Steam and condensate use the same main pipes. The condensate flows in the opposite direction of the steam.
Air valves are necessary for evacuating air during start-up.
The system is simple but the heat emission in radiators or in heat exchangers are hard to control. Modulating the heat will cause the heating elements to be partly filled with air. The system can work properly in applications where the heat can be modulated directly in the boiler, as applications for warehouses, garages and so on. The system should be avoided where it is necessary to individually modulate each radiator or heat exchanger.
The condensate draines back to the boiler and during stops the system fills with air. This makes the design suitable for temporarily heated application working under water freezing conditions.