British Gas BG330+ Boiler F4 Fault Code Lockout

This also applies to the Glow-Worm Flexicom hx boiler

New British Gas BG330+ boiler ignition and sensing electrode set beside a broken set
New BG330+ ignition and sensing electrodes. New set on left, broken set on right

We were called to a British Gas BG330+ boiler which had developed an F4 fault code.
After being re-set the boiler would make 5 ignition attempts before locking out again, showing the F4 fault.
On each ignition attempt the boiler would fire on low rate but as soon as it attempted to move up to high rate it would shut down again.
For the F4 fault code, the fault finding chart in the Installation and Servicing Instructions uses the term “Flame goes out whilst lit during a demand“.

The gas tap to the boiler was fully open. It’s also worth checking the emergency control valve at the gas meter, to ensure it hasn’t been knocked into a largely-closed position.
The wires and connectors to the gas valve, igniter unit and ignition electrodes seemed properly tight and undamaged. The earthing connection appeared fine too.
Gas inlet pressure was normal at about 22mbar.

The fact that the boiler initially lit each time suggested no fault in the igniter unit.
Cobwebs in the air inlet duct can cause problems with the air/gas ratio so we had brushed through the air inlet duct from the flue terminal end to remove any cobwebs.
The small amount of debris in the condensate trap was below the “service fill level” mark so the condensate trap did not require cleaning.
The boiler did not stay alight long enough for us to check the gas valve adjustment.

Spark Electrodes for BG330+ boiler or Glow-worm Flexicom hx boiler

Our next check was the spark ignition electrode set. This is one unit with two electrodes; the ceramic insulated ignition electrode and the bare earthing electrode.
We had isolated the boiler from the gas and from the electrical supply. We didn’t remove the plastic flue hood ducting as this might have required replacement of the sump seal.
With both the HT lead to the electrode and the earthing lead to the bracket removed, the electrode set is only held in place by two 8mm bolts.

We found that the ignition electrode had broken off within the ceramic and the metal end section had fallen off. The photo shows the broken electrode set on the right and a new set on the left for comparison.
The break in the metal was about 5mm inside the ceramic insulator tube. We have seen an identical fault before, on either a British Gas BG330+ boiler or a Glow-worm Flexicom hx boiler. Both boilers are made by Glow-worm.

It seems that the high voltage ignition spark was able to jump to the end of the ceramic tube and across to the earthing electrode, lighting the boiler. However, when attempting to go to high rate, the boiler electronics were unable to sense the required flame rectification current. The ignition sequence was then re-started, each time failing to complete.
Surprisingly, the lighting of the boiler each time was not explosive, despite the broken ignition electrode

We replaced the electrode set (Glow-worm part No.0020020731) which included a new gasket. The spark gap between the electrodes was correctly set at 4mm.
With the electrical power and gas to the boiler re-instated, and the boiler re-set (small recessed reset button on boiler fascia), the F4 fault code had disappeared and the boiler operated normally.

Servicing the British Gas BG330+ boiler (or servicing a Glow-worm Flexicom hx boiler)

The position of the boiler and the route and condition of the flue/air duct were all OK. We had brushed through the air duct to remove cobwebs as these can significantly reduce the air intake and affect the combustion.
There was only a small amount of debris in the condensate trap and there was no risk of it blocking the trap.
The gas rate was measured at the meter and was close to the required figure.

Using a 2.5mm allen key to alter the max rate air/gas ratio on a BG330+ boiler
Adjusting the throttle to alter air/gas ratio for max rate

We connected an electronic combustion gas analyser and checked the CO/CO2 ratio once the boiler had warmed up.

With the boiler set to max rate the CO2 level was too high. Using a 2.5mm allen key we adjusted the throttle (clockwise to decrease) until we had a stable CO2 reading of 9.3%. It’s important not to take too long doing this as the boiler may reach full temperature and reduce its output rate, giving a spurious reading.

We then toggled from High to Low, to force the boiler into the Low output rate.

When the combustion had settled down and we were getting steady readings from the analyser, we checked the CO2 reading at Low rate. This reading was too low so we used a 4mm allen key to turn the Offset screw (this time clockwise to increase) until a stable CO2 figure of 9.3% was reached. These high and low figures are different for different boilers and are not always the same as each other. Having set the Low rate we re-checked the High rate and it had remained correct.

Using a 4mm Allen key to adjust the Offset screw to set the low combustion ratio in a British Gas BG330+ boiler.
Using the Offset screw to adjust the gas/air ratio at low combustion rate