ASHRAE Journal - January 2020 - 34

ASHRAE - CELEBRATING 125 YEARS

Beyond Commercial Systems

While most of the applications for primary/secondary pumping have been in large commercial systems
such as chilled water campus systems, dual temperature change-over systems and freeze protection for
makeup air systems, hydronic system designers are
increasingly discovering the value of primary/secondary pumping systems in residential and light industrial applications.
Residential hydronic systems today are often more
sophisticated than those in large commercial buildings. Beyond space heating, these multi-load/multitemperature systems may incorporate a range of loads,
from domestic water heating, to radiant panel heating,
to snow melting, to intermittent garage heating, to pool
heating.
Primary/secondary pumping is particularly relevant
for radiant heating systems, including underfloor systems. Typically, the design specifications for underfloor
heating loops require much lower water temperatures
than are supplied by the boiler; however, boiler return
temperatures must be kept above their minimum. Water
might be exiting a secondary radiant loop at 90°F, but
the minimum return temperature of a domestic boiler
might be 140°F. In a primary/secondary system, cooler
water exiting the secondary mixes with hotter primary
loop water, ensuring that water returning to the boiler is
warmer than the minimum.8

Other Methods Emerge

Although valued for their simplicity and flexibility, primary/secondary pumping systems may not be
appropriate for all projects. While the design typically
has low first costs, has flexibility and reduces the complexity of the chiller or boiler staging and control by
eliminating on/off valves and minimum/maximum
flow requirements, efficiency is limited because the
zone nearest to the primary loop is susceptible to
over-pressurization.
In the last decade, manufacturers have developed
more sophisticated HVAC equipment. Today's chillers
and boilers are equipped with their own variable frequency drives (VFDs), enabling a significant range of
flow so equipment can be under-flowed or over-flowed
and still function optimally. The advances in controls,
chiller and pump technologies ultimately led to the
development of the variable primary pumping method.

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This technique, which features staged pumps and chillers, relies on variable speed secondary pumps and uses
two-way valves.
In a primary/secondary system, a chiller and its primary pump typically operate in tandem. In contrast, the
variable primary flow system can separate pump control
from chiller sequencing.
The variable primary system design is relatively
simple, with only one set of pumps creating flow for the
entire system. There is still a common pipe between
the chiller or boiler pumps and the system distribution,
but there are no secondary pumps. A control valve is
installed in the common pipe. Throttling this valve creates a pressure drop through the common pipe, which
forces more water into the distribution piping. The control valve is the only means of regulation in this type of
system.9
Like the secondary pump in a primary/secondary
system, the pumps in a standard variable primary flow
system operate to maintain a target differential pressure at a specific point in the system. This pressure difference tends to decrease when the air-handler control
valves open in response to increasing loads. To restore
differential pressure across the system, the pump controller increases the speed of the pump. Conversely,
when the air-handler control valves close in response
to decreased coil loads, the pump controller slows
the pump speed to maintain the target differential
pressure.10
A major benefit of the variable primary flow system
is that low Delta T can be managed by controlling the
pump flow rate, and neither a neutral bridge nor threeway valves are required. Using variable speed pumps,
chiller flow is adjustable to match system Delta T, and
the number of operating pumps does not need to match
the number of chillers.
Variable primary flow designs use fewer pumps and
piping connections than primary/secondary systems,
which means fewer electrical lines and a smaller
footprint for the plant. These factors may reduce the
initial cost of the chilled water system. Since fewer
pumps are needed with this method, the capital
investment is less, energy costs are lower and less
space is required.11
Although the variable primary flow method can be
an ideal solution for new chiller plants, the pumping

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ASHRAE Journal - January 2020

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