Home / Duct Cleaning / Guide
Duct cleaning - ventilation basics
A ventilation system moves air in a few different ways, and the terms can blur together - but supply, extract and recirculated air are three distinct things, doing three different jobs. Knowing the difference helps you understand what a system is doing with the air, and why the distinction between fresh and recirculated air matters especially. Here is the difference explained.
The short answer
Supply, extract and recirculated air are three distinct roles in a ventilation system. Supply air is fresh air brought into a space from outside, to provide the fresh air the people need. Extract air is stale, contaminated or humid air taken out of a space and exhausted, to remove what needs removing. Recirculated air is air taken from a space, cleaned or filtered, and returned to the same space, rather than exhausted outside - reusing the air instead of replacing it. The key distinction is between fresh air (supply) and recirculated air: supply air is genuinely fresh outdoor air, while recirculated air is the space's own air cleaned and put back. This matters because recirculated air, being the space's own air, does not bring in fresh air or dilute what fresh air dilutes - so a system relying heavily on recirculation may save energy (not having to condition as much fresh air) but provides less fresh air, which matters for air quality. And recirculated air must be properly cleaned, since it is returned to the space. So knowing the difference helps you understand what a system does with the air, and why fresh versus recirculated is an important distinction for air quality.
Supply air: fresh air in
Supply air is fresh air brought into a space from outside - the air a ventilation system supplies to provide the fresh air the people in the space need. A building needs a continuous supply of fresh outdoor air for the people in it, to provide oxygen, dilute the carbon dioxide and other things the occupants produce, and keep the air fresh and healthy. Supply air is that incoming fresh air: drawn from outside, possibly filtered and conditioned (heated or cooled) on the way, and delivered into the space. So supply air is the fresh-air input to a space - the genuinely new, outdoor air coming in.
The defining feature of supply air is that it is fresh outdoor air - air from outside, new to the space, bringing the freshness that only outside air can. This is what distinguishes it from recirculated air, discussed below: supply air is genuinely fresh, whereas recirculated air is the space's own air reused. The amount of fresh supply air a space gets is a key determinant of its air quality - more fresh air means better dilution of the occupants' CO2 and contaminants, and fresher air, while too little fresh air means a stuffy, poorly ventilated space. So supply air is the fresh-air side of ventilation, and how much of it a space gets matters for how well it is ventilated. Understanding supply air as the incoming fresh outdoor air is the first of the three roles.
Extract air: stale air out
Extract air is stale, contaminated or humid air taken out of a space and exhausted - the air a ventilation system removes to take away what needs removing. Where supply air brings fresh air in, extract air takes used air out: the air that has been in the space, now carrying the occupants' CO2, any contaminants, humidity, heat, smells, or (in a kitchen) grease-laden cooking air. This air is drawn out of the space by the extract system and exhausted, usually outside, removing it and what it carries from the space. So extract air is the output side of ventilation - the used air leaving.
Extract air and supply air are the two halves of the basic ventilation exchange: supply brings fresh air in, extract takes stale air out, and together they replace the air in the space, keeping it fresh. The extract removes what needs removing - the contaminants, humidity, heat and stale air - while the supply brings in the fresh air to replace it. In a kitchen, the extract has the particular job of removing the grease-laden, hot cooking air, which is why kitchen extract ductwork accumulates grease and needs cleaning. So extract air is the stale, used air being taken out and exhausted - the counterpart to supply air's fresh air coming in. Between them, supply and extract are the fresh-in, stale-out exchange that is the core of ventilation. The third role, recirculated air, is different, because it is not exhausted but returned.
Recirculated air: cleaned and returned
Recirculated air is the one that is genuinely different: it is air taken from a space, cleaned or filtered, and returned to the same space, rather than exhausted outside. Instead of the take-out-and-replace of extract and supply, recirculation reuses the space's own air: the air is drawn from the space, passed through filters or cleaning, and put back into the same space, circulating the air rather than replacing it with fresh. This is done, in many systems, to save energy: conditioning fresh outdoor air (heating or cooling it to the right temperature) takes energy, so reusing the already-conditioned indoor air, rather than replacing it all with fresh air that must be conditioned from scratch, reduces the energy needed. So recirculation is largely an energy-saving measure - reusing the conditioned air.
But recirculated air, being the space's own air returned, is fundamentally not fresh air - and this is the crucial distinction. Recirculated air does not bring in fresh outdoor air, so it does not dilute the CO2 and contaminants that fresh air dilutes, or provide the freshness that fresh air provides. It just re-circulates the existing air (cleaned of particles by the filters, but still the same air). So a system relying heavily on recirculation, with little fresh supply air, provides less fresh air to the space, which matters for air quality - the CO2 and any contaminants the filters do not remove build up, because they are not being diluted by fresh air. This is why the fresh-versus-recirculated distinction matters: recirculation saves energy but provides less fresh air, so the balance between fresh supply air and recirculated air affects the air quality. And recirculated air must be properly cleaned, since it is returned to the space - if the filtering is inadequate or the system is contaminated, recirculation spreads that back into the space. So recirculated air is a distinct, energy-saving but fresh-air-limited role, quite different from supply and extract.
Why the difference matters
Understanding the difference between supply, extract and recirculated air matters most for the distinction between fresh and recirculated air, because it affects air quality. A ventilation system's air-quality performance depends heavily on how much genuinely fresh supply air it provides, since fresh air is what dilutes the occupants' CO2 and contaminants and keeps the air healthy. A system that provides plenty of fresh supply air ventilates well; one that relies heavily on recirculation to save energy, with little fresh air, provides less ventilation in the fresh-air sense - which can show up as raised CO2 and a stuffier, less healthy environment, even though air is being moved. So knowing whether a system's air movement is fresh or recirculated tells you about its actual ventilation quality, not just that air is moving.
This became widely appreciated in the context of airborne infection, where fresh air (which removes and dilutes airborne contaminants) is protective, while recirculation (which can move air around without removing contaminants, unless well filtered) is less so - making the fresh-air proportion important. So the balance between fresh supply air and recirculated air is a real design and operational consideration, trading energy saving against fresh-air provision. And it connects to the cleaning of the system: recirculated air is returned to the space, so the filters and ductwork it passes through must be kept clean, or recirculation spreads contamination back into the space - making the cleanliness of a recirculating system's ductwork and filters important for the air it returns. So the difference between the three - and especially fresh versus recirculated - matters for understanding and managing a system's air quality. Supply brings fresh air in, extract takes stale air out, recirculation reuses the air; knowing which a system is doing, and in what balance, is knowing what it is really doing for the air quality.
The takeaway
Supply, extract and recirculated air are three distinct roles. Supply air is fresh outdoor air brought into a space, providing the fresh air the people need. Extract air is stale, contaminated or humid air taken out of a space and exhausted, removing what needs removing (in a kitchen, the grease-laden cooking air). Recirculated air is air taken from a space, cleaned or filtered, and returned to the same space rather than exhausted - reusing the air, largely to save energy on conditioning. Supply and extract are the fresh-in, stale-out exchange; recirculation reuses the space's own air.
The key distinction is fresh versus recirculated. Recirculated air, being the space's own air returned, is not fresh - it does not bring in fresh outdoor air or dilute the CO2 and contaminants that fresh air dilutes. So a system relying heavily on recirculation saves energy but provides less fresh air, which matters for air quality, and its filters and ductwork must be kept clean since the air is returned to the space. So knowing the difference - and especially whether a system's air movement is fresh or recirculated, and in what balance - tells you what it is really doing for the air quality, not just that air is moving. Supply brings fresh air in, extract takes stale air out, recirculation reuses the air: understanding which, and in what proportion, is understanding the ventilation.
Questions
Fresh air brought into a space from outside, to provide the fresh air the people need - drawn from outside, possibly filtered and conditioned on the way, and delivered into the space. Its defining feature is that it is genuinely fresh outdoor air, new to the space, which distinguishes it from recirculated air. How much fresh supply air a space gets is a key determinant of its air quality.
Stale, contaminated or humid air taken out of a space and exhausted - the used air carrying the occupants' CO2, contaminants, humidity, heat, smells or (in a kitchen) grease-laden cooking air. It is drawn out by the extract system and exhausted, usually outside. Extract air is the output side of ventilation, the counterpart to supply air's fresh air coming in.
Air taken from a space, cleaned or filtered, and returned to the same space rather than exhausted outside - reusing the space's own air instead of replacing it with fresh. It is done largely to save energy, since reusing already-conditioned indoor air takes less energy than conditioning fresh outdoor air from scratch. But it is not fresh air, which is the crucial distinction.
Because a system's air-quality performance depends on how much genuinely fresh supply air it provides - fresh air is what dilutes the occupants' CO2 and contaminants and keeps the air healthy. Recirculated air, being the space's own air returned, does not do this. So a system relying heavily on recirculation saves energy but provides less fresh air, which can show up as raised CO2 and a stuffier environment even though air is moving.
Not inherently - it saves energy and, well filtered, can be part of a sound system. But it provides less fresh air than supply air, so the balance between fresh supply and recirculation matters for air quality. It became particularly relevant for airborne infection, where fresh air is protective while recirculation moves air without removing contaminants unless well filtered - making the fresh-air proportion important.
Yes - recirculated air is returned to the space, so the filters and ductwork it passes through must be kept clean, or recirculation spreads contamination back into the space. So the cleanliness of a recirculating system's ductwork and filters is important for the quality of the air it returns - a reason to keep such systems properly cleaned and their filters maintained.
Recirculated air is returned to the space, so the ductwork and filters it passes through must be clean - or recirculation spreads contamination. Our duct cleaning keeps the system clean, whatever the air it moves.
