I have the exact same problem with mine and spoke to Vokera.
The Domestic Hot Water (DHW) part is triggered by a diaphram valve which "senses" a tap opening and uses the mains water pressure to push on an arm to operate both the diverter valve and a microswitch to fire the main burner. The diaphram inside the valve is apparently renowned for failing, meaning the arm doesn't get pushed enough, or by a consistent amount. Its a really cheap part (
Craig Shearstone
February 2009
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