There can be a number of reasons for this but the most common is the removable filters are blocked
This usually implies that there is a lack of good ventilation
This type of problem happens when the grease particles from the cooking range, like fryers etc are not being removed from the kitchen environment via the extraction hood.
By ensuring that all the open sides of the cooking equipment have the correct amount of canopy overhang
This usually happens when there is a lack of good ventilation from both the extract and input air.
If the extract system is working properly then the next step would be to fit an air input system matched to the rate of extraction.
This very much depends on the amount and type of cooking being done. At the very least once a week, for light cooking.
One area of concern that we often see are holes in the actual filter body, I.E. the mesh is no longer present in the filter, this is often the result of people cleaning the filter in the dish washer and then not rinsing them properly, subsequently the aluminium is then etched away, creating holes in the filter, so now the grease in the air is deposited in the duct, creating a fire hazard
This is dependent on the position of the gas equipment within the cooking range, but in most cases you should have a 200mm overhang at both ends of the hood and 300mm at the front
There are a number of ways to correct this the first is to terminate the duct at a high level if this has not been done, after this it may require a system that cleans the air prior to discharge.
This would indicate that the filters are not working I.E. as mentioned above there maybe holes in the filters allowing grease to pass through to the outside, another reason for this is the duct work requires cleaning which is also a fire hazard
This happens when the extract system is not working properly. There can be a number of reasons for this, the most common are, the filters are blocked, there is a restriction in the ductwork, or an incorrectly specked fan.
If you are installing new gas equipment and have no current air replacement, yes, this will be required for you to comply with corgi gas regulations.
Most extract systems are updated or replaced when new equipment is introduced, if gas, it will be required to maintain the correct over hangs.