A fuel cell stack assembly comprising: a stack of fuel cells, each fuel cell having a cooling air conduit with an input/output ventilation aperture disposed on a ventilation face of the stack, the ventilation apertures forming an array over said ventilation face of the stack; a first fan (7, 44b) arranged in a first air guide chamber to direct air flow through a first portion of the ventilation face and a second fan (7, 44a) arranged in a second air guide chamber to direct air flow through a second portion of the ventilation face, a partition wall (22, 45) lying between the first and second air guide chambers; a reconfigurable plenum in fluid communication with said first fan (7, 44b) and said second fan (7, 44a), the reconfigurable plenum having: a first end proximal to said first and second fans (7, 44b, 44a) and a second end distal to said first and second fans (7, 44b, 44a), a variable occluding member at the second end switchable between an open configuration and a closed configuration, in the open configuration, the first and second fans (7, 44b, 44a) are configured to direct air through the first and second portions (50b, 50a) of the ventilation face in the same direction to facilitate exit of air flow from the plenum at the second end, and in the closed configuration, the first fan (7, 44b) is configured to direct air through the first and second portions (50a, 50b) of the ventilation face in opposing directions thereby forcing return of at least some air from the first fan (7, 44b) to the second fan (7, 44a), wherein the partition wall (22, 45) is configured to ensure separation of forward and reverse airflows (43d, 43c). a stack of fuel cells, each fuel cell having a cooling air conduit with an input/output ventilation aperture disposed on a ventilation face of the stack, the ventilation apertures forming an array over said ventilation face of the stack; a first fan (7, 44b) arranged in a first air guide chamber to direct air flow through a first portion of the ventilation face and a second fan (7, 44a) arranged in a second air guide chamber to direct air flow through a second portion of the ventilation face, a partition wall (22, 45) lying between the first and second air guide chambers; a reconfigurable plenum in fluid communication with said first fan (7, 44b) and said second fan (7, 44a), the reconfigurable plenum having: a first end proximal to said first and second fans (7, 44b, 44a) and a second end distal to said first and second fans (7, 44b, 44a), a variable occluding member at the second end switchable between an open configuration and a closed configuration, in the open configuration, the first and second fans (7, 44b, 44a) are configured to direct air through the first and second portions (50b, 50a) of the ventilation face in the same direction to facilitate exit of air flow from the plenum at the second end, and in the closed configuration, the first fan (7, 44b) is configured to direct air through the first and second portions (50a, 50b) of the ventilation face in opposing directions thereby forcing return of at least some air from the first fan (7, 44b) to the second fan (7, 44a), wherein the partition wall (22, 45) is configured to ensure separation of forward and reverse airflows (43d, 43c). a first end proximal to said first and second fans (7, 44b, 44a) and a second end distal to said first and second fans (7, 44b, 44a), a variable occluding member at the second end switchable between an open configuration and a closed configuration, in the open configuration, the first and second fans (7, 44b, 44a) are configured to direct air through the first and second portions (50b, 50a) of the ventilation face in the same direction to facilitate exit of air flow from the plenum at the second end, and in the closed configuration, the first fan (7, 44b) is configured to direct air through the first and second portions (50a, 50b) of the ventilation face in opposing directions thereby forcing return of at least some air from the first fan (7, 44b) to the second fan (7, 44a), wherein the partition wall (22, 45) is configured to ensure separation of forward and reverse airflows (43d, 43c).