Air conditioning cools the air; it does not renew it

The air goes round the room; only the heat leaves

Follow the air in a cooled bedroom for an evening. The indoor unit pulls it in through the grille along the top, drags it across a cold coil and pushes it back out at the front a few degrees cooler. Then the same air comes round again. No outdoor air ever joins it, because an ordinary domestic split has no route to the outside for air at all.

What does leave is the heat, and it leaves as heat rather than as air. A refrigerant sealed in a loop between the two units picks it up at the indoor coil and releases it at the outdoor one, out on the balcony or the light well. That loop is closed. It never meets the air you breathe, and if it ever did, that would be a fault.

This is why the hole through the façade is so small. Through it pass two insulated copper lines, a thin condensate hose and an electrical cable. A duct capable of carrying a useful amount of outdoor air would need an opening of a completely different order, because air occupies volume and needs cross-section, while refrigerant travels down a pipe the thickness of a finger.

So the appliance can run all evening in a shut room, and the air in that room at midnight is the air that was there at six o'clock, minus some heat and some water vapour.

Why the phrase misleads English speakers

The confusion is built into the vocabulary, and it is worse in English than in Spanish.

In building engineering, conditioning air means treating it: temperature, humidity, cleanliness, movement. Large systems in offices, hotels and shopping centres do all of that and handle outdoor air as well, drawing it in at an air-handling unit, mixing, filtering and distributing it. The phrase "air conditioning" comes from that world. A wall-mounted domestic split belongs to the branch of the family that treats air without transporting any: it is a heat pump with a fan in front of it. Anyone who has lived with a ducted central system elsewhere, where a share of outdoor air is introduced at the air handler, is carrying over an assumption that does not survive the move.

Spanish and Catalan keep the two ideas further apart than English does, which is useful once you notice it. Climatización, in Catalan climatització, means heating and cooling. Ventilación, ventilació, means renewing the air. On specifications, quotations and building paperwork they appear as separate headings, because they are separate installations.

Then the remote control undoes the good work:

Mode on the remote	What it changes	Outdoor air introduced
Cool, Frío	Coil below room temperature; cools and drains off water	None
Dry, Deshumidificar	Cools more slowly so more vapour condenses on the coil	None
Fan, Ventilador, Ventilación	Coil off, fan on; stirs the air already in the room	None
Heat, on a reversible unit	Loop runs the other way, collecting heat from outdoors	None
Ion, plasma and similar extras	Act on the air already in the room; this site makes no claim for them	None

The third row is the trap. A remote sold in Spain may label that button Ventilación, the same word the technical code uses for something else entirely.

Cool, dry and moving: three sensations that read as fresh

Walk into a cooled room and you register three things at once: fewer degrees, less water in the air, and a current moving across your skin. Those same three arrive together when you open a window on an October morning, so the brain files them under one heading and calls it fresh air.

Something simpler reinforces it. A home's own smell stops registering after you have been inside a while — it has not gone anywhere, you have stopped noticing it. A visitor arriving from the street picks it up immediately and you do not. So a closed room gives you no warning through the nose either.

None of these sensations carries information about what is in the air. Temperature does not record how many people have been breathing in the room, how many hours it has been shut, or what was fried at lunchtime. Air can be pleasantly cool and six hours old; it can be uncomfortably warm and have come through the window a minute ago.

What it does well, including the water dripping into the light well

This page is not an argument against the appliance. It does two things properly, and one of them usually goes unnoticed.

It moves heat out of the room. Not hides it, not masks it: transports it to the street. In a Mediterranean summer that is a real service, and no ventilation habit substitutes for it on the afternoons when it is hotter outside than in.

It dries the air, whether or not you asked it to. The indoor coil runs below the dew point of the room's air, so water vapour condenses on it, collects in a tray and leaves through the drain hose. The trickle you hear landing in the light well or on the balcony below was in your living room a few minutes earlier: breath, the shower, the pan on the hob. This is why a cooled room feels drier, and why on this coast the same temperature is far more bearable with the unit running than without it.

The filter is a separate matter. The washable mesh catches lint and coarse dust, and its main job is to protect the coil behind it. Cleaning it helps the machine work as intended; it does not turn the appliance into a treatment for your air.

One caution belongs here, because the dryness is genuinely useful and it is easy to over-read. Removing moisture while the unit runs is not the same as resolving a damp problem. It does nothing about a leak, nothing about a wall that wets whenever the unit is off, and nothing about mould already visible on a surface. World Health Organization guidance on dampness and mould supports cautiously linking persistent indoor dampness and mould with respiratory symptoms, and that is a situation for finding the cause rather than lowering the temperature.

What more running time will never change

Everything that accumulates in a closed flat accumulates just the same with the unit on, apart from heat and part of the water vapour.

Carbon dioxide. It is exhaled by the people inside, and no recirculating appliance takes it out. The only thing that lowers it is replacing the air with air from outside. Cool it, dry it, filter it, blow it about faster: the CO₂ stays.

Odours. These are more deceptive, because cooling does mute them for a while and then redistributes them colder. Frying smell is not eliminated by degrees. It is diluted when new air arrives, and much better dealt with where it is produced — which is why DB-HS3 asks for a specific extraction provision in the kitchen for cooking fumes, on top of the dwelling's general ventilation.

Renewal itself. The Spanish code describes a route: air admitted in the dry rooms, crossing the dwelling, extracted from the wet rooms, out. A split takes part in none of those three steps. It admits nothing, transfers nothing and extracts nothing.

An August night in a single-aspect flat

The situation where all of this becomes concrete is always the same one. A bedroom, two people, windows shut to keep the cool in, the blind down and the unit on night mode.

By three in the morning it has done exactly what it was asked to do: the room is cool and the air is drier than it was at bedtime. It has also gone five hours without being changed. What you notice on waking is not heat but stuffiness — which is why the reflex response, one more degree down on the thermostat, changes nothing. It acts on the only variable that was already right.

The shape of the flat decides how far it goes. Where the openings are all on one façade there is no cross route at all when you finally do open up: air comes and goes on the same side. A bedroom giving onto an interior light well — pati de llum, patio de luces — draws on air that moves little and is often humid, so the same open window achieves less than one onto the street.

With everything shut, the route that does renew is the one the code describes, and it depends on two things the air conditioner never touches. First, that the bathroom and kitchen extraction is genuinely pulling. Second, that air can reach it: an extractor only removes air from a room if replacement air can get in, and a close-fitting door with no gap underneath and no transfer grille is an easy-to-miss reason a bathroom fan can appear to do nothing at all.

Two rule books, and where this site stops

Keeping cooling and renewal apart is not an editorial preference; in Spain the two sit under different instruments.

Renewing the air of a dwelling is dealt with by Documento Básico HS 3, Calidad del aire interior, of the Código Técnico de la Edificación — the air path just described. Fixed thermal installations in a building, ventilation systems among them, fall under the RITE, Real Decreto 1027/2007 as amended, which sets criteria for design, assembly, maintenance and inspection. Ventilation is not an accessory of the climate equipment. It has rules of its own.

Equipment that does deliberately bring outdoor air into a dwelling exists — balanced mechanical ventilation, with or without heat recovery. That is a ventilation installation, not a setting on the air conditioning. Where a climate unit includes an outdoor-air intake, the intake is sized and assessed as ventilation, with an air flow rate: the caudal you will see on a specification sheet.

The sealed loop is the other boundary, the one around this site, and it is not arbitrary either. Intervening in it — charging it, emptying it, tracing a leak, taking a unit down to move it — is certified work. Real Decreto 115/2017 and the European F-Gas framework require certification of the personnel and the companies that handle these circuits.

So air conditioning appears on this site only where it clears something up about ventilation. There is nothing here about installation, breakdowns, servicing, gas top-ups or models. If a unit cools less than it used to, ices up, makes a noise it did not make before or drips indoors, that is a question about the equipment rather than about the air in your home, and it is answered somewhere else.

Reading a flat that already has air conditioning

When a listing advertises A/A or aire acondicionado, that line tells you about cooling. It says nothing about how the flat renews its air, and the second question is worth asking separately — at a viewing, in the first week after moving in, or before signing off a refurbishment. Four things to look at, none of which needs tools:

1. In each bedroom and living room, some way for outdoor air to arrive: a slot or grille in the window frame, an aireador, a vent through the external wall — or nothing.
2. Under the internal doors, a gap, or a transfer grille in the leaf. Where a door sweeps the floor, the path stops there whatever else is installed.
3. In the bathroom and the kitchen, extraction that visibly moves air, plus a kitchen provision for cooking fumes separate from the general ventilation.
4. Where that extraction goes: an individual duct, an older shared vertical duct — a shunt — or a grille onto a light well. Shared ducts, façade openings and roof outlets are communal elements, and what may be altered is decided by the building rather than by the occupant.

A flat can have cooling in every room and none of the four. If that is what you find, the appliance on the wall was never going to fix the air, and it is not the thing to spend the next summer adjusting.