Chapter 4
Pumped Return Systems
There are circumstances where a gravity return
system isn’t practical.
These include:
• When gravity isn’t able to return the condensate
due to long return pipe runs.
• When the “A” dimension is inadequate.
• When zone valves are used - when zone
valves close, the boiler pressure pushes
water approximately 30" up the return for
every psi in the boiler. In a typical system
operating at 2 PSI the return water would
rise to 5' above the boiler’s water level.
• When there is a long condensate return
time lag.
• When a replacement boiler has a higher
water level than the boiler being replaced.
Pumped return systems have additional components
not found in gravity return systems,
including:
• Thermostatic traps (2 pipe systems only)
• Float & thermostatic (F&T) trap(s)
• A condensate receiver & boiler feed pump
Thermostatic traps – are typically installed
on heat emitters in 2 pipe systems that don’t
utilize air vents. They allow air and condensate
to pass, but prevent steam from passing into the
return. In addition, to prevent condensate from
flashing into steam in the dry return, they only
allow condensate to pass through when it is at
least 10°F cooler than the steam temperature.
Figure 1.
24 National Oilheat Research Alliance
Figure 1
Float & thermostatic (F & T) traps - F&T
traps prevent steam from passing through,
control condensate flow to a receiver and allow
air to pass through to a vent in the condensate
Figure 2