Underfloor heating and controls

Electric underfloor heating is a heating cable laid under the floor finish, usually tiles. A mat has the cable fixed to a mesh at a set spacing, so it rolls out quickly over a regular floor. Loose cable is laid by hand at a spacing to suit the room, which makes it better for awkward shapes and the gaps between fittings.

Either way, it goes only in the open floor. It is never laid under the bath, the WC, a basin pedestal or kitchen units, where the heat cannot get out.

Wet underfloor heating, with water pipes from a boiler or heat pump, is a different system. The pipework is for your heating engineer; the wiring centre, pump and thermostats are electrical, and that part is ours.

On a concrete floor, a lot of the heat would go down into the slab instead of up into the room. Insulation boards laid first stop that: the floor warms faster, and the heat goes where you want it.

Boards add height, so the build-up is agreed with the tiler before anything is laid, to meet the door threshold and the floor of the next room.

The thermostat needs to know how warm the floor is, and the floor probe tells it. The probe is laid between two runs of cable, an equal distance from each, usually in a thin conduit so it can be replaced without lifting tiles.

It also protects the floor. Wood, laminate and vinyl all have a maximum temperature, and the thermostat uses the probe to stay below it.

The heating cable is tested before it is laid, once it is down, and again after tiling: its resistance should match the maker’s figure and its insulation should be sound. A damaged cable is far easier to deal with before the tiles go on.

Underfloor heating gets a dedicated circuit from the consumer unit, sized for the heating’s load, with 30 mA RCD protection. Heating cables have an earthed metal screen, so damage to the cable trips the RCD instead of leaving the floor live.

A new circuit is notifiable under Part P and gets an Electrical Installation Certificate with its test results. If the board has no spare way or no RCD protection, the quote says what is needed first.

Most electric underfloor heating goes into bathrooms and en-suites, so the bathroom zones apply. The heating itself is made to go in the floor under them. The thermostat is not: it goes outside the zones, usually on the wall just outside the door, with the cold tail and the probe running to it in the wall.

Work in a bathroom’s special location is notifiable under Part P, and the notification is made for you.

A room thermostat switches the heating by the air temperature. An underfloor heating thermostat usually reads the floor probe, the air, or both. Programmable thermostats add times and temperatures for each day. Smart thermostats do the same from an app, and can be grouped so that each room is controlled on its own.

Most of the wiring is simple. What matters is that the thermostat suits what it controls: an underfloor heating thermostat for an electric floor, one that works with your boiler for a wet system, and a smart thermostat where it has the signal it needs.

A boiler-fed heating system is held together electrically by a wiring centre: a junction box where the boiler, pump, motorised valves, room thermostat, cylinder thermostat and programmer all meet. The two common layouts are known as S‑plan, with a two-port valve for each zone, and Y‑plan, with one three-port valve shared by the heating and the hot water.

Replacing an old programmer with a smart control, adding a heating zone, or finding out why the hot water comes on but the heating does not, are all jobs on the wiring centre. Each starts with tracing what is already there, because older wiring centres have often been altered more than once.

We do the wiring, not the gas. Anything on the gas supply or the boiler’s gas side is for a Gas Safe registered engineer.

On a new boiler or a new heating system, the electrical side is ours: the boiler’s fused supply, the wiring centre, valves, pump, thermostats and controls, planned with the heating engineer so the two halves meet. If your heating engineer has asked for an electrician, send us their wiring diagram or the boiler’s make and model, and we will tell you what is involved.