Boiler reapir blog – Main combi 24he with intermittent flame failure

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I got a call to repair an intermittent fault on Main 24HE combination boiler. Information I was given was that boiler had been intermittently falling to flame failue (middle of 3 led lights flashing on the boiler) over longer period of time and that recently it had started to become more frequent.

Generally flame failure can be caused bya number of issues such as:

• rectification electrode faulty or failling
• ignition electrode failing
• ingition or rectification wirign faulty
• faulty gas valve
• faulty fan or weak fan
• faulty air pressure switch
• faulty PCB (Printed circuit board)

All of the above can cause an intermittent fault but some components are much more likely to be intermittent than others. With intermittent faults my first port of call would be as follows: PCB, air pressure switch, electrodes, weak fan. If boier did not start at all then I would start by looking at fan or gas valve.

Fault finding logic in this case would be to follow boilers sequence to try to establish at which stage boiler fails when it does go into the ‘flame failure’

Sequence of Main 24HE is as follows:

• call for heat from thermostat or call for hot water
• fan starts
• air pressure switch makes contact and send singal that fan is OK to PCB
• PCB send signal to gas valve to open
• Ignition electrodes get voltage to spark ignition (from spark generator controlled by PCB)
• Burner ignites and flame sensing electrode send singal to PCB that flame is OK

What I was looking at is stage at which boiler fails. When on site boiler went into ‘flame failure’ around 70% of the time so it was relatively easy to see what was happening. In this instance what was happening was:

• call for heat or hot water
• fan starts
• boiler fails to ignite, there is no sound of sparking

From that information I knew that I can rule out follwoing components:

• sensing electorde – as boiler does not ignite
• fan – fan starts and differential pressure is correct

Whaterver componet is failing it will be after the fan. I will be looking at compotnets and testing them in following order:

• air pressure switch
• PCB
• gas valve

My tests confirmed faulty component quickly and boiler got repaired the same day.

Have you come accross similar fault on Main / Baxi / Potterton boilers? What was the outcome? Let me know.

I recorded a short video from this repair that you can watch below.

For gas boiler repairs please contact me through my website here: plumber Crystal Palace