How-to guide
Underfloor Heating Thermostat Wiring: Electric and Water Systems
By Ismail Farooq · Updated 2026-10-30
Underfloor (radiant floor) heating comes in two very different kinds, and the thermostat for one won’t work for the other. Electric systems are heating mats or cables under the floor, controlled by a mains thermostat that switches the heating current itself. Water (wet, hydronic) systems are pipes fed from a boiler or heat pump through a manifold, and their thermostats only send a signal. Work out which you have before buying anything.
Which system do you have?
| Clue | Electric | Water (hydronic) |
|---|---|---|
| Where the heat comes from | Mats or cables under tiles or flooring | Pipes fed from a boiler or heat pump |
| What’s in a cupboard or under a floor | Nothing; maybe a spur or breaker labelled “UFH” | A manifold with flow meters and small actuators on top, and usually a pump and wiring centre |
| Wires at the thermostat | Heavy mains wires plus a thin two-wire floor probe | Thin wires or a mains switched live to a wiring centre; or nothing if wireless |
| Typical rooms | Bathrooms, kitchens, one or two rooms | Whole floors, new builds and extensions |
Electric floor heating thermostats
These thermostats have three groups of terminals:
- Supply (L, N in) from the fused spur or breaker. In North America, 240 V models take two hots instead.
- Load (L, N out) to the heating mat’s cold tails. The mat’s earth braid must connect to earth.
- Floor probe: a thin two-wire sensor laid in the floor between heating cables, usually in a conduit so it can be replaced later.
The two things that catch people out when replacing one:
- Match the floor probe. Probes are NTC sensors with different values (10k, 12k and 15k are common). A thermostat expecting a different value reads the floor temperature wrong and can overheat it. Many thermostats let you choose the probe type in settings; otherwise replace the probe or buy a thermostat that supports it. You can measure the probe with a multimeter on resistance: a 10k probe reads about 10 kΩ at 25 °C.
- Check the load. Add up the mats’ wattage and divide by the voltage. Thermostats in the UK and Europe are commonly rated 16 A, about 3,680 W at 230 V. Larger areas need a contactor or a second thermostat.
Electric floor heating needs RCD protection (30 mA) in the UK and Europe. In North America, floor heating in bathrooms and kitchens must be GFCI protected, and many floor thermostats have a GFCI built in with a test button. A new circuit for floor heating in a bathroom is notifiable work under Part P in England and Wales, so use a registered electrician.
Water underfloor heating
Each room or zone has a thermostat and one or more actuators, small heads on the manifold that open that room’s loop. A wiring centre next to the manifold links them all:
- When a room thermostat calls for heat, the wiring centre powers that zone’s actuators.
- It also switches on the UFH pump and sends a boiler or heat pump demand through a volt-free relay (or switched live).
- Actuators are usually normally closed, so they take a few minutes to open after a call. That delay is normal.
Wiring centres come in 230 V and 24 V versions, and the thermostats and actuators must match the wiring centre’s voltage. A 230 V wiring centre normally takes a switched live from each room thermostat; a 24 V one supplies its own low voltage. Wireless thermostats pair with a receiver in the wiring centre instead.
Making water UFH smart
- Swap the room thermostats for smart ones that match the wiring centre (same voltage, or a compatible wireless receiver). Many UFH brands sell their own smart thermostats for exactly this.
- Single zone fed by a boiler: a general smart thermostat with a switched-live output can drive the wiring centre’s zone input. See UK smart thermostat wiring for switched live and receivers.
- Mixed radiators and UFH: the radiators and the underfloor zones each need their own call for heat. Whole-home systems with UFH controllers handle both.
Settings for underfloor heating
- Slow response. A screeded floor takes hours to warm up. Use the thermostat’s optimum start or early-start feature and keep setbacks small (2 to 3 °C) rather than switching off overnight.
- Floor temperature limit. European standard EN 1264 sets a maximum floor surface of 29 °C in rooms people spend time in, and floors such as wood, laminate and vinyl often have a lower limit set by the flooring maker. Electric thermostats with a probe enforce it; set the floor limit to match your flooring.
- Floor, air or both. Bathrooms often run on floor temperature (warm feet). Living spaces usually run on air temperature with a floor limit. “Combined” or “air with floor limit” mode is the safe default for timber floors.
- Heat pumps: UFH suits heat pumps well because it works at low water temperatures. Set the heat pump’s flow temperature as low as keeps the rooms warm.
When it isn’t heating
- Electric: check the thermostat isn’t in a floor-limit or probe-fault state, then measure the mat’s resistance against the value on its label. An open circuit means a damaged cable; call the mat maker’s installer.
- Water, one room cold: the actuator may have failed or isn’t getting a signal. Many have a visual indicator showing open or closed. Try swapping actuators between zones to narrow it down.
- Water, all rooms cold: check the pump is running and the boiler or heat pump receives the demand from the wiring centre.