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©1999 AAF-HermanNelson
Page 4 / IM UV-3-202D (Rev. 6/99)
SYSTEM BALANCING
The recommended method, and the one most commonly
used, for balancing a system is called "proportionate balanc-
ing." Figure 6 shows a water system consisting of reverse
return mains feeding three branch mains. Branch A and
Branch C are designed as reverse returns, while Branch B is
designed as a direct return upfeed riser. Each of the branch
circuits has a balancing valve in the line going back to the
system return main. In addition, each unit has a balancing
valve in its return line. The prescribed method of
proporationate balancing is as follows:
1. Open all valves fully.
2. Beginning with Branch A, take readings to determine the
GPM flowing to each unit in the branch.
3. Determine the ratio of the actual/design flow for each of
the coils in Branch A. This is called the proportionate flow
rate. For instance, if Coil A-1 had a design flow rate of 10
GPM and the measured flow rate was 15 GPM, then its
proportionate flow rate would be 1.5.
4. Assume Coil A-1 has the lowest proportionate flow rate,
A-2 the next lowest, and so on. Leaving the balancing valve
to Coil A-1 wide open, begin to throttle the balancing valve
on A-2 until the two coils have the same proportionate flow
within an allowed tolerance. (This usually is set by the bal-
ancing contract at around 5 percent.) Proportionate bal-
ance now has been achieved between these two coils.
5. Proceed to Coil A-3 and establish proportionate balance
between it and Coil A-2 by the same procedure. Coil A-2
need not be read. It will change in direct proportion to the
change in A-2 and will remain in balance with it.
6. Adjust the balancing valve in Coil A-4 until it is in propor-
tionate balance with Coil A-3. Likewise, bring Coil A-5 into
proportionate balance with Coil A-4. This is the end of
Branch A. All coils on this branch will be proportionately
balanced and any increase or decrease in the total system
GPM, or the branch, will increase or decrease the GPM at
each coil proportionately. They will remain in balance with
one another.
7 By the same process, achieve a proportionate balance of
all coils on Branches B and C.
8. The next step is to balance the branches. To do this, select
at random one coil on each of the three branches. Use the
same procedure as for coil balancing and proportionately
balance the branches against one another, using the se-
lected coils. Note the balancing valve on the lowest pro-
portionately flowing branch will be left wide open.
9. The final step in the procedure is to adjust the flow from
the pump to the system to bring all coils to their design
flow rate. As was previously discussed, this can be done
by imposing additional resistnace at the pump by means
of a balancing valve and throttling the system back until
the flow rates are equal to those called for by the design,
or decreasing the output of the pump by either changing
the impeller or changing the pump.
Figure 6.
System
Pump
Branch C
Branch B
Branch A
TYPICAL WATER SYSTEM CONSISTING OF REVERSE
RETURN MAINS FEEDING THREE BRANCH CIRCUITS
UNIT BALANCING VALVE
BRANCH CIRCUIT
BALANCING VALVE
A-1
A-2
A-3
A-4
A-5
B-1
B-2
B-3
B-4
C-1
C-2
C-3
C-4
