ASHRAE Journal - September 2024 - 46

TECHNICAL FEATURE
FIGURE 3 Suggested flow tolerance (%) for 97% heat transfer.
±5%
-0/+10%
260
240
220
200
180
160
140
120
100
80
70
60
50
40
difference, will deliver 97% of the rated coil heat capacity
at just 80% of design flow. Supply water temperature
and design temperature drop will define acceptable
flow tolerance. Application of these relationships can
prove beneficial to evaluate current hydronic system
components and their potential reuse.
For some applications, changing the energy source
inherently alters the system pumping strategy, which
subsequently impacts the piping layout required for
the correct proportional distribution of system fluid
throughout the building. A possible consequence,
especially in a centralized pumping design, is the
creation of an individual piping circuit whose combined
friction losses at design flow for pipe, valves, fittings
and heat transfer coils is exceptionally higher than any
other circuit within the system. This is known as the
critical circuit and will dictate the total differential head
the main pump must create at total system design flow.
Figure 4 offers a simple example of a variable-primary
centralized pump
design that highlights
an important
takeaway from this
scenario; the pump is
extremely oversized
for a majority of
the remaining
circuits within the
building, making the
control and balance
valve sizing and
the final hydronic
proportional balance
of the entire system
more challenging.
Total Pump Head
25 ft + 60 ft = 85 ft
S
ASD
20 Branches Total
25 ft PD
Source
Mechanical Room
(Incl S&R Mains)
25 ft PD
Note: Critical circuit piping shown in green.
46
ASHRAE JOURNAL ashrae.org S E PTEM B ER 2024
40 Branches Total
60 ft PD
±10%
±15%
±20%
Another potential scenario is if this new head loss
value complicates reuse of the existing pump or requires
a new pump that is larger than expected, the designer
has the option to integrate primary-secondary pumping
as a problem-solving strategy. First conceived in 1954,
the primary-secondary pumping method can be
used to break up the existing HVAC system into more
manageable subzones. This is done by adding a low
pressure drop " common " pipe across the supply and
return main to create hydraulic separation of specific
circuits in the system. Because flow in one circuit cannot
create flow in the other, smaller energy-saving pumps
are used with flow and differential head capacities
specific to the zone they serve.
Figure 5 shows the basic application of primarysecondary
to the system shown in Figure 4, where the
heat source and associated mechanical room piping and
accessories are a zone, and the high pressure drop area
that formed the critical circuit is now a separate zone.
Notice the secondary distribution pump is rightsized to
manage the other zones, as their pressure drops are very
similar.
Primary-secondary pumping offers two important
advantages:
* Less energy is required to move water through the
entire system (rather than one large circulator, small
energy-efficient circulators can be used to overcome
the friction and inertia-or pressure drop-of their
respective loops).
* More control can be taken over zones (and each
zone can operate at its own optimum temperature).
FIGURE 4 Variable-primary centralized pump design example.
20 Branches Total
20 ft Pressure Drop
(PD)
30 Branches Total
27 ft PD
20 Branches Total
20 ft PD
Supply Water Temperature, °F
Bypass
Cooling
Heating
80°F ∆T
60°F ∆T
40°F ∆T
15°F ∆T
30°F ∆T
12°F ∆T
10°F ∆T
8°F ∆T
20°F ∆T20°F ∆T
6°F ∆T
10°F ∆T
20°F ∆T
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ASHRAE Journal - September 2024

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