Boiling and condensation
The boiler changes liquid water into steam. Steam moves through piping because of pressure differences and air removal, condenses at radiators and gives up latent heat.
Air must get out
Steam cannot efficiently fill piping and radiators that remain occupied by air. Venting is therefore central to steam distribution performance.
Condensate must return
After steam condenses, water must return to the boiler by the system's intended arrangement. Piping pitch and near-boiler configuration can affect system behavior.
Low pressure is normal
Residential steam heating is fundamentally a low-pressure application. Pressure controls and low-water safety devices are not homeowner adjustment points.