Next to the boiler's heating element there is a cartridge slot to insert the thermostat. Therefore it is the place to usually insert the Nectarsun S thermometer. If there is no such slot, inside of the boiler there is a mechanical thermometer, which needs to be pulled out. Then there is space for the Nectarsun S thermometer. After the insertion of the Nectarsun S thermometer, the mechanical thermometer needs to be returned to it's old place, covering the new one.