Mechanical ventilation systems have been adopted in airtight energy- efficient houses in Canada to provide fresh air, remove moisture and indoor pollutants and provide a comfortable environment for the home-occupants. Homes constructed under the R-2000 Home Program are equipped with mechanical ventilation with heat recovery. Since 1984, the performance of approximately 700 R-2000 Homes has been monitored on an annual basis. This monitoring has included the measurement of indoor levels of formaldehyde and the documentation of ventilation system operation.