• Air is slowly exhausted through the individual
heat emitter’s vents.
• water rises higher in the return as the pressure
drop increases.
• condensate begins to form.
Steam pushes into the heat emitters
Figure 4
• the emitters heat up and cool the steam.
• the steam begins to condense.
• condensate starts to flow back into the piping
through the supply valves.
• condensate runs down the branch piping,
against the flow of steam.
• the condensate load increases the pressure
difference in the piping, raising the water
level in the return to its highest point during
the cycle.
• The water level in the boiler drops slightly
because water has been “steamed off” and
the condensate has not yet returned.
22 National Oilheat Research Alliance
Steady state heating
Figure 5
• steam fills the heat emitters until it reaches
the vents
• vents close when exposed to steam temperature.
• the boiler continues to fire until the thermostat
is satisfied or the limit control setting
is reached.
• the water level in the return drops as the
start-up condensate load flows back to the
boiler.
• boiler water level drops because of the
condensate out in the system.
Note: the water level may drop
out of the gauge glass when the
boiler shuts down because the
steam bubbles collapse and the
condensate has not yet returned.
Figure 4-Steam pushes
into heat emitters
Figure 4-Steady state