National Oilheat Research Alliance 17
Figure 5
In the two pipe direct return systems,
one pipe carries water from the boiler to the
heat emitters and another pipe brings water
back to the boiler. Heat emitters on these systems
are connected to the supply and return
piping like the rungs on a ladder.
As you can see in Figure 6 below,
the heat emitter closest to the boiler has
the shortest run of piping while the heat
emitter farthest away from the boiler
has the longest piping run. In this example,
the water must flow a total of:
38' to travel from the boiler,
through the first heat emitter and back
to the boiler.
58' to travel from the boiler, through
the second heat emitter and back to
the boiler.
78' to travel from the boiler, through
the third heat emitter and back to the
boiler.
Less water will flow through the heat emitters
that are further away from the boiler and
they will generate less heat. On these systems,
balancing valves must be installed on each heat
emitter’s piping to increase the resistance of
the shorter piping circuits to even out the flow
of water through each circuit.
The terminal units in a
two pipe reverse return
system are piped in “parallel”
to separate supply and return
pipes. For proper operation,
these systems rely on a “first
supplied, last returned” piping
arrangement.
Figure 7, on following
page, illustrates that the supply
connection of the first terminal
unit is the closest one to the
boiler and the return connection
is the furthest one from the
boiler. The first unit supplied
Figure 6