I have a Vaillant combi boiler and seem to have a problem with the
diverter valve. The hot water only comes on when the heating is on. I
have taken the diverter valve out of the boiler to study it and can
see that the diaphram is supposed to push up a pin that forces a
hammer to fall and make a contact that ignites the burner and diverts
water into the hot tap system.

When I turn the hot water tap on you can see the hammer try to move
down to make the switch contact, but it does not have the energy. It
would seem that there is not enough pressure to push it hard enough.

Does anyone out there understand what i'm talking about, or, even
better, know what the problem might be?

Kind regards.