In the interests of this thread not devolving into yet another slanging match
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If you have an inkling of doubt about the behaviour of your unit, even after your installer's attempts to reassure you, then you are entitled to a second opinion and should call a Vaillant technician in. Explain your intention to your installer and try to keep it all friendly because your main concern is identifying a "possible factory fault" which "might" affect your warranty if left unreported! It might end up costing you for a callout but peace of mind sometimes comes with a price tag!