How do I turn on the heater if it has been turned off for more than a week?
If the heater is turned off for over one week, re-enable the BOOST function and allow water to reach its set temperature of at least ten minutes. Open hot taps until cold water runs out before reheating (page 29).
From the official Itho Daalderop 120l Smart Upp Boiler manual, page 5
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