the incoming supply.

A greatly missed detail during the survey, is the build up to the finished floor height. An existing subfloor to be covered with a vinyl tile flooring covering would need an average build-up of approximately 24mm, including thermal board, cable, self-levelling compound and floor finish, increasing the floor height to well above any sub-floor.

Different floor finishes can vary a lot, as tiles come in a number of thicknesses, as would their adhesive beds to level the existing floor. There should be contact with the flooring contractor as to whether the final product will work with underfloor heating and, if so, what depth coverage they require. There have been several cases where the floor covering has not been deep enough and cables have started burning through the floor.

It’s a good idea to get a layout of the room before designing the system. Thermal blocking (placing of rugs, bean bags or similar) over the floor can reduce the ability of the cable to disperse its heat, and even cause the system to fail. Units where food is to be stored shouldn’t have underfloor heating installed beneath them as it can cause reduced lifespan and unwanted odours.

After checking the availability of supply, allowing no diversity for the electric heat source, it’s worth noting the existing RCD protection of the system. It is recommended that a Type A RCD is fitted.

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Further safety considerations

When designing a system, it’s worth noting that one 30mA RCD should protect no more than 7.5kW/230V or 13kW/400V. Further safety considerations can be found in BS7671 chapter 753 Heating cables and embedded heating systems. The system itself will comprise of: a local isolator, a thermostat suitable for electric underfloor heating (usually rated at 16A max), an underfloor heating cable and a floor sensor.

The underfloor heating cable has a cold tail (part of the cable that doesn’t get hot), the heating cable itself and a resistor joint at the end. It’s good practice to install a secondary floor sensor to allow for potential failures in the future. The floor sensor should also not be located near other sources of heat which could affect its readings. The cold tail joint, end of cable resistor and floor sensor should be completely embedded in an adhesive or compound to prevent overheating. No cable should cross another and the cables should ideally be spaced no less than 40mm and no more than 110mm apart (check with manufacturer’s instructions).

The floor surface itself should be no hotter than 35˚C (compliance with CENELEC guide 29) and it can be achieved by following manufacturers’ installation instructions. The thermostat is best fitted to a 47mm deep pattress or flush box to allow suitable spacing for connection and supplementary floor sensor. The cables should be terminated with good practice, remembering that in accordance with IEC 60228 class 5 and 6, conductors should be ferruled. The supplementary floor sensor can be left isolated in the box for future use.

The heating cable will usually have a protective earth shield surrounding the line and neutral conductors. To test the cable, a continuity test should be carried out and recorded for future maintenance and fault-finding works. The measured test results should mimic the readings provided by the manufacturer and should be tested:

a) before removing the cable from the box (to check it has arrived in good condition),

b) after installation, and

c) after the flooring contractor has carried out their work.

This would identify any faults and if so, what stage of the installation they were likely damaged. The floor sensor should also be tested, and a measurement recorded to prove their functionality. Insulation resistance tests should be carried out between the live and earth conductors at 500V, unless otherwise stated by manufacturers. An ‘as installed’ drawing showing dimensions of the room and the hot areas is good practice to avoid future damage, for example where people may drill into the floor to install door stops.

When commissioning the system, it can pay to have a thermal camera or thermochromic sheet to speed up the process. These instruments will identify the changing temperatures of the surface and even show the locations of the cable beneath to a reasonable degree of accuracy. By following the best practices, you can proactively ensure that the installed system will last for many years, providing your client with an effective underfloor heating solution.

In his spare time, Pete co-produces the Hit The Lights podcast, providing electricians with the chance to chew the fat about the latest issues and share stories from the trade. Follow the podcast on Twitter @HittheLightsPod

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