Chapter 3
How Typical One Pipe Gravity
Return Steam Systems Work
National Oilheat Research Alliance 21
Standby mode
When the system is in a standby mode the
water level in the boiler is the same as in the
return. The piping and heat emitters are full of
air. Figure 1.
When the thermostat calls for heat:
Figure 2:
• the burner fires and the water in the boiler
boils, creating steam.
• the boiler water level rises slightly as the
steam bubbles displace water.
• the steam pressure builds as steam pushes
against the air in the distribution system.
• the pressure pushes air out through the vents.
• water begins to rise in the return riser be
cause flow causes a pressure drop in the piping.
(Pressure is lower than it is at the boiler.)
The main vent (Figure 3) quickly exhausts
air from the piping and closes when steam
reaches it, then:
• all branches receive steam at about the same
time.
Figure 1- Standby mode
Figure 2-Thermostat
calls for heat
Figure 3-Quickly
exhausts air