Hello, This is due to swapping different thermostats but not swapping the sensor cable. The sensor types are not compatible between thermostats so you are required to replace the sensors when changing thermostats. If this is not impossible, but the thermostat is installed in the same room as the underfloor heating, you can use the built in room sensor. If installed outwith the room, you would be required to install a room sensor, in the same room as the underfloor heating. Please contact us on 01887 822020 if this is required.