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Baxi Bermuda 551 and 552 Back Boiler Faults, Problems and Fault Finding

Baxi Bermuda 552 Boiler Fault

Baxi Bermuda 552 pilot holds but gas valve locks out a few seconds after the main burner opens.

This fault is common to many gas boilers, particularly back boilers

Customer reported that the gas valve on this Bermuda 552 back boiler would lock out. Following the lighting instructions they could re-light the pilot which would stay alight until they turned up the boiler thermostat to make the main burner light. The main burner would light up but then shut down, locking out the gas valve.


Back boilers are open flued and draw their air for combustion and dilution from the room. Because they are at floor level and generally in rooms with carpets, soft furnishings (and maybe pets, which drop fur) they are at greater risk of drawing in the dust, fluff, lint and fur.

This back boiler was well overdue for a service and the pilot burner needed blowing clean. The pilot flame was too small and barely touching the thermocouple. When the main burner fired, the pilot flame was pulled slightly further away from the thermocouple which cooled, locking the gas valve out. All this back boiler needed was thorough servicing.


Occasionally we come across another cause for this type of fault. If the gas supply to the boiler becomes inadequate the pilot may already be reduced in size but when the main burner opens there is a sudden drop in gas pressure to the pilot. The pilot flame becomes either too short or mis-shapen, failing to keep the thermocouple hot.


Gas pressure problems may be caused by a blockage in the gas supply pipe or even by a partly closed gas isolation cock or emergency control valve (ECV) at the meter. One further place to look if gas pressure at the burner is too low is the inlet port of the gas valve itself. Two or three times we have seen a heavy build up of metallic scale deposits which have come from the inside of the iron gas supply pipe. There can be so much scale that you can “pour” it out of the end of the gas valve. Blowing the inlet port of the gas valve clean of these scale deposits and then (carefully!) allowing the gas supply to blow the scale out of the gas supply pipe sorted the problem for us.

The customer had a Baxi Bermuda 552 back boiler which developed a loud roaring noise which happened  whenever the main burner lit up. This roaring noise was very obviously different from the normal burner noise.


The fault with this boiler was caused by the Honeywell V4600C gas valve which had performed faultlessly for the previous 25 years or more. The gas valve had started open and close very slowly, sometimes failing to close completely. This caused a popping noise on closing as the flame momentarily lit back inside the burner. When the gas valve failed to close perfectly it allowed a tiny flame to be retained at the main burner injector. The next time the gas valve opened, this flame caused the gas to light back inside the burner, causing the roaring noise.


Light-back (flash-back, strike-back) inside the main burner can also be caused by damage to the burner (see  below).

This old Baxi Bermuda 551 back boiler had a damaged main burner caused by leakage from the heat exchanger. The heat exchanger had been changed but the heavy rusting visible in the photos had caused perforation of the burner wall allowing the burner to light back inside. This produced the characteristic roaring noise.


Lighting back inside a main burner can also be caused by gas valve problems (see above).

Click on an image for a bigger picture

Baxi Bermuda 552 Boiler Fault

Baxi Bermuda 552 back boiler burner makes a roaring noise.

This fault can occur on almost any older gas boiler but is most obvious on back boilers and other open flued boilers (chimney type boilers).

Photo of a new Honeywell V4600C gas valve, also called a multifunctional gas control.

Honeywell V4600C gas valve (also called a multifunctional gas control)

Baxi Bermuda 551 Boiler Fault

Baxi Bermuda 551 back boiler burner makes a roaring noise.

Photo of a heavily rusted main burner from a Baxi Bermuda 551 back boilerDetailed photo of the main burner from a Baxi Bermuda 551 back boiler. The photo shows heavy rusting round the air entrainment ports.

Heavily rusted burner from a Baxi Bermuda 551 back boiler. This boiler was 30 years old

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