A ventilation system for a building, wherein a. said building comprises: i. a stairwell; ii. a fire cell separated from said stairwell by a wall; b. said ventilation system comprises iii. a fresh air intake; iv. a stale air exhaust; v. a means for providing air flow in the ventilation system; vi. a heat exchanger arranged to exchange thermal energy between fresh air coming in from the air intake and stale air going out to the exhaust; vii. a ventilation duct arranged for transferring stale air from the fire cell to the stale air exhaust via the heat exchanger; viii. a ventilation duct arranged for transferring fresh air from the fresh air intake to the stairwell via the heat exchanger; ix. a fire proof air transfer unit arranged as a conduit for transferring air between the stairwell and the fire cell; c. wherein said ventilation system is configured such that during operation: x. the air pressure in the stairwell is at least 1 Pa higher than ambient atmospheric pressure; xi. the air pressure in the fire cell is at least 1 Pa lower than ambient atmospheric pressure; xii. a fresh air flow corresponding to at least 40% of the flow of stale air withdrawn by the ventilation system from the fire cell is provided to the fire cell from the stairwell via the air transfer unit. a. said building comprises: i. a stairwell; ii. a fire cell separated from said stairwell by a wall; i. a stairwell; ii. a fire cell separated from said stairwell by a wall; b. said ventilation system comprises iii. a fresh air intake; iv. a stale air exhaust; v. a means for providing air flow in the ventilation system; vi. a heat exchanger arranged to exchange thermal energy between fresh air coming in from the air intake and stale air going out to the exhaust; vii. a ventilation duct arranged for transferring stale air from the fire cell to the stale air exhaust via the heat exchanger; viii. a ventilation duct arranged for transferring fresh air from the fresh air intake to the stairwell via the heat exchanger; ix. a fire proof air transfer unit arranged as a conduit for transferring air between the stairwell and the fire cell; iii. a fresh air intake; iv. a stale air exhaust; v. a means for providing air flow in the ventilation system; vi. a heat exchanger arranged to exchange thermal energy between fresh air coming in from the air intake and stale air going out to the exhaust; vii. a ventilation duct arranged for transferring stale air from the fire cell to the stale air exhaust via the heat exchanger; viii. a ventilation duct arranged for transferring fresh air from the fresh air intake to the stairwell via the heat exchanger; ix. a fire proof air transfer unit arranged as a conduit for transferring air between the stairwell and the fire cell; c. wherein said ventilation system is configured such that during operation: x. the air pressure in the stairwell is at least 1 Pa higher than ambient atmospheric pressure; xi. the air pressure in the fire cell is at least 1 Pa lower than ambient atmospheric pressure; xii. a fresh air flow corresponding to at least 40% of the flow of stale air withdrawn by the ventilation system from the fire cell is provided to the fire cell from the stairwell via the air transfer unit. x. the air pressure in the stairwell is at least 1 Pa higher than ambient atmospheric pressure; xi. the air pressure in the fire cell is at least 1 Pa lower than ambient atmospheric pressure; xii. a fresh air flow corresponding to at least 40% of the flow of stale air withdrawn by the ventilation system from the fire cell is provided to the fire cell from the stairwell via the air transfer unit.