Chapter 5
How Two Pipe Pumped Return
Steam Systems Work
Standby mode
When the system has cooled and is in a
standby mode, the water level in the boiler
is at its normal fill line. The supply piping,
heat emitters and return risers are full of air.
Thermostatic traps are open, the F/T trap
thermostatic valve is open and the float valve is
closed. Figure 1.
32 National Oilheat Research Alliance
When the thermostat calls for heat:
• 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 through the piping
and heat emitters.
• since the thermo
static heat emitter traps
and thermostatic element
of the F&T trap
are open, air flows
through the heat emitter
traps into the return
piping and out through
the condensate tank
vent piping.
• the thermostatic traps
stay open until they are
exposed to steam tem
perature. Figure 2.
The F&T trap thermostat
element quickly exhausts
air from the main
• The boiler water level
drops slightly because
Figure 1- Standby mode