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 (p2) in the stairwell is at least 1 Pa higher than ambient atmospheric pressure (p0); xi. the air pressure (p1) in the fire cell is at least 1 Pa lower than ambient atmospheric pressure (p0); xii. a fresh air flow (f1) corresponding to at least 40% of the flow of stale air withdrawn by the ventilation system from the fire cell is provided to the fire cell from the stairwell via the air transfer unit .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 (p2) in the stairwell is at least 1 Pa higher than ambient atmospheric pressure (p0); xi. the air pressure (p1) in the fire cell is at least 1 Pa lower than ambient atmospheric pressure (p0); xii. a fresh air flow (f1) 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 (p2) in the stairwell is at least 1 Pa higher than ambient atmospheric pressure (p0); xi. the air pressure (p1) in the fire cell is at least 1 Pa lower than ambient atmospheric pressure (p0); xii. a fresh air flow (f1) 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 . <EOS>