Chapter 7
Sizing a Condensate Return Unit
Understand the cycle
When dealing with pumped return systems
it is imperative that the condensate return unit
must be properly sized.
Before attempting to size a condensate return
unit, it is necessary to understand how a steam
heating system cycles:
• When the burner fires the boiler water temperature
National Oilheat Research Alliance 37
rises to 212°F.
• Steam starts being generated and the boiler
water begins to expand, increasing the volume
in the boiler.
• The water volume expands rapidly as the
steam moves from the boiler into the header
and system piping.
• The water level in the boiler drops as water
becomes steam.
• If the system was equipped with a condensate
return unit that had not collected sufficient
condensate from the system, it could
not pump condensate back to the boiler.
• If the water level drops to an unsafe level
and the condensate does not return to the
boiler, the low water cutoff will shut off
power to the burner.
• The options at this point are to wait for the
condensate to come back to the boiler (while
generating no heat), or to add water directly
to the boiler after it cools down enough so
that water can be added safely without causing
damage to the heating system, home or
people.
Start with the load
A properly sized condensate return system
can help to avoid having to utilize either of
these options by pumping water from the
condensate tank into the boiler before the water
drops to an unsafe level.
To properly size a condensate return system
one must start with the load.
Steam boiler ratings show how much steam
they can produce in terms of pounds per hour
(PPH), horsepower (BHP) and BTUH (British
Thermal Units.)
A steam boiler puts ½ gallon of water per
minute (in the form of steam) into the system,
per 1,000 square foot of EDR (Equivalent
direct radiation.) When the steam cools it
turns back into water (condensate) at a rate of
0.000496 gpm per square feet of EDR.
The easiest way to calculate EDR is to divide
the net BTUH of the boiler by 240.
The boiler label shown in Section 1 was
rated for 651,000 BTUH, the calculation for
EDR is:
EDR = BTUH/240
EDR = 651,000 BTUH / 240 = 2,712.5
Since condensate is typically generated at
a rate of 0.000496 gpm per SQ FT EDR, this
system generates condensate at a rate of:
0.000496 X 2,712.5 = 1.35 gpm
With a system condensate return time of ten
minutes, this system would call for a 13.5 gallon
tank.
For more information see: http://unitedstates.
xylemappliedwater.com/2015/05/20/
how-to-size-a-condensate-return-unit/
Note: If there is any likelihood that the make-up water
supply to the feeder may be accidentally shut off for
any reason, it is desirable to mount a float operated
low water cut-off switch in the lower level of the tank.
The low water cut-off, connected to the motor starter
of the boiler feed pump, would stop the pump should
a low water condition occur, thus preventing running
a dry pump.