Double-flow ventilation in Spain: two duct networks, not one box

What a balanced system is balancing against

In an extract-only system — simple flujo in Spanish, simple flux in Catalan — only one half of the exchange is mechanical. A fan pulls air continuously out of the bathrooms and the kitchen, the flat sits slightly below outdoor pressure, and replacement air arrives through whichever gap is most willing: a window trickle vent if the flat has any, otherwise the gap under the front door, a disused flue, a shared vertical duct, the perimeter of an old frame. The total volume of air moved can be exactly as designed while the air itself arrives from somewhere nobody chose.

Double flow — doble flujo, doble flux, sold in English-language material as MVHR or heat recovery ventilation — closes that loose end by making supply mechanical too. One duct network delivers outdoor air into the bedrooms and living rooms, the other extracts from the wet rooms, and the two flow rates are set close to one another. With both halves working, the dwelling stays near outdoor pressure and stops sourcing its own replacement air.

Balanced does not mean identical to the litre. It means the difference between what goes in and what comes out is small and deliberate rather than accidental. That single property is what makes double flow a different architecture rather than single flow with a component bolted on.

What does not change is the route through the flat. DB-HS3, the indoor air quality section of Spain's technical building code, sets out the same circuit for any dwelling: air admitted in the dry rooms, crossing the circulation space, extracted from the wet ones. Double flow does not redraw that route; it drives both ends of it mechanically instead of one.

The supply side is the half you cannot buy separately

A system that only extracts has no say over the incoming air. It arrives through whatever aperture exists, at whatever temperature and with whatever it is carrying. Duct the supply and three decisions appear that did not exist before.

Where the air is taken from. The intake is positioned on purpose: the quieter façade, the roof, well away from the system's own exhaust and from any flue outlet. For a single-aspect flat with bedrooms over a busy street, or one whose openings give onto a patio de luces — the interior light well, pati de llum in Catalan, a narrow shaft where the air is humid and barely moves — that choice matters more than any specification on the equipment datasheet.

What happens to it on the way in. Supply air crosses a filter before it reaches a room, and what the filter holds back depends on the class specified. A passive inlet filters nothing; whatever the street is offering comes straight through.

What temperature it arrives at. The supply passes through the unit, which tempers it using heat from the outgoing stream. The consequence at system level is the part that matters here: air can be delivered directly into a bedroom without a cold draught landing on the bed. With a passive trickle vent above the headboard there is nothing to adjust, and the January temptation is to tape it shut — at which point the extract fan is pulling against a sealed flat. How the exchanger achieves the tempering, and how much it recovers, is a separate subject with its own page.

Two things this does not do. It does not replace the specific extraction for cooking fumes that DB-HS3 requires in the kitchen on top of the dwelling's general ventilation, so the hood keeps its job. And it is not air conditioning: a ventilation unit is sized to renew air, not to heat or cool the flat, and it is not a dehumidifier either.

What two duct networks ask of a flat that already exists

This is where double flow is won or lost, and it is decided by the plan of your flat rather than by a product comparison.

What the system needs	What that means in an existing flat
Two duct routes, supply and extract	A continuous void, usually a dropped ceiling along the corridor, with height for both networks where they cross. Measure the clear corridor height and the gap between the door heads and the ceiling
Somewhere for the unit to live	It has weight, it vibrates and it has to be opened. It needs an access hatch, a condensate drain within reach and a position that does not share a partition with the head of a bed
Two openings to the outside	Intake and exhaust, far enough apart that the system does not re-inhale what it has just expelled. Façades and roofs are communal elements, so that conversation belongs at the start
Transfer openings between rooms	Supply leaves the air in the bedrooms, extract collects it in the bathroom and kitchen, and doors sit in between. Without an undercut or a grille — abertura de paso — the circuit breaks indoors however good the ducts are
Acoustic separation	Three kinds of noise: the unit itself, duct-borne noise reaching the terminal, and crosstalk between two rooms joined by the same duct. The most sensitive bedroom is the judge
A decision about existing shafts	Whether an old communal shunt serving stacked bathrooms can be reused is a technical question and a community one. Never open or alter a communal duct yourself

Field studies of balanced-ventilation retrofits in Spanish dwellings, including buildings under heritage protection, keep landing on the same constraint: the hard part is getting ducts and penetrations through a building that was never designed for them. The technology is not the bottleneck.

Commissioning is not the same as switching it on

The design assigns each room a caudal — an airflow rate, the word that will appear on every quotation and specification you are shown. An installation does not deliver those figures by itself. Each branch has its own length, bends and resistance, and a fan sends air down the path of least resistance, not the fairest one.

Balancing — equilibrado, part of what a Spanish installer calls puesta en marcha — means measuring at each terminal with an instrument, adjusting the terminal cones and the regulating dampers, measuring again, and repeating until the distribution approaches the design. The two totals, supply and extract, are then set against each other. It is a day of work that leaves no trace in any photograph of the finished job.

Skip it and the system can fail in two ways at once:

• Badly distributed. The totals look right, but one bedroom takes most of the supply and whistles, another receives almost nothing and stays as stuffy as before, and the bathroom extracts less than intended and still takes hours to dry. The impression is that the system does nothing, when in fact it is doing a great deal in the wrong place.
• Out of balance overall. Supply well above extract leaves the dwelling pressurised, pushing humid indoor air into every gap in the construction. Extract well above supply reproduces exactly the situation double flow was meant to end: unfiltered, untempered air arriving from the landing, the light well or a communal shaft.

So ask for the measured values in writing, terminal by terminal, rather than the manufacturer's datasheet. A sheet of measured flow rates is the difference between a system that has been handed over and one that has merely been installed. Those figures also describe a clean machine: as filters load, resistance rises and the balance moves with it, and units with constant-flow control compensate up to a point rather than indefinitely.

Administratively, an installation of this kind is a fixed thermal installation within the scope of RITE, Real Decreto 1027/2007 in its consolidated text, which sets duties for design, installation, maintenance and inspection. In Catalonia, companies carrying out RITE-scope work must be registered in RASIC. That company register must not be confused with RITSIC: the documentation and registration procedure for an installation depends on its type and the power thresholds stated by the administration. The practical consequence for an owner is to ask which documents apply to this design and who signs them, rather than treating “legalisation” as automatic wording for every system.

Doble flujo or simple flujo: the comparison that actually decides it

The useful question is not which system is better in the abstract. It is which architecture fits this flat, this setting and this household.

Your situation	Points towards	Why
Bedrooms over a busy street, or openings onto an enclosed light well	Double flow	The intake can be sited deliberately and the incoming air filtered
Tight ceiling heights, little room for a dropped ceiling	Single flow	One network passes where two will not
Nobody will take charge of filter changes	Single flow	Loaded filters cost the system the flow rate that justified it
Winter comfort of the incoming air is the priority	Double flow	Supply arrives tempered and can be delivered inside the room
A flat lived in throughout the works, disruption to be kept down	Single flow	Fewer ducts, fewer penetrations, less equipment
A tightly refurbished flat with high occupancy	Either, done properly	What is missing is continuous renewal, and both architectures provide it

Climate does real work in that table. On a coast with mild, humid winters the equipment performs as designed, but it has fewer hours of useful heating season in which to justify itself than in a continental climate — the machine is not worse, the weather simply offers it less to recover. Hot, humid summers raise a different question: filtered renewal, yes, but the moisture in August outdoor air does not disappear because it travelled through a duct. Measured work on ventilation in Spanish dwellings looks precisely at that pairing of air renewal and humidity control, and it is the pairing worth asking about locally rather than importing an answer from a colder market.

Cost and disruption follow the same logic: what drives both is the length and difficulty of the duct route through your particular flat plus the balancing at the end, and what you live through is the dropped ceiling and the penetrations rather than the unit itself.

Which leaves an honest limit. If the actual complaint is that the bathroom takes days to dry and the kitchen does not pull, a well-executed single-flow system with genuine air inlets and doors that let air through fixes what needed fixing. Double flow buys something else: control over the air that comes in. It earns its complexity when that control is what you lack.

What you are signing up to for the next ten years

Both sides of a balanced system carry filters, and both have to be changed. The interval is set by the manufacturer and by the setting: a quiet inner courtyard is not the same environment as a façade over dense traffic, nor as a flat close to the sea, where the incoming air carries salt all year.

Two design decisions become a decade of consequences. Whether the unit has a hatch you can genuinely open — a machine sealed inside plasterboard does not get maintained, and what is not maintained loses flow until it stops doing its job. And whether the condensate drain was given a route from day one, because retrofitting one is always worse. Supply-duct hygiene matters more than extract-duct hygiene, for the plain reason that supply air goes straight into the bedrooms.

If you rent, establish early whose job the filter changes are and whether the unit can be reached from inside the flat at all. If you own, keep the commissioning figures and the maintenance records together: they are what let anyone compare the system as it runs now with the system that was designed.

What to ask the person designing it, and what a good answer sounds like

None of these questions requires you to know anything about exchangers. They are about routes, flows and access, which is where a balanced installation succeeds or fails.

• Where exactly will the intake and the exhaust be, and how far apart? A good answer names two specific positions and gives a reason for each. "On the façade" is not an answer.
• What caudal is each room designed for, and will I receive the measured figures at handover? A good answer is yes, terminal by terminal, after balancing — not the equipment datasheet.
• Where does the unit go, how do I open it, and where do the condensates run? A good answer points at a specific location, includes a hatch and traces the drain to a specific outlet.
• What filter class on the supply side, how often replaced in this setting, and are they a standard size? A good answer treats filters as part of the system rather than as aftercare.
• What happens to the corridor ceiling, and how much clear height is left? A good answer is a number in centimetres, measured on site.
• Which parts of this touch communal elements, and what does the community of owners need to approve? A good answer puts that at the beginning of the programme rather than the end.

If the answers are specific, you are talking about an installation. If they are all about the model of unit, the conversation has not reached your flat yet.