What a VMC is, and what it asks of an existing flat

The word that matters in "VMC" is "controlled"

VMC stands for ventilación mecánica controlada — ventilació mecànica controlada in Catalan, which is the name the Generalitat's own guidance for dwellings is published under. You will also meet ventilación mecánica and ventilación forzada, in listings, in quotations and from tradespeople. In a dwelling, treat all three as the same idea. Forzada is not a separate technology; it is a blunter way of saying that a fan, and not the weather, is what moves the air.

The word carrying the weight is controlada. A natural system moves whatever air the wind and the indoor-outdoor temperature difference happen to offer that day. A mechanical system moves a number somebody chose: the caudal, the design flow rate in cubic metres per hour, worked out room by room from DB-HS3, the part of the Código Técnico de la Edificación that governs indoor air quality. That number is the product. The fan is only how it is delivered.

Hold onto that when you read a quotation. "A more powerful extractor" describes a component; "continuous extraction at the design caudal for these rooms, with admission here and transfer there" describes an installation, and only the second can be checked once the work is done.

One caution for anyone arriving with prior experience. France uses the same three letters for the same family of systems, and English-language material calls the branches continuous mechanical extract and MVHR. The hardware is recognisable across all of them; the rulebook is not. In a Spanish dwelling the requirements come from DB-HS3, and for a fixed installation from RITE.

Why a VMC is built to run when nobody is home

Moisture, carbon dioxide and smells are produced on a continuous schedule. Two people asleep for eight hours in a closed bedroom, a bathroom still drying two hours after a shower, laundry on a rack in a room with the door shut — none of that waits for someone to press a switch.

So a mechanical system is designed around a low rate that runs all the time, with occasional boosting on top: the cooker hood while you cook, a run-on timer in the bathroom, or grilles that open further when humidity rises. The alternative — a strong fan operated by hand once the mirror has already fogged — treats air renewal as a reaction to a symptom that has, by then, already happened.

Continuity also buys quiet. Moving a small volume slowly through generously sized ducts is close to silent; pushing the same daily volume through in a few minutes needs velocity, and velocity is noise, at the grille and along the duct.

Two consequences follow, and both surprise people:

• Switching it off is not a saving, it is a different flat. A dwelling that has been refurbished and fitted with airtight windows has very little accidental leakage left. Turn the system off overnight for quiet and the planned route stops, with nothing behind it.
• More outdoor air is not dehumidification. On a humid coastal night in August, running the system harder renews the air without drying it. A VMC replaces indoor air with outdoor air. In a Mediterranean summer that is worth remembering before expecting it to solve a damp feeling.

Extract-only or balanced: the fork that decides everything else

	Extract-only (simple flujo)	Balanced (doble flujo)
How outdoor air gets in	Through passive inlets in the dry rooms, drawn by the slight depression the fan creates	Ducted and fan-driven into each dry room
What is ducted	The extract side only	Both sides: two separate networks
Incoming air	Outdoor air, at outdoor temperature and humidity, with little or no filtration	Filtered at the unit before it reaches the rooms
Heat	Leaves with the extracted air	A large share can be transferred to the incoming air by an exchanger
What the flat gives up	One duct route, a fan, an outlet	Plant space, two duct routes, filter access, sometimes a condensate route

These are not two grades of the same product. Extract-only solves renewal. Balanced solves renewal and additionally tempers and filters what comes in, at a much larger cost in building work. A flat with no route for two duct networks has, in practice, already chosen.

Extract-only (simple flujo), described properly

This is the lighter of the two branches, and the one an existing flat is likelier to be able to accommodate, so it is worth setting out in full. It consists of extract terminals in the wet rooms, a duct from them to an outlet, a fan — one central unit, or a small one per wet room — passive inlets in the dry rooms, and a transfer path between the two.

The passive inlets are the part people forget. They are trickle vents in the window frame, aireadores, or purpose-made through-wall inlets in bedrooms and living rooms. The fan holds the dwelling very slightly below outdoor pressure, so outdoor air is pushed in wherever it can find a way. That is both the elegance of the design and its whole vulnerability: it works only if the ways in are the ones that were intended.

The failure to watch for is the half-installed version. The extract goes in; the inlets never do. The flat then takes its replacement air from wherever is easiest — the stairwell, the gap around the front door, or backwards down a duct shared with other flats. The signs are recognisable: internal doors that pull themselves shut, a whistle at a window on a windy day, cooking smells that belong to somebody else's dinner.

One item to raise early rather than late. Where a combustion appliance takes its air from the room it stands in — a gas water heater in a kitchen or on an enclosed galería — its interaction with a system that deliberately depressurises the dwelling has to be assessed by whoever designs the installation. It can change the answer, so it should not surface after the ducts are in.

Inlets and grilles that respond to humidity

Humidity-controlled ventilation — higrorregulable — is extract-only with variable openings: inlets, extract grilles or both open further as relative humidity rises and close back as it falls. More air when a bathroom is in use or a bedroom has been occupied all night, less when the flat is empty. This is a recognised system subtype in Spain, with a technical suitability document — a documento de idoneidad técnica, or DIT — issued by the Eduardo Torroja institute.

Where it earns its place is a dwelling whose occupancy swings: a coastal flat used at weekends and empty midweek, a spare bedroom shared some nights and unused for months. A fixed rate has to be set for the demanding case and then runs at that rate regardless of whether anyone is in.

Two honest limits. It modulates flow along an air path; it does not create one, so the inlets, the transfer openings and the extract all still have to exist and still have to be right. And a technical suitability document assesses a system, not your dwelling.

The balanced branch, in short

Balanced ventilation — doble flujo — ducts both directions. Filtered outdoor air is delivered to bedrooms and living rooms while stale air is drawn out of the bathrooms and kitchen, both sides fan-driven. Residential units of this type generally include an exchanger that passes a large share of the heat from the outgoing stream to the incoming one without the two mixing. The unit itself, the duct layout it demands and what heat recovery is genuinely worth in this climate are subjects with their own pages; this one stops at the fork.

What your building decides before you do

Extracted air has to leave the building, and in a flat that exit is rarely yours alone to choose.

• A façade outlet alters the appearance of the façade, which is a communal element.
• A shared vertical duct — a shunt, serving stacked bathrooms or kitchens up through the block — is communal by definition. Connecting to it, or discovering that it no longer draws, is a building question rather than a flat question.
• A roof terminal means work on, and access across, the communal roof.
• An interior light well — patio de luces, pati de llum — is a narrow shaft that several flats already discharge into. Whether it can take another extract, and at what height, is decided for the building as a whole.
• A single-aspect flat, with every opening on one façade, may end up with its inlets and its outlet on the same face. Where they sit relative to each other stops being a detail.
• If you rent, none of the above is yours to decide at all.

The practical move is cheap: ask the administrador de finques, the building's managing agent, what the vertical ducts serve and what the community of owners has allowed in other flats. One answer can remove a whole branch from the table before you have looked at a single product.

What a retrofit takes out of the flat itself

What the system needs	What that costs in an existing flat
A duct route from wet rooms to the outlet	A false ceiling over the corridor and bathroom, and the ceiling height it eats; or boxing run along a wall
A permanently live electrical supply	A circuit that stays on. A fan meant to run continuously cannot sit behind a light switch
Somewhere to house a central unit	Reachable space with a hatch. A unit sealed above plasterboard is a maintenance problem for the life of the installation
Quiet at the continuous rate	Anti-vibration mounting, duct sizing, attenuation, and enough separation that one duct does not carry conversation between rooms
Access for cleaning and filters	Grilles and inlets you can reach, and in a balanced system a filter change on a schedule

Noise deserves separating out, because it decides whether the system is ever left running. A system that is inaudible in the hallway and audible from a pillow at three in the morning gets switched off, and a system that is switched off is not a system. Ask where the fan sits relative to the bedrooms, and ask what happens to sound travelling between two rooms served by the same duct.

Timing matters as much as specification. All of this is ordinary work while ceilings are open during a refurbishment and disproportionate afterwards. If a reform is already planned, the ventilation decision is at its cheapest now; if the flat is finished, expect the honest answer to be a lighter, more local solution.

Regulated work, and what that makes you as the client

A fixed ventilation installation falls within the scope of RITE, the Reglamento de Instalaciones Térmicas en los Edificios, Real Decreto 1027/2007 as amended. RITE covers design, installation, maintenance and inspection duties, and companies carrying out RITE-scope installation or maintenance work in Catalonia must be entered in the RASIC industrial-safety register. That company registration is separate from the documentation and, where the official thresholds apply, registration of the installation in RITSIC. Whether a particular residential job needs that procedure depends on the installation and its declared power; “fixed ventilation” alone does not answer it.

Read the shape of that rule carefully: it is written about companies, and about fixed installations. Swapping a bathroom extractor for a similar one is not the same scale of work as ducting an entire flat, and where a particular job sits is a judgement for the professional in front of you, not something to settle from a screen.

What it changes for the person paying:

• The design flow rates, room by room, are a reasonable thing to ask for in writing before work starts.
• So is the question of how those rates will be verified once the system is in.
• A fixed installation generates documentation. That paperwork is what a future buyer, or a future dispute, gets read against.
• Maintenance belongs to the design rather than to the afterthought: filters, grilles and the fan all have a schedule, and a neglected system drifts away from the figures it was commissioned at.

DB-HS3 sits on the other side of the same job. It fixes what the dwelling has to achieve — which rooms admit air, which rooms it is extracted from, how it transfers between them, and the separate provision kitchens require for cooking fumes, over and above the general ventilation of the home. The Catalan architects' association publishes an application guide to it, which is the reading professionals here work from.

What to settle before anyone opens a catalogue

Six questions, in this order, and none of them needs a specialist:

1. Is there any designed way for outdoor air to reach the bedrooms and living rooms? If there is not, extract-only has nothing to pull, whatever fan is fitted.
2. Do the internal doors still have a gap or a grille underneath, or did a refurbishment quietly close them?
3. Does anything actually leave the existing wet-room extracts, and where does that duct end up?
4. What will the building permit as an outlet?
5. Is the flat occupied steadily, or in bursts? Steady occupancy suits a fixed rate; irregular occupancy is where humidity-controlled openings earn their keep.
6. Is a refurbishment coming anyway? Ducts are ordinary while the ceilings are open and expensive once they are closed.

A torch, an afternoon and one conversation with the managing agent will answer most of them. Each answer that comes back removes options rather than adding them, and that is the only direction in which this decision ever gets smaller.