Problem first began with A & B flashing together when demand from either hw or ch. Boiler guy changed out the pub then air pressure switch, then the fan plus new seals. Still no joy. Pump and fan run fine but APS is not being activated. He can short the APS and the boiler will fire up and run. Flue appears to be clear, it's a short length out the wall. Venturi was confirmed to be unblocked. Strange things is there appears to be a slight suction on the venturi, however the APS needs a. Blow to activate. Any ideas please? We're both flumuxed with it.
hmm, there's just the single tube from the venturi to the APS. On the APS its attached to the port furthest from the electrical connections.