The venturi does not need to be 'blocked' not to work, it only takes a little bit of dirt or corrosion. However your fault doesn't sound much like that. Dirty venturi cause the hot water to run hot-cold-hot. If the problem is for both hot water and CH not firing up on demand I would guess the flow micro switch is either broken or not being actuated. A corgi man should sort it.
c
May 2009
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