Age of air is the average time the air at a point has spent inside a room since it came in as fresh air. Young air has just arrived. Old air has been circulating around the room, and the people in it, for a while. Ventilation rates tell you how much fresh air goes into a room. Age of air tells you where it goes, which makes it one of the clearest ways to show a client how well a design works where people actually sit.
1The idea in one picture
- Young air: fed directly with fresh air
- Older air: the well-mixed open plan
Imagine stamping every parcel of fresh air with the time it came out of a supply diffuser. Wherever you stand in the room, the air around you is a mix of parcels that arrived at different times. Their average age is the local mean age of air at that spot.
Colour a floor by age of air, at seated head height, and you get a map of where the fresh air reaches first and where it reaches last. The picture under this heading is exactly that. The rooms fed by new fresh-air diffusers read young, and the open plan, served by mixing fan-coil units, sits at an older, more uniform age.
2Why air changes per hour isn't enough
- Same fresh-air rate, much older air
The traditional yardstick is air changes per hour (ACH): how many times an hour the fresh-air supply could replace the whole room volume. Turned round, it gives the nominal time constant:
For a room getting 6 air changes an hour, that's 10 minutes. If the room mixed perfectly, the air everywhere would be the same age, equal to the nominal time constant.
Real rooms don't mix perfectly:
- Air can short-circuit, running from a supply diffuser straight to a nearby extract grille without passing anyone.
- Corners, spaces behind tall furniture and the far ends of long rooms can be stagnant, with much older air.
- Warm supply air thrown along the ceiling in winter can stay up there and never reach the seated people below.
Two rooms with identical ACH can have very different age-of-air maps. That is the information a ventilation rate hides.
3Ventilation effectiveness
Comparing the age of air where people breathe with the perfectly mixed figure gives a measure of how well the air distribution works. It has several names (air-change effectiveness, ventilation effectiveness, the local air change index), but the reading is the same:
- About 1: the room behaves as if it were well mixed. This is typical of good ceiling-supply systems in cooling mode.
- Above 1: fresh air reaches the occupants better than mixing would. Well-designed displacement ventilation, which supplies cool air slowly at low level, is the classic example.
- Below 1: some of the supply is wasted, short-circuiting to the extract or bypassing the occupied zone.
Ventilation standards build this in. ASHRAE Standard 62.1, for example, uses a zone air distribution effectiveness to adjust the outdoor air a zone needs. It treats ceiling supply of cool air as fully effective, and warm air supplied and returned at ceiling level as less effective, so more outdoor air is needed for the same result.
A design can meet its fresh-air rate on paper and still fall below 1 in the rooms that matter. Diffuser choice, position and throw are usually the reason.
4What changes the map
- Transfer grille: a way out for the air
- Extract kept apart from the supply
- Diffuser type. Swirl diffusers mix quickly near the ceiling. Linear slot diffusers throw air along a surface. Four-way cassettes blow out of four sides. Each gives a different pattern, and the same flow can perform differently through each.
- Where the extract is. An extract grille right next to a supply diffuser is the classic short-circuit. Separating supply and extract, or extracting from the far side of the room, usually lowers the age where people sit.
- Partitions. A meeting room built across an open-plan layout can end up with supply but no sensible way for air to leave, or with air arriving only second-hand from a neighbour through a transfer grille. Both age the air in the room. Cat A vs Cat B fit-out covers why this happens so often.
- Heating versus cooling. Warm supply air rises and cool air falls, so the same diffusers can give a better map in summer than in winter.
- Occupancy and furniture. People and equipment add heat, which drives local air movement, and high-backed furniture and screens create pockets of older air.
5How age of air is measured
On site, age of air is measured with a tracer gas: a harmless gas released into the supply, with its concentration logged at points around the room. Two common approaches:
- Step-up: start dosing the supply at a steady rate and record how quickly each point's concentration rises.
- Decay: fill the room with tracer, stop, and record how quickly each point clears.
The rate of rise or decay at each point gives its local mean age. International standard ISO 16000-8 describes the method for buildings.
Tracer tests are precise but take time and equipment, so they are mostly used to diagnose problems or to validate a design, rather than as routine commissioning.
6How age of air is modelled
At design stage, age of air comes from airflow modelling. A full CFD (computational fluid dynamics) study solves the airflow in detail, including heat, buoyancy and turbulence, and is the right tool for critical spaces.
Lighter, indicative models solve the airflow from the terminal positions and design flows on a coarser grid. They are quick enough to run for every scenario, such as Cat A against Cat B or a room with and without a new branch, and good enough to show where fresh air goes and where it doesn't. They are not a substitute for CFD where precise figures matter. They are a way to start the right conversation early, while changes are still cheap.
7Age of air, CO₂ and fresh air per person
The three measures answer different questions:
| Measure | Question it answers |
|---|---|
| Fresh air per person | Is enough outdoor air being delivered for the people here? |
| CO₂ | Is the air in this room actually being refreshed for the people in it? |
| Age of air | Where in the room does the fresh air reach, and how quickly? |
A room can get the right fresh air per person and still show high CO₂ in one corner, because the air doesn't reach it. Age of air shows why. For the rates and CO₂ levels themselves, see how much fresh air an office needs per person.
8Showing it to a client
Age of air is abstract on paper and obvious in colour. A slice through the room at seated head height, shaded from fresh to stale, lets anyone see in seconds which seats get the fresh air first and which get it last. Moving particles that leave each diffuser and change colour as they age make the same point even more directly.
HVAC Viewer's airflow view shows an indicative age-of-air slice and those moving particles. The figures come from a steady-state calculation driven by the design figures, not a full CFD study, and each model says so.
9Frequently asked questions
What is the age of air?
The age of air at a point is the average time the air there has been inside the room since it arrived through a fresh-air supply. Low values mean fresh air reaches that point quickly. High values mean the air there has been circulating for a while.
What is the difference between age of air and air changes per hour?
Air changes per hour describes how much fresh air a room gets relative to its volume, as a single number for the whole room. Age of air varies from point to point and shows where in the room the fresh air actually reaches.
What is a good air-change effectiveness?
About 1 means the room is as well ventilated as a perfectly mixed room, which is typical for ceiling supply in cooling. Above 1 is better than mixing, as with displacement ventilation. Below 1 means some fresh air is wasted, for example by short-circuiting to an extract.
How is age of air measured?
With a tracer gas released into the supply air and logged at points around the room, using a step-up or decay test. ISO 16000-8 describes the method. At design stage, age of air is estimated with airflow modelling.
Why does a room feel stuffy if it meets its ventilation rate?
Because the rate only says how much fresh air enters the room, not where it goes. If supply air short-circuits to the extract, or a corner is stagnant, people there breathe older air than the rate suggests. An age-of-air map shows where.