Skyscraper design requires some sort of draft block, such as revolving doors, to prevent the chimney effect of the tall structure from sucking in air at high speed at the base and ejecting it through vents in the roof while the building is being heated, or sucking in air through the vents and ejecting it through the doors while being cooled, both effects due to convection. Modern revolving doors permit the individual doors of the assembly to be unlocked from the central shaft to permit free flowing traffic in both directions, but this feature is typically used only during emergencies, or to admit oversize objects. Normally, the revolving door is always closed so that wind and drafts cannot blow into the building, to efficiently minimize heating and air conditioning loads.