Boiler Problem Gradually Getting Worse

My sister has a problem with her central heating boiler - it's getting worse and she is getting no satisfaction from the service engineer (recommended by the Irish Agent) - his original answer is that it was a problem with the wind and she should put a guard of some sort on the chimney. He current suggestion is that they create an additonal opening through the external wall and put in an additional air intake.
The boiler is a Geminox Megalithe (French make) about 4 years old. Oil fired, with built in domestic water tank. The unit is indoors in a back hallway off the kitchen.
During the summer, when heating domestic water only, there are no problems. When the heating is switched on (radiators upstairs + underfloor downstairs) the boiler cuts out (at least once a day - sometimes up to 4 times a day). It must be reset (holding down switch on burner unit).
I don't believe wind is a problem - it blows in both the summer and winter around these parts and the flue output is not in a particularly windy location.
The obvious theory is that the additional load - and probably longer burn times is causing the problem. My suspicion would be a heat sensor may be too sensitive and is cutting the system off but the service engineer has ignored this suggestion.
What do folks think of the suggestion of opening up an air vent. The hallway where the unit is situated should have plenty of airflow. It has a back door opening to the outside, and is about 15 feet long with doors to a scullery and downstairs toilet. There does not appear to be any specific inlet on the unit for air. The Geminox website is not very informative but seems to indicate that the system does not support a balanced flue. How can I tell if the flue is a balanced or normal flue. My understanding of balanced flues is that they serve both exhaust and inlet functions. Is this correct? Can a balanced flue be installed on a unit that doesn't specifically support it? The current flue has a 90 degree bend and goes through an external wall. If it is not a balanced flue I assume that air is taken from the general surroundings.
Any pointers or tips would be greatly appreciated - because of access etc. opening up a hole for an air intake will be a horrible job - especially if it doesn't fix the problem
Thanks
Frank Davis Email is Davisfie in yahoo^*CO*(UK
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Frank Davis wrote:

Possibly the flame detector is sooted up. I had one that did that regularly due to poor flue. cleaning the sensor and replacing teh carboned up nozzle were regular occurrnecs.
Also check fuel filters - water and crap in teh lines can cause flameouts under high dflow rates as well, as can simply a restructed flow due to a bent pipe
I'd do the whiole oil feed over - first of all disconnect from boiler and check how fast oil comes out via gravity. If it stops, may be an airlock? run out of oil has it recently? Or if its slow or bubbly could be ater/air or crap in teh system. Once oil flow is well priven, then its either the pump, the nozzle or the sensor.
There isn't that mmuch IN a boiler really that can go wrong./

Yes
Not to my understanding.

The boiler room or room the boiler is in.

Don't think it will. Think its someting else.

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