Air changes per hour: the measure cooling does not give
A room's air changes per hour describes how often outdoor air replaces what is in it. Cooling equipment moves air without ever exchanging it, so a split unit contributes nothing to it. Once a premises carries an occupancy figure, that gap becomes somebody's obligation.
By Team Snowflake | Updated 7 Aug 2026
What does air changes per hour actually measure?
Air changes per hour, written ACH on a drawing, counts how many times the air in a space is replaced by air from outside in an hour. It is a rate, and it is always measured against the volume of the space. One air change means a volume of outdoor air equal to the room has come in. The same volume of indoor air has gone out.
The word replaced is doing all the work in that sentence. Air that leaves a vent, travels around the room and returns to the same vent has been moved, not replaced. Circulation and ventilation both describe air in motion. Only one of them changes what the air is made of.
Volume is why the same equipment scores differently in two rooms. A fan pulling a fixed flow gives a high air change rate in a small store room and a low one in a showroom of the same footprint with twice the ceiling height. A figure quoted without the space it was calculated for is not yet information.
The measure also says nothing about the quality of the air arriving. It describes speed of exchange, not what is being exchanged. When the PSI climbs, more air changes bring more of the outside in. Ventilation and filtration answer different questions, and a high figure is not automatically the better one.
Why a split system delivers none of it
A split system scores zero on this measure, at every fan speed. What the indoor unit takes in at its return grille is what it blows out again, colder. Nothing joins that loop to the outdoors, so working the unit harder leaves the count untouched.
The confusion is a units problem more than a physics one. An indoor unit publishes its airflow in cubic metres per hour or litres per second, which is exactly how a ventilation rate is written. Divide that airflow by the room volume and out comes a number shaped like an air change rate. It is a recirculation rate. The same air is being counted again on every pass.
That is why the usual levers move nothing here. A larger unit, a higher fan setting and a freshly cleaned filter all make the loop faster. None of them opens the loop. Cooling capacity is a heat number and ventilation is an air-replacement number, and no amount of the first buys any of the second.
Households arrive at the same fact from a different direction. The question there is does aircon bring in fresh air, and the answer sets out what a shut bedroom collects and what clears it. This guide stays with the measure itself, and with the premises that have to answer for it.
Where outdoor air enters a shop or office
Outdoor air arrives through a provision that somebody drew, sized and installed. In a Singapore commercial unit the candidates are few, and the cooling equipment is not among them. Naming which one a premises actually has is the whole exercise.
The openable window is the first candidate, and it is often missing. Shop units, clinics and office suites are frequently sealed by design, with fixed glazing and a shopfront kept shut so the cooling holds. Where a window does open onto a mall corridor rather than the outside, it is moving air between two indoor spaces.
Extract fans are the second, and they work backwards from how people picture them. A toilet fan or a kitchen hood pulls air out. Something has to flow back in to replace what left, so an extract sets up a draw through the rest of the premises, or through a make-up air supply installed to feed it. The extract is the reason air moves, and the replacement path is what decides whether the result counts.
A dedicated outdoor air unit is the third, and larger systems use it. A pre-treated air unit, written PAU on most drawings, takes outdoor air, cools it and dries it. It then delivers that air to the floor, or into the return of the indoor units. It is a separate machine with its own duct to a louvre outside. On a roof it is easy to mistake for another condenser.
Leakage is the fourth, and nobody designs it. Gaps around the shopfront, the door swinging as customers come and go, and unsealed service penetrations all pass some air. It is uncontrolled, unmeasured and weather-dependent, which is precisely why a code that wants a number refuses to count it.
| Where the air comes from | How to spot it on site | Whose scope it sits in |
|---|---|---|
| Where the air comes fromOpenable window or door to outside | How to spot it on siteIt opens, and what is beyond it is genuinely outdoors | Whose scope it sits inThe occupier. Nothing installed, nothing maintained |
| Where the air comes fromToilet or kitchen extract fan | How to spot it on siteA grille that pulls. Hold paper near it and it is drawn in | Whose scope it sits inUsually the fit-out contractor, with its own upkeep |
| Where the air comes fromPre-treated air unit or fresh air fan | How to spot it on siteA duct that runs out to an external louvre | Whose scope it sits inBase building, or a mechanical ventilation contractor |
| Where the air comes fromSplit, ducted or cassette indoor unit | How to spot it on siteSupply and return grilles both sit in the same room | Whose scope it sits inThe aircon contractor. Contributes nothing to the count |
| Where the air comes fromLeakage around the shopfront | How to spot it on siteNothing visible. It rises and falls with door traffic | Whose scope it sits inNobody. Not a provision, and not accepted as one |
Which Singapore rules put a number on a space
Requirements in Singapore attach to named spaces for named reasons, never to a premises as a whole. There is no single air change figure for a shop, a clinic or an office. Asking for one produces a number somebody read somewhere, rather than one in force.
NEA's Code of Practice on Environmental Health, shortened to COPEH, gives the clearest example. Under its 2024 Edition, a public toilet with an extractor or exhaust fan has to reach a minimum of 20 air changes per hour. A naturally ventilated one is held to a different test. It needs openable windows or openings adding up to at least 5% of the toilet floor area.
SCDF's Fire Code 2023 puts figures on escape routes, for a different purpose entirely. Some exit staircases and internal exit passageways are mechanically ventilated rather than naturally ventilated. Those run supply mode only, at not less than 4 air changes per hour. A smoke-free lobby or fire lift lobby is set at not less than 10. The numbers are there to keep smoke out of a route people leave by. They say nothing about the air in the office beyond that door.
Outdoor air rates for occupied spaces sit in SS 553. That is Singapore's code of practice covering air-conditioning and mechanical ventilation in buildings, shortened to ACMV. Its outdoor air provisions are set out by occupancy type. That is why this lands as a business question and not a household one. A flat has no headcount attached to it. A clinic waiting room, a classroom and a restaurant floor all do. The figure follows the use and the number of people, not the cooling gear. The rates themselves belong with the standard.
Treat any figure quoted on site as needing a source, because editions move. The 2020 Edition of that same NEA code set a different pair of numbers. It put food shop kitchens at 20 air changes per hour where mechanical ventilation was employed, and public toilets at 15. The 2024 Edition raised toilets to 20, and swapped the natural ventilation figure for an openable-area one. Its food shop section no longer states a kitchen number at all. It sets siting and exhaust-treatment requirements, then refers Qualified Persons to SS 553 instead. The older kitchen figure still circulates. Ask which document and which edition anything quoted came from, before it reaches a drawing.
| The space | What applies to it | Where it comes from, and why |
|---|---|---|
| The spacePublic toilet with an extract fan | What applies to it20 ACH minimum | Where it comes from, and whyCOPEH 2024 Edition. Odour, and keeping floors dry |
| The spacePublic toilet ventilated naturally | What applies to itOpenings totalling 5% of floor | Where it comes from, and whySame code, same clause. No fan guaranteeing it |
| The spaceExit staircase or internal exit passageway | What applies to it4 ACH minimum, supply only | Where it comes from, and whySCDF Fire Code 2023. Keeping an escape route usable |
| The spaceFire lift lobby and smoke-free lobby | What applies to it10 ACH minimum, supply only | Where it comes from, and whySCDF Fire Code 2023. The point firefighters go in |
| The spaceThe occupied floor people work on | What applies to itOutdoor air by occupancy type | Where it comes from, and whySS 553, the ACMV code. What occupants breathe |
Which toilets the NEA code calls public
The definition turns on access, not on ownership or payment. A toilet is public where the general public can walk in without being a resident, student, staff member, guest or regular client. Charging for entry to the premises does not take it out of scope.
The code then names the premises types it covers, and the range surprises most operators. Toilets in all of these are treated as public ones.
- Shopping malls and centres, including the shop floors of commercial buildings
- Supermarkets and wet markets
- Restaurants, coffeeshops, hawker centres, food courts, bars, pubs and nightclubs
- Conference, convention and exhibition halls, cinemas and theatres
- MRT stations, bus terminals and interchanges, petrol stations, stadiums, parks and public pools
What to establish before signing a cooling scope
Settle who supplies outdoor air before the cooling equipment gets chosen. On a tenanted office floor it is often the base building, arriving through a duct nobody in the tenancy ever sees. In a strata shop unit it is frequently nobody at all, and that is a finding to work with rather than a fault to fix.
A cooling quote priced on capacity is answering a different question. Capacity is about heat, and outdoor air is about replacement. A quote can be correct on the first and completely silent on the second, and both sides can sign it without noticing, because nothing in it was ever wrong.
The fit-out drawing settles most of it in one look. Mechanical ventilation appears there as its own system, with a fan, a duct and a louvre on an external wall. If the drawing shows indoor units and a condenser and nothing more, the premises has whatever the base building hands it and no more than that. The submission is also where each space got assigned a provision, or did not.
This surfaces at licensing or fit-out because that is when a Qualified Person commits it to a drawing. By the time a servicing contractor is standing in the unit, the provision either exists or it does not. Putting one in afterwards is building work. There is a wall penetration, a louvre and an approval behind it, and no quantity of aircon work substitutes for that.
Once a premises is open, a carbon dioxide reading is the usual way to check. That is a measuring job rather than an equipment one. What a cooling contractor can settle is narrower, and still worth settling before anyone starts guessing.
- Whether any duct in the premises runs to a louvre or opening on an external wall
- Which spaces were given a ventilation provision in the licensing or fit-out submission
- Who maintains that provision, given a cooling contract usually stops at the indoor units
- Whether the toilets on the premises fall inside the public toilet definition in the NEA code
- Which document, and which edition of it, any air change figure quoted to you came from
What a cooling contractor can and cannot answer
An aircon contractor can name the gear in a room and trace where a duct goes. That covers which indoor units only recirculate. It covers whether anything on site is truly joined to the outdoors, and which grille belongs to which system. Those answers are usually enough to show whether a ventilation question exists at all.
Sizing a fresh air supply against a code is a different trade, and a different signature. It sits with the mechanical and electrical consultant, and with the Qualified Person on the submission. Anyone offering to clear a licensing condition by servicing the cooling has misread which machine the condition is about.
Ready to get started?
Tell us what’s going on. Symptoms, setup, photos, anything we should know. We’ll assess and come back with the right next step.