In recent years, the need to improve living standards and the quality and comfort of buildings has grown significantly, together with the need to limit heat losses as much as possible and therefore reduce energy consumption.
As a result, both in new buildings and in the renovation of existing ones, we are seeing an increasing use of solutions such as better thermal insulation, greater airtightness of the building envelope, higher-performance windows and doors, and similar technologies.
As a side effect of using ever more efficient materials and technologies, the spaces we live in are becoming more and more sealed.
In sealed environments, indoor air naturally becomes humid and polluted, due to numerous factors such as household cleaning products, fumes and odours from cooking, humidity and emissions from bathrooms, CO2 produced by human respiration and plants, and dust and bacteria brought in from outside. All of this contributes to an indoor air environment that can potentially be unhealthy and allergenic.
Ventilating indoor spaces therefore becomes a priority.
Until recently, air renewal was either a voluntary, manual operation – typically by opening windows – or a secondary effect of openings in the building (doors, chimneys, etc.) and air leakage through the building envelope and window frames.
However, the common habit of opening windows has several drawbacks:
- significant loss of thermal energy;
- entry of excessively hot air in summer or very cold air in winter;
- entry of outdoor pollutants (pollen, dust, particulate matter, etc.) that then remain indoors.
The modern alternative for air renewal without opening windows is Mechanical Ventilation with Heat Recovery (MVHR), also known as Mechanical Ventilation with Heat Recovery units (HRUs) or Controlled Mechanical Ventilation (CMV). Several building certification schemes for high-efficiency buildings (A+, A, B or equivalent classes) already consider the adoption of suitable MVHR systems as essential, and in some cases mandatory.
Mechanical ventilation systems consist of units that renew the air in a space by extracting stale air and supplying fresh, high-quality air. Air renewal takes place continuously and in a controlled way – i.e. keeping parameters such as temperature, relative humidity and pollutant concentration under control – without opening the windows, thus significantly reducing energy consumption.
It is therefore the quality of the supplied air and the ability to maintain that quality over time that distinguishes MVHR from “traditional” air renewal by opening windows.
An MVHR system makes it possible to keep the microclimate of the dwelling under control through an imperceptible but constant air change. In a typical configuration, the supply air flows at very low speed through all rooms via a series of inlet diffusers, and leaves the spaces through a series of extract grilles.
In general, MVHR systems can be distinguished according to the following scheme:
Centralised systems
A centralised system allows full coverage of the entire floor area of the dwelling or building. Air renewal is provided by one or more fans.
Centralised systems can be of the single-flow or double-flow type.
Single-flow systems extract air from the rooms and discharge it outside – generally through the roof – via ductwork. Fresh outdoor air enters through dedicated wall vents on the building façade. To ensure that all relevant rooms are traversed by the airflow, transfer grilles are provided in the internal doors.
In residential applications, extraction usually takes place in wet rooms (bathrooms, kitchens).
Double-flow systems simultaneously and automatically provide both the supply of fresh air and the extraction of stale air.
The supply and extract duct networks are separate from each other and each is equipped with its own fan and terminal diffusers in every room. The supply (fresh) air is taken from outside and pushed by the fan along the supply ductwork, while the stale air is extracted from the rooms and discharged outside through the extract ductwork.
Extraction normally takes place in rooms with a higher presence of pollutants or humidity (bathrooms, kitchens), while supply is provided in the so-called “noble” rooms (living rooms and bedrooms).
Double-flow systems can be equipped with a heat recovery unit: in this case, the extract air and the supply air pass through a heat exchanger where thermal energy is transferred from one stream to the other.
In a well-insulated and efficiently conditioned building, continuous extraction of warm (or cool) indoor air would otherwise increase heating or cooling energy demand. Heat recovery helps to avoid this, reducing the building’s overall energy consumption.
Another important feature of this type of system is the presence of high-efficiency filters, whose function is to deliver a high level of cleanliness for the supply air, removing outdoor pollutants and particulate matter before they enter the building.
Pocket filters are preferable, as they offer a large dust-holding capacity and ensure lower pressure drops. They are also easy to maintain and, since they are readily accessible, can be replaced without the need for specialised technicians.
The advantages of double-flow systems are therefore numerous compared to single-flow systems: the main one is the ability to treat, filter, heat or cool the outdoor air while ensuring constant air renewal and recovering heat from the exhaust air.
Decentralised (room-based) systems
Unlike centralised systems, decentralised (or single-room) units provide a solution for individual rooms. They are particularly suitable when an air-change system has to be installed in existing, already occupied dwellings or during renovation, where it is not possible to install ductwork or where building works would be excessively invasive or expensive.
Technical standards for MVHR systems
Historically, the main Italian technical standard for the design and construction of mechanical ventilation systems was UNI 10339:1995 “Air-conditioning systems for thermal comfort in buildings – General, classification and requirements”. This standard provided guidance on the classification and definition of minimum system requirements and on the reference values for key parameters during operation.
As of July 2024, UNI 10339:1995 has been officially withdrawn without direct replacement. Today, the primary reference for the design of ventilation systems and indoor air quality in Europe – and therefore also in Italy – is the series of standards UNI EN 16798, in particular UNI EN 16798-1, which defines indoor environmental input parameters and minimum ventilation rates according to building type and use.
These standards:
- define several indoor environmental quality categories;
- set minimum outdoor air flow rates when spaces are occupied;
- provide design values and methods for both residential and non-residential buildings;
- support demand-controlled ventilation strategies.
The goal is to maintain, in the volume conventionally occupied by people, suitable thermal and hygrometric conditions and acceptable indoor air quality, while at the same time keeping energy consumption under control.
In residential buildings, alongside constant-flow systems, variable-flow mechanical ventilation has been developed: these systems adjust the air renewal rate based on indoor parameters such as humidity and occupancy. When the rooms are occupied or indoor humidity is high, ventilation increases; conversely, when no one is present and relative humidity is low, the air change rate is reduced.
In the tertiary (commercial and office) sector, demand-controlled ventilation is still less widespread, even though modern standards such as UNI EN 16798-1 explicitly allow designers to size systems based on real occupancy and air-quality needs rather than fixed, worst-case assumptions derived from old occupancy indices.
For further technical insights on variable-flow mechanical ventilation in the tertiary sector and on some of the technologies currently available on the market, see the paper by Valentina Raisa and Marco Zecchin.