We investigate ventilation heat recovery with buoyancy flow, arranging interior spaces in an open thermal loop with heat exchange through partition walls. We draw inspiration from a Victorian-era thermosiphon scheme after finding it in Montreal’s former Royal Victoria Hospital and tracing the concept back to the original Centre Block of Canada’s Parliament Hill. We corroborate the archival evidence with analog models to visualize how the original hospital’s ventilation system worked. We use saline baths to verify how the siphon functioned and digital Schlieren to confirm how the full flow cycle worked with heat recovery. Then, to generalize these observations, we develop a mathematical model showing how heat recovery with buoyancy ventilation can work in simple room arrangements. We define a criterion for stable, unidirectional flow and a heat recovery efficiency limit of 50%. Finally, we validate the mathematical model experimentally, demonstrating stable, one-way flow close to the heat recovery limit.