Intermitten fault,My boiler is reluctant to fire up when turning the hot tap on or the central heating, sometimes wer'e trying on and off all day only to find it comes on ok the next day and continues to work.
The venturi is not blocked and the fan is clean, The diaphragm operates the micro switch and the fan cuts in. But the burner will not light
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.
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