Having been involved in the opening of T5 (albeit the supply chain so clearly not an expert), I do remember having it explained that air circulation around the floors is a fundamental part of the building design. I also vaguely remember a comment around the temperature control system, which i believe for the main section of the terminal relies heavily on passive air circulation rather than pure AC, effectively becoming useless in the rare case of sequential days over a certain temperature - although i can't remember the source of this so it may well be codswallop..

On an off topic, I do remember having a very strange 'hello how are you' type conversation with Willie Walsh shortly after 4am when the terminal effectively became active on the day of opening, as well as lots of BA staff asking me for directions to certain areas which clearly was a precursor to the carnage that kicked off later that day!