Your flue venturi most probably needs cleaning. This is a tube in the flow from the fan situated at the top of the boiler. Airflow causes a pressure drop between the inside of the precisely machined tube and the surrounding flue. An air pressure switch senses the drop and tells the boiler the fan is ok. A small build up of deposit on the tube will spoil the aerodynamics and stop it working reliably. If you were to measure the voltage (carefully) across the wires going from the control box to the air pressure switch when the boiler flame cuts out you would see the APS go open circuit and a voltage appear (240V ac as I remember). See figure 37 of the manual.
See fig 38 for the control box connections.
Figure 36i, component 6338 holds the venturi, this is the section you need to extract to clean the venturi. Figure 36j component 7070 is the APS. You can not clean the venturi by poking objects down the pipes that the silicone tubes going to the aps push on to. The venturi need to come out.
The boiler probably needs a service, which should be more than hoovering dead insects from the bottom of the combustion chamber, and should include cleaning of the heat exchanger, fan and venturi, along with various wearable seals and diaphragms being replaced (all in vokera kit t0019).
Safety stuff:Repairing gas boilers is potentially dangerous work, you must be a 'competent person' to do it (on your own boiler), and may not do it as a job unless you are corgi registered. If you dont reassemble it properly you could get carbon monoxide leakage, if you are not careful probing the aps circuit on a live boiler you could electrocute yourself, etc etc. You are probably best off (if unsure) getting a repairer familiar with the model to do the work, phone Vokera and ask for a local bod. Corgi generalists can spend ages beating around the bush, and not do the full service with pressure checks (expansion tank, burner gas pressure, flue pressures), or kit t0019 parts etc.
An alternative explanation is that the secondary heat exchanger is blocking up, the primary loop water is over heating and causing the boiler to cut off the heat, this is an unlikely but expensive possibility (domestic hot water heat exchanger extraction is a more significant job). Check the flue venturi first, it will be the culprit.
If you dont have the boiler manual Vokera may email you a pdf copy. There are two models of 80sp, make sure you get the correct manual (or both!).
Ian
January 2007
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