Abstract: A heat recovery ventilator for use in ventilating a building, the ventilator having a single two-position damper operable such that in one damper position, heat is exchanged between a first flow of fresh air passing through the first path and a second flow of exhaust air passing through the second air path. When placed in the second damper position, fresh air is blocked from entering the ventilator and exhaust air is recirculated, passing through both air paths, and exiting through the fresh air outlet. In this mode of operation, exhaust air is inhibited from exiting through the exhaust outlet due to the relative pressures within the ventilator, thereby inhibiting the formation of a negative pressure within the building.