I've got a baffling problem with my Vokera combi boiler (it is a 20-80 RS
Flowmatic, which sounds like something out of Wallace and Gromit to me).

When I turn on the hot tap the boiler doesn't light unless it is already
warm. Typically, the only way I can get hot water is by turning the
central heating on first. Once the boiler has lit and I have the hot tap
running, I can turn off the central heating and the hot water will carry
on flowing all day. Interestingly, the trick of turning the heating on
doesn't work if the hot tap is already running - in fact it stops the
heating from coming on too.

I have verified that the flow sensor is working (I can see it make
the switch contact and I hear the resulting relay clicks). However, at
that point, the fan in the flue should speed up - it doesn't. When the
boiler is warm and the hot water system is working as it should, I can
hear the flue fan speedup a few seconds after the tap is opened. It only
takes a couple of seconds for the boiler to light once the flue fan is
going.

Any ideas?

Thanks

- Andrew