A defective thermostat in your boiler can cause severe damage in your home.
The job of the thermostat is to shut off the system at a preset temperature to prevent overheating. This saves in electrical costs (uneccesary heating) and prevents joints from leaking or bursting from overheating.
We have clients that experienced flooding as a result of forgetting to shut off the boiler.
We recommend always using a timer (have an electrician install one if you don't have one).
We also recommend installing a quality thermostat that is less likely to fail (the blue thermostat in the picture).
Lastly, a thermostatic mixing valve installed on the boiler will ensure that no water above 50C will enter the home, ensuring that no pipes will burst due to overheating.