The vacuum-air-line system is a variation of the one-pipe steam-heating system. Radiator air vents arereplaced with air valves, the outlets of which are connected to a return air line. A vacuum pump is included toexhaust the system of air. Air-line valves are of the thermostatic type.
Advantages of this system include:
1. Steam circulation is more rapid.
2. Radiators heat efficiently at lower pressures.
3. Air vents are not required.
There are some disadvantages:
1. The steam may be noisy at times because the steam and condensate flow in the same pipe.
2. Piping and radiator valves must be oversized to accommodate the flow of both steam and condensate.