VMC · Selection guide · Vayron

How to choose the right heat recovery VMC system: a technical guide for installers

Updated September 2026 · 9 min read

Family relaxing on the sofa in a living room with mechanical ventilation

What heat recovery VMC is and how it differs from extract-only ventilation

Before going into the selection criteria, it is worth being clear about the fundamental difference between the two main types of VMC (mechanical ventilation):

  • Extract-only VMC (single-flow): removes stale air from inside the building using a central fan. Fresh air enters through passive air inlets in the façade. It is the most economical and widespread solution in existing homes.
  • Heat recovery VMC (double-flow): extracts stale air and supplies filtered fresh air to the interior simultaneously and in a controlled way, passing both airstreams through a heat exchanger that transfers the energy from the extracted air to the incoming air. It recovers between 70% and 90% of the heat.

Heat recovery VMC is more expensive to install, but it is the only solution that simultaneously meets the indoor air quality, energy efficiency and thermal comfort requirements demanded by the most exacting projects and by current regulations for new buildings.

The four key criteria for selecting the right system

1. The required airflow: the essential starting point

The airflow the system has to move is the first figure you need to calculate, and it must be done in accordance with the CTE DB-HS3 (Spanish Building Code, indoor air quality section). Sizing errors at this stage are the most common and the hardest to correct after installation.

Parameters that determine the required airflow:

  • Floor area and number of rooms in the dwelling or building.
  • Expected number of occupants per room.
  • Type of room: wet (kitchen, bathroom) or dry (bedroom, living room).
  • Building use: residential, offices, hospitality, etc.

As a general rule, for a residential home with 3-4 bedrooms and 4 occupants, the required ventilation airflow is usually between 150 and 250 m³/h. However, you should always calculate it specifically for each project, never by estimate.

2. The efficiency of the heat exchanger

The performance of the heat exchanger is the parameter that has the greatest influence on the system's real energy savings. It is expressed as a percentage of heat recovered and varies significantly between models:

  • Cross-flow exchangers: efficiencies between 60% and 75%. They are the most economical and compact. Suitable for projects with a tight budget or limited space.
  • Counter-flow exchangers: efficiencies between 80% and 92%. They are the current standard for high-efficiency projects and nearly zero-energy buildings (nZEB).
  • Enthalpy exchangers (with moisture recovery): in addition to heat, they recover part of the moisture from the extracted air. Especially recommended in very dry or very humid climates.

3. The noise level: the factor that generates the most complaints

Noise is the number one cause of complaints in domestic VMC installations. A technically correct system that causes acoustic nuisance is a failed system from the customer's point of view.

  • The central unit should have a sound power level below 35 dB(A) for installations in habitable areas. Some manufacturers offer especially quiet versions below 25 dB(A).
  • The location of the unit is decisive: installing it in a plant cupboard or service area significantly reduces noise transmission.
  • Using acoustically insulated flexible duct in the sections close to the air valves reduces airflow noise in rooms.

4. The space available for the installation

The dimensions of the central unit and the routing of the duct network are physical constraints that determine which systems are feasible in each project. In renovations in particular, the available space is usually the limiting factor.

  • Compact ceiling or wall units: ideal for renovations where there is no suspended ceiling or the installation space is limited. They allow a shorter and simpler duct layout.
  • Cupboard units: the most common in new builds, they are installed in a plant cupboard or utility area and distribute the air through a duct network in the suspended ceiling.
  • Decentralised VMC: in projects where a centralised installation is not feasible, decentralised systems (individual units per room) are an alternative that does not require a duct network.

The most common selection mistakes and how to avoid them

  • Choosing the model on price without checking the maximum airflow: many installers select the cheapest model without checking whether its maximum airflow covers the needs of the project. An undersized system cannot be adjusted to make up for the lack of capacity.
  • Not allowing for the pressure losses of the duct network: the airflow the unit moves on the test bench is not the same as what reaches the air valves. Pressure losses can reduce the effective airflow by 15% to 30% if the network is poorly designed.
  • Installing a residential system in a commercial project: domestic units are not designed to run continuously in heavy-use environments. They suffer premature failures and do not meet the RITE (Spanish building services regulation) requirements for non-residential buildings.
  • Ignoring outdoor air filtration: in urban areas with high pollution or environments with a high dust content, it is essential to fit class F7 or higher filters on the outdoor air intake. Failing to do so causes air quality problems and shortens the service life of the exchanger.

Maintenance as part of the selection

One criterion that many installers overlook when selecting the system is ease of maintenance. A technically superior system with hard-to-reach filters or replacements that are difficult to source will cause more problems in the long run than a more modest but well-supported one.

When selecting a heat recovery VMC system, consider:

  • Availability of replacement filters: choose brands with established distribution in Spain and filters available without long lead times.
  • Ease of access to the exchanger for cleaning: some models allow the exchanger to be removed without tools, which greatly simplifies periodic maintenance.
  • Built-in maintenance indicators: units with visual or app alerts when the filter needs changing reduce call-outs caused by a lack of maintenance by the end user.

Conclusion

Selecting the right heat recovery VMC system means analysing the project from four angles: the airflow required by the regulations, the efficiency of the exchanger, the acceptable noise level and the space available for the installation. Add ease of maintenance to that and you will have every criterion you need to always recommend the most suitable solution.

Technical contact: tecnico@gtsystems.es · Web: https://vayron.es/