PAU Explained: Why Outdoor Air Gets a Machine of Its Own
The air outside a Singapore office holds more water than a room unit can strip out on top of its own job. Larger buildings treat that outdoor air separately, before it reaches a fan coil, and delete the step and the damp arrives with nobody's name on it.
By Team Snowflake | Updated 16 Sept 2026
The job a pre-treatment unit does before the air arrives
A pre-treatment unit conditions outdoor air on its own. That happens before the air meets anything in the occupied space. NEA's ACMV handbook lists it as a primary air handling unit, written PAU on paperwork, with its duty given as cooling and dehumidifying outdoor fresh air.
Feeding it undiluted outdoor air is the reason it exists as a separate box. The same handbook notes that a PAU handles an airstream made up entirely of outdoor air, which carries a lot of moisture, so its coil works on the wettest air in the building.
The conventional alternative is to mix outdoor air into a much larger stream of return air and send the blend across one coil. That arrangement works against itself. Outdoor air of high moisture content joins a large volume of drier return air, and since the outdoor share is smaller, the blend comes out relatively dry. The handbook states the consequence: the potential to dehumidify that mixed air drops, which pushes designs toward colder water and deeper coils.
Why a fan coil cannot absorb it on the way past
A fan coil is not refusing the work; its control loop is answering a different question. A technical paper on outdoor air systems in Singapore describes the units downstream of a PAU as reacting to internal heat loads rather than to the humidity of the airstream. They stop when the room reaches temperature, and whatever water remains in that air stays in the room.
Capacity is the second limit, and it hides better than the first. NEA's handbook shows two coils with identical total cooling capacity removing very different proportions of sensible and latent heat. It concludes that such a system can hold the preset space temperature while missing the desired relative humidity. A correct capacity figure says nothing about which half of the job it does.
Why does outdoor air get counted as a separate load?
Two loads land in every conditioned space, and they originate in different places. The room makes one through its occupants, lighting, equipment and the sun on its glass; outdoor air brings the other in with it, and that share arrives whether the room is busy or empty.
NEA's handbook sets out how a chilled water design divides the two. Fan coil units are described as mainly carrying the sensible heat load of the spaces, while a central primary air handling unit cools and dehumidifies the outdoor fresh air and delivers it through ducts. That conditioned air supplies the ventilation and carries away the moisture from the latent load.
The wider literature names the arrangement. A dedicated outdoor air system is defined as two parallel systems: one dedicated to the ventilation air and handling both its latent and sensible load, the second handling the mostly sensible loads generated indoors. Singapore practice reaches the same shape through the PAU.
Where pre-treatment is present, the handbook says the downstream coils are then designed to handle mainly sensible cooling and a relatively small latent load. That sentence is the part worth carrying away: the terminal equipment on a fit-out was selected on the assumption that the water had already been taken out upstream. Remove the assumption and every selection below it is wrong.
| What generates it | Where it is meant to be handled | What shows up when it is not |
|---|---|---|
| Heat given off by people and equipment | The unit serving that room | The room drifts warm once it fills up |
| Sun landing on glass through the afternoon | The unit serving that room | One side of the floor never quite settles |
| Water riding in with the outdoor air | A pre-treatment unit, upstream of both | Air that reads cold and still feels sticky |
| Water given off by the occupants themselves | Whichever coil is doing the drying | Grilles and glazing collecting droplets |
| Ventilation air that arrives untreated | Nothing was ever assigned to it | Damp that tracks headcount, not weather |
- What generates it
- Heat given off by people and equipment
- Where it is meant to be handled
- The unit serving that room
- What shows up when it is not
- The room drifts warm once it fills up
- What generates it
- Sun landing on glass through the afternoon
- Where it is meant to be handled
- The unit serving that room
- What shows up when it is not
- One side of the floor never quite settles
- What generates it
- Water riding in with the outdoor air
- Where it is meant to be handled
- A pre-treatment unit, upstream of both
- What shows up when it is not
- Air that reads cold and still feels sticky
- What generates it
- Water given off by the occupants themselves
- Where it is meant to be handled
- Whichever coil is doing the drying
- What shows up when it is not
- Grilles and glazing collecting droplets
- What generates it
- Ventilation air that arrives untreated
- Where it is meant to be handled
- Nothing was ever assigned to it
- What shows up when it is not
- Damp that tracks headcount, not weather
Why the second machine is not a spare
Failure behaviour separates them cleanly, and it is the test to apply. Lose the units in the space and the room warms up while the air coming into it stays dry. Lose the pre-treatment unit and the room can hold its temperature perfectly while every surface turns damp. Two unlike complaints, two unlike machines, and only one of them reachable from inside the tenancy.
Where the unit sits, and what it hands its air to
A pre-treatment unit sits upstream of the space and never inside it. Everything it handles comes from outdoors, so it needs a duct to a louvre or intake. It also needs floor area for a filter stage, a coil, and a fan strong enough to push treated air out to wherever it is owed.
Delivery goes one of two ways, and the handbook names both: cold, dehumidified air leaving a PAU can be supplied straight to the conditioned spaces, or fed into the inlet of the air handling units. Which is in use decides where a shortfall becomes visible.
Straight delivery gives the treated air its own diffusers, running in parallel with whatever the room unit is blowing. Feeding an intake hides it completely. The outdoor air is already inside the supply air before it leaves the plant, and an occupant sees one set of openings with no way to tell what is in them.
Premises carrying a large ventilation requirement are where these units concentrate. NEA's handbook offers hotel guest rooms as the example: many small spaces, each owed outdoor air, each with a terminal unit doing nothing beyond holding a temperature. Offices with dense occupancy, clinics and function rooms sit in the same category.
None of this vocabulary describes a household split system. A flat's indoor unit takes room air, chills it and gives it back, with nothing upstream treating anything. Pre-treatment belongs to buildings where somebody was obliged to bring outdoor air in, and where the size of that obligation made it its own machine.
Telling treated air from recirculated air at the ceiling
Opening count is the first clue, and it needs no drawing. A space fed only by its own unit has a supply and a return belonging to the same box, and the air runs in a closed loop between them. A space also receiving pre-treated air has a supply opening with no matching return, because that air came from outdoors and leaves through an extract or the building itself.
Behaviour once the room unit stops is the confirmation. Where treated outdoor air is delivered straight to a space, something keeps blowing after the terminal unit shuts down, often on the building's own schedule rather than the wall controller. Where everything goes quiet together, the space has no independent supply.
What a space does when the provision falls short
Undersizing or deleting a pre-treatment unit produces no fault anyone can point at. It produces a building that holds its temperature and cannot hold its moisture, and the complaints that follow get blamed on almost anything except the machine nobody built.
The failure path is documented. A Singapore technical paper describes engineers assuming the cooling downstream of a PAU will finish dehumidifying the outdoor air. It states that this is rarely what happens, because those units answer to heat and not to the moisture in front of them. When they ramp up on a temperature rise, the cold air they deliver takes the space air below the dew point, and condensation and mould follow.
What an occupier reports is never a shortfall on an outdoor air unit. It is a space that reads cool on a thermometer and sticky to everyone standing in it. It is skirting or gypsum staying damp at the perimeter long after anyone has stopped looking for a leak. It is a stack of small complaints that never point at a machine.
The thermostat is the wrong lever in both directions. Set it lower and the terminal units run longer, pulling out more water while also bringing surfaces closer to the temperature where condensation forms. Set it higher to save energy and they run less, so more of the incoming water stays behind. Neither setting reaches the air arriving from outside.
Push back on a scope that answers a damp complaint by cleaning the units in the space. Servicing returns a coil to the duty it was selected for, and on a fit-out with pre-treatment in the design, that duty was mostly temperature. A quote written as though a clean coil will resolve the moisture has priced the wrong machine.
Complaints that get filed under the wrong machine
- A meeting room that turns clammy once it fills and recovers on its own when it empties.
- Droplets on diffuser faces and on the ceiling tiles immediately around them.
- A musty smell returning soon after every clean, strongest near the supply openings.
- Paper, labels and card stock that refuse to lie flat in one part of the floor.
- Mould appearing on the cool side of a partition while the warm side stays clear.
Questions that pin down the air arriving in a space
Establish whether anything treats outdoor air before it reaches the space, and establish it before a cooling scope is signed. Where a mechanical drawing exists the answer is on it; where none exists, it is a short conversation with whoever manages the building, and the answer is either a named unit or silence.
Ownership follows immediately from the answer. A pre-treatment unit almost never sits inside a tenancy: it serves several occupiers at once through shared ductwork, so its filters, coil and controls belong to the building. An occupier with a moisture problem is therefore often reporting plant that is not theirs to open, and a scope written from inside the leased area prices around it without saying so.
Ask any contractor pricing a humidity complaint which machine the water is coming off. If every line in the answer names equipment inside the leased area, nothing in that quote has accounted for the air entering it.
Three things are answerable on site with no paperwork. Does any ceiling opening blow without a return grille near it, and does the damp track headcount or the weather outside? Did neighbours on the same floor start noticing it at the same time? None of those name the machine, but together they say whether the problem stops at the tenancy boundary.
- Whether any unit outside the leased area was assigned to treat outdoor air for this floor.
- Which ceiling opening, if any, that unit's air actually arrives through.
- Who maintains it, under what arrangement, and where that arrangement stops.
- Whether the damp complaint follows occupancy or follows the weather.
- What the space was drawn to be used for, and what it is being used for now.
| What to ask | What a clear answer sounds like | Why it changes the scope |
|---|---|---|
| Is outdoor air treated before it reaches this floor | A named unit, with its own tag and duct | Decides whether the moisture has an owner |
| Where does that unit's air enter the space | Its own diffusers, or another unit's intake | Sets where a shortfall becomes visible |
| Who holds the upkeep on it | The building, under its own arrangement | Puts those filters outside a tenancy scope |
| What was it selected to deliver | A duty stated on the mechanical schedule | Shows what the fit-out was allowed to add |
| What has changed here since handover | Headcount, partitions, or a different use | Load added later sits outside the design |
- What to ask
- Is outdoor air treated before it reaches this floor
- What a clear answer sounds like
- A named unit, with its own tag and duct
- Why it changes the scope
- Decides whether the moisture has an owner
- What to ask
- Where does that unit's air enter the space
- What a clear answer sounds like
- Its own diffusers, or another unit's intake
- Why it changes the scope
- Sets where a shortfall becomes visible
- What to ask
- Who holds the upkeep on it
- What a clear answer sounds like
- The building, under its own arrangement
- Why it changes the scope
- Puts those filters outside a tenancy scope
- What to ask
- What was it selected to deliver
- What a clear answer sounds like
- A duty stated on the mechanical schedule
- Why it changes the scope
- Shows what the fit-out was allowed to add
- What to ask
- What has changed here since handover
- What a clear answer sounds like
- Headcount, partitions, or a different use
- Why it changes the scope
- Load added later sits outside the design
Why the damp often shows up long after handover
Fit-outs change what a space does, and the outdoor air provision was sized against what the drawing said it would do. Turning a store room into a meeting room, or a quiet back office into a call floor, raises occupancy without anyone touching a machine. Occupancy is the variable that moves the water: more people means more outdoor air owed and more moisture given off inside, and neither reaches the cooling equipment as a signal it can act on. The gear still delivers what it was selected for; what changed is the room it was selected against, and that mismatch surfaces as damp instead of heat.
Common questions
What is a PAU in aircon?
What is the difference between a PAU and an FCU?
Why does an office feel clammy when the thermostat reads cold?
Can a PAU be removed from a fit-out?
Who maintains a PAU?
Sources
- Air-Conditioning and Mechanical Ventilation Systems
National Environment Agency · Checked
Primary air handling units and their outdoor-air dehumidification role.
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