Underfloor heating loads over 15 amps will require switching through a suitably sized contactor. Due to the inductive nature of the contactor coil, a sudden interruption of current flow often leads to a sharp rise in voltage across the device creating a spike that can often cause an electronic device such as the underfloor heating thermostat to lose its programming and rest it back to factory default.
To overcome this a component called a suppressor (or a snubber) will be required.
The Suppressor is not polarity conscious and is connected across the live and neutral terminals of the contactor coil.
All underfloor heating installations are required to be protected by a 30A RCD and all electrical work should be carried out by a suitably qualified electrician complying with current IEE regulations.