Im also annoyed at my lodgers who crank the digi stat up to 26c-30c . Unfortunately the only stat I have found with mechanical limit stops is the potterton ptt2. Given the amount of gas that is being wasted trying to heat the house (sauna) up to 30c it might be worth me actually installing it before I do much more real paid work
Industrial set point thermostats have no external dial and some have covers that are locked so the internal adjustment cannot be tampered with.
Wire one of these in series with the present stat to open the demand ciruit when the maximum temperature you set is reached.
Don't use one of the type that opens at the set temperature and has to be manual reset ( with a key ). That would leave your tenants in the cold the first time they set the temperature too high.
As another poster said, move it somewhere where they can't access it - though look out for the 'override' button on the SCR (receiver fitted by the boiler).
Alternatively, put up the rent to cover the cost of the additional gas.
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