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Ductwork - supply and extract
Ventilation has two directions, and they are not the same. Supply ductwork blows fresh air into a space; extract ductwork draws stale or contaminated air out. Because they carry different air in different directions, supply and extract ducts pick up different loads and need different care - which is worth understanding to look after each properly.
The short answer
Supply ductwork delivers fresh air into a space, so it carries relatively clean incoming air and its concern is keeping that supplied air clean. Extract ductwork draws used, stale or contaminated air out - kitchen grease, general stale air, or process contaminants - so it carries the load being removed and fouls accordingly. Because the two carry different air in different directions, they pick up different loads and need different care: extract generally needs more attention because it carries the contamination, while supply needs keeping clean so it does not deliver dirt into the space.
Two directions of ventilation
Ventilation works in two directions, and distinguishing them is the key to this. Supply ductwork carries air into a space - it is the delivery side, bringing fresh or conditioned air in to ventilate the area. Extract ductwork carries air out - it is the removal side, drawing used, stale or contaminated air away and out of the building. Same ductwork in appearance, opposite jobs: one supplies, one extracts, and the direction of the air is the fundamental difference between them.
That difference in direction leads directly to a difference in what each duct carries and therefore needs. Because supply brings air in and extract takes air out, they handle different air - supply the relatively clean incoming air, extract the used or contaminated air being removed. So they accumulate different loads and face different concerns, which means they need different care. Recognising that ventilation has these two distinct directions, and that supply and extract are not interchangeable, is the starting point for understanding what each actually needs.
Extract carries the load
The extract side generally carries the greater cleaning load, because it is drawing out exactly what needs removing - and that is what fouls it. In a kitchen, the extract carries the hot, grease-laden air from the cookline, so grease builds up inside it, with all the fire risk that brings. In general ventilation, the extract draws out stale air, dust and whatever the space produces. In an industrial setting, an extract or LEV draws out the process contaminant. In every case, the extract handles the used, contaminated or loaded air, so it accumulates the substance being removed and fouls accordingly.
This is why extract ductwork generally needs more attention than supply. It is on the receiving end of whatever the space or process generates - grease, contaminants, dust - so it builds up a real load that has to be cleaned, and in the kitchen case that load is a fire risk cleaned to a fire-safety standard. The extract's job of removing contamination means it carries that contamination, which is what makes it the side that fouls and the side that typically needs the more frequent, more demanding cleaning. What the extract draws out is what it then holds until it is cleaned.
Supply needs to stay clean
The supply side has a different concern: because it delivers air into the space, its job is to keep that supplied air clean, so it does not become a source of contamination itself. Supply ductwork carries relatively clean incoming air, so it does not foul the way extract does with removed contaminants - but it still accumulates dust and debris over time, and if it does, it can deliver that dirt into the space along with the air. A dirty supply duct blows its dirt into the room it is supposed to be freshening, which undermines the whole point of supplying clean air.
So supply ductwork needs keeping clean specifically so that the air it delivers stays clean. Its care is about the quality of the air being supplied - ensuring the delivery side is not adding dust or contamination to the incoming air. This is a real concern, particularly for the air quality of occupied spaces, even though the supply side does not carry the heavy contaminant load the extract does. The supply's job is to bring clean air in, and keeping it clean is what ensures it does that rather than becoming a source of the dirt it is meant to help remove.
Why they need different care
So supply and extract need different care because they carry different loads for different reasons. The extract, carrying the contamination being removed, generally needs more attention and, depending on what it carries, cleaning to the appropriate standard - a kitchen grease extract to the fire-safety standard, a general extract for air quality and hygiene. It fouls because that is its job, so it has to be cleaned to control what it accumulates. The extract is usually the more demanding side to look after because of the load it handles.
The supply, carrying cleaner incoming air, needs care focused on keeping that air clean - cleaned to keep it from delivering dust and contamination into the space, as part of maintaining air quality. It generally accumulates less than the extract but still needs attention so it stays a source of clean air. So looking after ventilation properly means recognising that supply and extract are different jobs with different needs - not treating them the same, and giving the extract the attention its contaminant load demands while keeping the supply clean enough to deliver clean air. Different directions, different loads, different care.
The takeaway
Ventilation has two directions that are not the same. Supply ductwork blows fresh air into a space, carrying relatively clean incoming air, so its concern is keeping that supplied air clean and not delivering dust into the room. Extract ductwork draws used, stale or contaminated air out - kitchen grease, general stale air, or process contaminants - so it carries the load being removed and fouls accordingly, generally needing more attention.
So supply and extract need different care because they carry different air in different directions. The extract, on the receiving end of whatever the space produces, is usually the more demanding side, cleaned to the appropriate standard for what it carries - the fire-safety standard for a kitchen grease extract. The supply needs keeping clean so it delivers clean air rather than dirt. Looking after ventilation properly means treating the two directions as the different jobs they are - which is the point of understanding what each needs and why.
Questions
Direction. Supply ductwork carries air into a space, delivering fresh or conditioned air to ventilate it; extract ductwork carries air out, drawing used, stale or contaminated air away. Same ductwork in appearance, opposite jobs - and they carry different loads as a result.
Because it draws out exactly what needs removing, so it carries that load and fouls accordingly - kitchen grease with its fire risk, general stale air and dust, or a process contaminant. It is on the receiving end of whatever the space produces, so it builds up a real load that has to be cleaned.
Yes - to keep the air it delivers clean. Supply carries relatively clean incoming air and does not foul like the extract, but it accumulates dust over time, and if it does, it can deliver that dirt into the space. Keeping it clean ensures it supplies clean air rather than contamination.
Because they carry different loads for different reasons. The extract, carrying the contamination being removed, generally needs more attention and cleaning to the appropriate standard; the supply, carrying cleaner air, needs care focused on keeping that air clean. Different directions, different needs.
A kitchen grease extract carries combustible grease with fire risk, so it is cleaned to the fire-safety standard (TR19 Grease). A general extract or supply for air quality is cleaned to the ventilation-hygiene standard. The care depends on what the duct carries.
Not well - they are different jobs with different needs. Treating them the same risks under-attending the extract's contaminant load or neglecting the supply's air quality. Looking after ventilation properly means giving each the care its direction and load demand.
We clean both supply and extract ductwork to what each needs - the extract to control its contaminant load, the supply to keep the air it delivers clean - because the two are different jobs.
