The trouble cooling-system of your boiler stove
Since a burning stove cannot be "switched off" at a moment's notice, the law states that the water heat-exchanger must be fitted with a thermal safety device. Should your water-heating stove overheat for whatever reason (e.g. the pump is not running because of a power failure) the thermal safety device ensures that the stove is cooled with cold water fed through the heat exchanger through a separate circuit.
All water-heating DROOFF boiler stoves are fitted with a thermal safety device by the manufacturer before delivery.
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