 Solution

At the heated bottom, thermal conduction first transfers energy into the adjacent gas. A warmer parcel ordinarily has lower density than its cooler surroundings at approximately the same local pressure, so buoyancy drives it upward. Cooler gas replaces it below, creating natural convection. Motion and mixing transport warm gas through the vessel, with conduction continuing across smaller temperature differences. In a sealed rigid vessel the mean pressure can rise as heating continues; “less dense at the same local pressure” is the buoyancy comparison, not a claim that the vessel's overall pressure stays fixed.

A useful control heats an otherwise similar vessel at the top: the resulting warm-over-cold arrangement is stably stratified, suppressing the overturning seen with bottom heating. Heat still spreads by thermal conduction; absence of bulk overturning does not mean absence of heat transfer.

For unwarmed cold outside air, use distributed high-level inlets or upper window openings that let the incoming stream mix with warmer room air before descending into the occupied region. A concentrated low-level cold stream can pass directly across people's feet without first mixing. Provide a separate route for outgoing air and avoid an inlet-to-outlet shortcut that leaves the occupied region poorly ventilated. The UK Health Security Agency's guidance specifically notes that a top window opening allows incoming air to warm as it mixes, reducing cold draughts.^[ref]

This is not a universal rule that every inlet belongs at the ceiling. A low inlet beside a heat source can warm the air before it rises, and deliberately designed displacement systems use other arrangements. Wind, opening heights, available pressure differences and the exhaust path determine the actual flow. The relevant principles are controlled air replacement, buoyancy and mixing, not height in isolation.