Combining heat recovery with natural ventilation is a relatively new topic of significant academic and commercial interest. The present study shows the performance of a recently developed Passive Ventilation system with Heat Recovery (PVHR) installed in a primary school building. The study includes (i) long term (12-month period) monitoring of the thermal environment and CO2 concentrations and (ii) intense short term monitoring (2-week period during the heating season) of the environmental conditions in two classrooms, including detailed monitoring of the temperatures and bi-directional air speeds within the system itself. Airtightness measurements using the Blower Door test method were performed, while time-varying ventilation rates within each classroom were estimated by using a form of continuity equation taking into account CO2 generation rates by occupants. Preliminary results show that average ventilation rates in the two classrooms ranged between 4.20 l/s/p and 5.93 l/s/p, above the recommended minimum set by BB101 (3 l/s/p). Furthermore, CO2 concentrations for the majority of the monitoring period were below 1500ppm, while future research steps are also suggested.