Helpful video reference. Heat Mat, a UK underfloor heating manufacturer, published "Heat Mat Underfloor Heating Smart Thermostat Installation" showing the swap from an older unit to their WiFi thermostat. The wiring process is the same regardless of brand, so this is a good reference even if your system is ProWarm, Warmup, Nuheat or another make.
1. Buy the right replacement thermostat
Two things must match before you order:
- Load rating. The thermostat switches the full current draw of your heating mat or cable. Find the wattage from the original installation documentation or from a label on the existing thermostat. A 150 W/m² mat in a 4 m² bathroom is 600 W - well within any domestic thermostat. A larger room at 200 W/m² over 10 m² is 2,000 W and needs a thermostat rated to handle it (most are rated to 3,600 W / 16 A, which covers almost all domestic installations).
- Floor sensor NTC value. The probe buried under your floor sends a resistance reading to the thermostat to control temperature. Most UK systems use a 10 kOhm NTC sensor, but some older installations use 6.8 kOhm, 12 kOhm or 15 kOhm. If the new thermostat uses the wrong NTC value it will display an incorrect temperature and control badly. Check the original thermostat's datasheet or call the manufacturer.
If you cannot find the NTC value, a multimeter on resistance mode across the sensor wires at room temperature (20-22°C) will read roughly 10 kOhm for a 10k sensor, 6.8 kOhm for a 6.8k sensor and so on. Compare to the chart in the new thermostat's instructions.
2. Isolate the circuit
Turn off the UFH circuit at the consumer unit - usually a dedicated MCB or RCBO labelled "underfloor heating" or "bathroom floor". If there is no label, turn off all circuits you are unsure about and test at the thermostat with a voltage tester.
Some thermostats are surface-mounted and some are flush-fitted in a back box. Remove the faceplate (usually two screws or a snap-fit) and test between every terminal and earth, and between live terminals, before touching anything.
3. Photograph the existing wiring
UFH thermostats have more terminals than a standard light switch. A typical layout includes:
- L - incoming supply live (brown from consumer unit or FCU)
- N - incoming supply neutral (blue)
- E - earth (green-and-yellow)
- L out or Load - switched live to the heating element
- N out or Load N - neutral to the heating element
- FS1 / FS2 (or similar) - floor sensor probe wires (no polarity)
Take a photo that shows every terminal clearly. On the new thermostat the labels may differ slightly, but the functions will be the same.
4. Remove the old thermostat
Loosen each terminal and remove the conductors one at a time, laying them aside in the same order. Do not pull by the cable - ease each conductor out of its terminal. The floor sensor wires are usually a thin two-core cable coming out of the floor; handle these gently as they can be fragile at the point where they exit the floor surface.
Undo any cable clips or ties holding the thermostat to the back box and pull it forward enough to see all the connections easily.
5. Fit the new thermostat back box (if needed)
Most replacement thermostats use a standard single-gang back box (25 mm or 35 mm deep). If your old thermostat used the same size, the new one usually mounts straight on. If the dimensions differ, you may need to fit a new back box or use a surface-mount adapter.
Thread the heating element cables and the floor sensor cable through the appropriate cable entries on the new back box before securing it.
6. Transfer the wiring
Using your photo as a reference, connect conductors to the new thermostat one at a time. Work methodically:
- Connect the incoming supply live (L) and neutral (N).
- Connect the earth (E).
- Connect the outgoing load live and neutral to the heating element.
- Connect the floor sensor wires to the FS terminals (no polarity on the sensor).
Tighten each terminal and tug each conductor. Strip lengths are given in the thermostat instructions - typically 6-8 mm.
7. Restore power and test
Refit the faceplate and turn the MCB back on. The thermostat should power up, run its startup sequence and display a floor temperature reading within a minute or two.
Set the temperature to 5°C above the current floor reading to confirm the heating element switches on. You should hear or feel the element begin to warm after a few minutes. If the thermostat displays a sensor error or an unusually high/low temperature, check the NTC value against the floor sensor spec.
8. Programme the schedule
Most modern thermostats allow a 7-day programme with different heating periods for weekdays and weekends. Set the heating to come on 30-45 minutes before you need the floor warm - electric UFH in a tiled floor takes time to come up to temperature through the screed.
A floor temperature limit of 27°C is recommended under most flooring types to protect the adhesive and tiles. A comfort temperature of 22-25°C at the sensor is typical for a UK bathroom.
When to call us
If the floor is cold after a thermostat swap, the element itself may have failed. Richard can test insulation resistance and continuity on the element from the back box without lifting the floor. A failed element is unfortunately expensive to replace, but the test is quick.
UFH problem in Sandwich?
Richard diagnoses and repairs electric underfloor heating faults, including element testing and thermostat replacement, at a fair local rate.
Contact Richard