Heat recovery ventilation in Spain: what you are actually buying

The phrase on the quotation also sells wood-burning stoves

Search in English and you get MVHR, or heat recovery ventilation, and everything you find is about renewing the air in a home. Search in Spanish or Catalan, or read a quotation drawn up here, and the words are recuperador de calor — a phrase that covers three unrelated machines.

• A fireplace insert or cassette, casete or insert de leña: it captures heat from a wood fire and blows it back into the room, moving air that is already indoors. It renews nothing.
• Industrial or process heat recovery, which takes heat off a hot fluid in a plant and returns it to the same process.
• The exchanger inside a residential ventilation unit, which is the only one this page is about.

That third machine sits inside a balanced system — doble flujo in Spanish, doble flux in Catalan — where one fan pushes outdoor air into the bedrooms and living rooms and another pulls stale air out of the bathrooms and kitchen. The exchanger is the block those two streams pass through on their way past each other. Its job is not to heat the flat, and it is never a standalone purchase: it makes continuous renewal something you can live with in February, in a dwelling where air is already being moved day and night whether anyone is at home or not.

Two air streams, one thin wall between them

Open a residential exchanger and you find a block of very fine channels, stacked and alternating: half carry the warm air leaving the flat, half the outdoor air coming in. Between any two adjacent channels there is a single thin sheet of plastic or aluminium. Heat crosses it. Air does not.

The separation is the whole point, not a technicality. Extract air is drawn from wet rooms — the bathroom after a shower, the kitchen after a fry-up. If the two streams touched, the unit would deliver back into the bedrooms exactly what it had just removed from the bathroom.

Geometry decides how much heat makes the crossing. In a cross-flow core the two streams meet at an angle and share a wall only over the square where they overlap. In a counterflow core they run parallel and in opposite directions along the full length of the block, so at every point on that journey there is still some temperature difference left to transfer — the incoming air is always meeting outgoing air slightly warmer than itself. That is where the top of the quoted efficiency range comes from, and it is also why counterflow cores are physically longer, which matters when the box has to disappear above a false ceiling in a hallway.

How that tempered air then reaches each bedroom, and how the stale air is collected, is a separate design question about ducts and airflows, and the core does not settle it.

What "90% efficient" is a percentage of

Manufacturer sheets commonly lead with a high percentage, but there is no universal range that describes every residential unit and every installed system. Before comparing two figures, pin down what each is a percentage of and the conditions under which it was obtained.

It is not a percentage of your bill. It is the share of the temperature difference between the extract air and the outdoor air that the incoming air has already made up by the time it reaches the room. On a cold morning, a high figure means air arriving near the temperature of the flat rather than near the temperature of the street. It means that, and only that.

Between the datasheet and a lived-in flat there is a gap the number does not describe:

What the declared figure assumes	What an occupied flat does to it
Supply and extract flows balanced	Two filters loading at different rates pull them apart
One stated airflow, the caudal on the sheet	The same unit at another setting does not repeat the number
The core on its own	Two fans run continuously on electricity, which the percentage ignores
Heat carried by the ventilation air	Nothing about walls, glazing or thermal bridges; where the envelope is weak, ventilation is one line of the balance
Clean, tested conditions	Duct leakage, extra bends and loaded filters move the result away from the catalogue

The two cited studies answer different questions. The 104-location Spanish study is an energy and cost model using manufacturer-declared efficiencies of 86 to 91%; it is not a measurement of one occupied flat. The other is a field study of one heritage retrofit in Spain, where mean apparent thermal effectiveness was around 74% and varied with conditions. Neither turns a catalogue figure into a universal promise.

Does the moisture come back with the heat?

A plastic or aluminium core transfers temperature and nothing else. This is sensible recovery: the water vapour in the extract air cannot cross the sheet, so it leaves the building with that air. An enthalpy core — entálpico, entàlpic — uses a vapour-permeable membrane instead, so part of the moisture crosses along with the heat.

On this coast that choice cuts both ways, and both edges are worth seeing at once. In a hot, humid summer, with the flat being cooled, indoor air is drier than the air outside, so a membrane sends part of the incoming moisture straight back out instead of letting it in: less humidity to deal with indoors. In winter here, the domestic problem is rarely air that has gone too dry; it is vapour generated inside — showers, cooking, laundry dried on a rack — that needs to leave. A membrane returns part of it to the flat at precisely the moment you wanted rid of it.

The familiar argument for enthalpy cores, that they stop a home drying out over winter, was built in colder and drier continental climates and does not transfer intact to a mild, humid Mediterranean one. If what pushed you towards ventilation was condensation on the glass, ask which core a proposed unit uses and why that one. Published work on heat recovery in a Spanish apartment has examined humidity behaviour and not only temperature, which is a fair sign the question is real rather than a sales distinction.

In a mild winter, comfort is the honest argument

The Catalan coast has mild, humid winters and hot, humid summers. That erodes the energy case on its own terms: the heat available to recover depends on the difference between indoor and outdoor temperature, and here that difference is smaller than in the continental winters the payback arguments were built on. A saving calculated in a colder country does not travel south inside the box.

What does hold up is the temperature at which air enters the room. Without recovery, outdoor air is admitted into the dry rooms through a façade opening — a trickle vent in the window frame, or a wall inlet, an abertura de admisión. In January it arrives at street temperature, and it arrives exactly where the opening is. What follows is predictable: tape over the vent, a cushion against it, a wardrobe pushed in front of it, especially when the opening sits above a bed. A blocked inlet renews nothing, so the system goes on looking installed while it quietly stops working, until one morning every window is streaming.

Recovery removes the motive for blocking it. Air arrives tempered, through a supply diffuser, and the cold draught stops being something you feel. In a single-aspect flat with every opening on one façade, in a bedroom giving onto an interior light well — the patio de luces, pati de llum, where the air is humid and barely moves — or on a street where opening the window in winter costs heat and noise together, that is a difference you meet every day of the season rather than once a year on a bill.

What it does on a hot, humid August night

Summer inverts the logic. When outdoor air has cooled overnight and the flat is still holding the day's heat, sending incoming air through the core is actively unhelpful: the warm air on its way out would heat it back up on the way in. That is what the summer bypass is for — bypass estival, sometimes sold as free-cooling. A damper routes incoming air around the core, so night air arrives at the temperature it actually has. Whether a unit has one, and what governs it — an automatic control reading temperatures, or a manual setting somebody has to remember — is not a minor line on the specification in a climate with hot summers.

Two limits belong with it. Night air on this coast can be cooler than the flat and still carry a great deal of moisture, so a bypass deciding on temperature alone can bring humidity in along with the coolness. And a bypass moves outdoor air; it does not chill it. Balanced ventilation is not air conditioning, and a unit that renews air beautifully can leave a west-facing flat in August exactly as hot as it was.

Filters and condensate: the part that never finishes

An exchanger brings two permanent obligations that simpler systems do not have.

Filters. A filter does its job by getting dirty, and as it loads it resists more and passes less air. The catch is that the supply filter and the extract filter do not load at the same rate — one collects street dust, pollen and salt air, the other kitchen grease and fabric lint — and the core knows nothing about it. Uneven loading unbalances the two streams, and a figure declared with balanced flows stops describing what is happening inside the box. Real recovery falls first, then the airflow, and eventually the system is only going through the motions of ventilating. Access decides whether any of this gets done: a unit behind a sealed plasterboard ceiling with no hatch is a unit whose filters will not be changed.

Condensate. Inside the core, the extract air is cooled below its dew point and leaves liquid water behind. That is not a fault; it is the unavoidable by-product of taking heat out of humid air, and it scales with how much heat you take. The water needs a drain and a trap — desagüe and sifón — and that drain has to still work years later. Blocked, or with a trap dried out over a long absence from a coastal second home, it fails without announcing anything.

None of this is discretionary housekeeping. A system like this is a fixed thermal installation and falls within the scope of RITE, Real Decreto 1027/2007 as amended, which sets duties covering design, installation, maintenance and inspection. In Catalonia, companies carrying out this kind of thermal installation and maintenance work must be registered with the competent industrial-safety register. The translation for someone living in the flat is blunt: the catalogue efficiency is a first-day figure, and it only survives while the core, the filters and the drain stay in the condition it was declared in.

Is recovery aimed at what is actually bothering you?

The exchanger is precise about what it touches. It changes the temperature — and with a membrane, part of the humidity — of air that a system is already moving. Everything else in a ventilation complaint sits outside its reach.

What is bothering you	Does the exchanger address it
Cold air pouring in at the window vent, so you blocked it up	Directly. This is the problem it exists to solve
You cannot open the window for noise, or the light well smells	Indirectly, but genuinely: renewal without opening anything
The bathroom stays wet for an hour after a shower	No. That is the extract path, and it fails or works without any exchanger
Windows streaming on winter mornings	Only as part of a system that renews air properly; the core alone changes nothing
The flat is unbearable in August	No. That is cooling, which is a different machine entirely

The rows that say no are not a detour. If the bathroom barely pulls, if new internal doors were fitted flush to the floor with no gap or transfer grille left underneath, if the kitchen has no effective extraction at the hob, then a heat recovery unit would be recovering heat from a flow that is not happening. Those repairs come first, they are usually far cheaper, and they are what makes the exchanger worth discussing at all.