HS 3: ventilation and indoor air quality

The section applies with its full apparatus to residential buildings, dwelling interiors, common areas, waste stores, storage rooms, car parks and garages; in other uses, air quality is referred to the regime of the RITE (Reglamento de Instalaciones Térmicas en los Edificios, the Spanish HVAC regulation), which sets air quality categories and flow rates per room type. This partition is worth keeping clear when budgeting mixed buildings: the dwellings follow HS 3, the commercial plinth follows the RITE, and the two regimes coexist in the same building with different logics and items.

The section's philosophy marked an era in Spanish construction: it definitively buried ventilation by infiltration, that of badly closing windows, and replaced it with a principle of organised circulation, air admission in dry rooms, bedrooms and living rooms, circulation towards the wet ones, kitchens and bathrooms, and extraction in the latter, with transfer openings guaranteeing the sweep. To this general scheme are added the complementary ventilations, natural support through windows, and the specific additional ones, the mechanical extraction of kitchen hoods with their own duct.

The section's operational heart is its table of minimum flow rates, expressed in litres per second, assigned by room type, with the logic of admission in dry rooms and extraction in wet ones and balancing rules between both flows for the dwelling as a whole. On those flow rates everything else is sized: admission openings, transfer openings, extract grilles and ducts, and the fans where they exist.

Two traits of the flow rate system matter to the design. The first is its continuous minimum character: the section requires the flow to be guaranteed, not merely possible, which points towards permanent, controllable systems. The second is its direct relationship with energy: every litre per second of outdoor air is thermal load the energy verification counts, and there lies the structural link between this section and the DB-HE that runs through this whole article.

The real repertoire of the Spanish dwelling is articulated in three families, admitted by the section with conditions. The choice between them conditions both the cost and the energy demand.

Hybrid ventilation combines natural draught, when conditions allow, with mechanical assistance when they do not: vertical ducts with support extractors, admission through façade openings. It is the solution with the lowest investment cost and the least control, sensitive to outdoor conditions and limited in tall or compact buildings.

Mechanical extract ventilation, the de facto standard of ordinary new construction, guarantees the flow rates with a permanent extractor, façade admission through air inlets, often self-regulating, and continuous extraction in the wet rooms. Reasonable control, moderate cost, and the energy load of renewing air without recovering its heat.

Double-flow mechanical ventilation with heat recovery adds the supply network and the recovery unit: the extracted air yields its heat to the incoming air, substantially cutting the ventilation load. It is the solution with the highest investment and maintenance cost, and the best energy and comfort behaviour, whose economic justification is decided at the crossroads with the DB-HE: in severe climate zones and in projects sailing tight through verification, the recovery unit pays for itself in compliance and in bills; in mild climates, its return stretches out.

Car parks and garages have their own apparatus within the section: natural or mechanical ventilation with its sizing conditions, and in the mechanical case, duct networks and fans with specific requirements, coordinated with carbon monoxide detection and with the fire protection regime. It is a budget block of its own, with sheet metal duct, fan and control items that in large basements reach very appreciable amounts, and whose coordination with the DB-SI compartmentation, ducts crossing compartments, dampers, is unavoidable.

As an indicative order of magnitude, the ventilation block sits in residential between 1.5 and 4 percent of the PEM (presupuesto de ejecución material, the direct construction cost), with the range governed by the choice of system, and with car parks as an additional package where they exist. In the method of the article on the cost of CTE compliance, the section offers the perfect example of a decision that must not be taken in isolation: the extra cost of double flow over simple extraction is only evaluated correctly by adding its effect on the energy verification and on the operating bill, and that account changes sign with the climate zone.

The characteristic items the quantities must capture without exception: admission inlets with their performance, transfer openings, undercut doors or grilles, duct network with its insulation where relevant, grilles, extractors or recovery units with their curves, independent hood duct, and where applicable silencers, because ventilation that can be heard is the user's first complaint and its later correction is expensive. Omitting any of them produces an incomplete system on site.

The first is inheriting the previous project's system without recalculating flow rates or verifying the specific building's energy fit. Flow rates depend on the specific dwelling's brief.

The second is forgetting the interior transfer openings, the humble detail, undercut doors, grilles, on which the dwelling's real sweep depends. Without them the air does not cross the dwelling however well the extractor works.

The third is budgeting the recovery unit while skimping on its ducts, insulation and silencers, delivering an expensive system that annoys and ends up switched off. The system is budgeted complete or not at all.

The fourth is treating the car park as an annex: its ventilation is among the basement's large items and among the most intertwined with fire protection. Its ventilation is coordinated with the compartmentation from the design stage.

The fifth is neglecting maintenance in the system comparison: filters and cleaning of double-flow systems are real recurring cost, and an unmaintained system loses precisely the performance it was paid for. The comparison is made on life cycle cost.