Mechanical ventilation with heat recovery (or MVHR) is a key component of passive houses. This type of system consists of a ventilation unit with a heat exchanger (and humidity exchanger if the unit is enthalpic), which transfers the energy from the air being taken out of the house, to the fresh air being supplied from outside (in summer this process operates in reverse). In this way, in winter, supply air is preheated, using only a small amount of electricity for the fans. In summer, when the indoor air is cooler than the outdoor air, the process is reversed: the air supplied to the interior is cooled.

The ventilation unit also allows the incoming air to be filtered, significantly improving air quality and reducing the amount of dust and other airborne particles.

The system operates continuously, at low airflow rates and without noise. Air is extracted from wet rooms such as bathrooms and the kitchen (eliminating the need to install separate extractor fans with roof ducts), and fresh air is supplied to dry rooms such as living rooms, dining rooms, and bedrooms.

Real‑life performance at Can Naiades

We have evaluated the real-life performance of a Zehnder ComfoAir Q450 ERV unit, installed by Fontalgar Instalaciones in the Can Naiades house, working at a flow rate of 200 m³/h. To calculate the actual performance of the machine, we’ve used the Passive House Institute’s efficiency equation:

The performance data of the Passivhaus Certificate is shown below (full data here). The heat recovery rate according to the certificate is 83%:

The Zehnder ComfoAir Q450 ERV model has not yet been certified for cooling performance. However, the ComfoAir Q350 ERV (being a very similar machine), has a cooling recovery certificate (complete data here). The cooling recovery rate in summer according to the certificate is 81%:

In both cases, the electrical consumption of the equipment is certified at 0.21 Wh/m3. In summary, the certified values are:

Field inputs and real-life performance

Temperature data were taken for a cold day in winter, and a hot day in summer. The temperatures in each case are as follows:

The electric term was calculated as follows:

The calculation of heat recovery efficiency in winter and summer is shown below:

Efficiency in winter:

Efficiency in summer:

Finally, electricity consumption, measured for 6 months of 2025 (the data available at the time of writing), was 150 kWh, shown below:

At a flow rate of 200 m³/h, for 4416 hours (June-December inclusive), that’s a total volume of 883,200 m³. The average specific fan energy consumption is therefore Wh/m³ = 883,200 ÷ 150,000​= 0.1698 Wh/m³.

In summary, the certified values compared to the measured values are as follows:

Conclusions

Regarding heat / cooling recovery, the comparison shows that the installed unit has a real-life performance very close to the certified values, both in winter and summer. As for electricity consumption, the measured value is lower than the value of the certificate.

For the moment analysed in winter, the MVHR unit pre-heats the supply air from -1.5ºC to 19.9 ºC, with a ΔT = 21.4 ºC and a COP = 42 (recovered heating power ÷ electrical power input). This translates into significant savings in space heating energy consumption.

We’d like to thank Fontalgar Instalaciones and Zehnder Group Ibérica for their support on this project.

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