Why gun ports were not ordinary ventilation openings

Did sailing warships open gun ports in calm weather to improve ventilation?

No, especially on the lower deck; ventilation relied on grated deck openings and devices that directed air down from above.

Opening gun ports for ventilation is rejected here, especially for ports on the lower deck. Gratings allowed air to circulate vertically through the decks. Crews could also rig equipment above to direct air below. These arrangements provided ventilation between decks while keeping the lower gun ports shut.