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Designers may be missing an opportunity to provide additional value for
their clients if their chiller-efficiency specifications prevent them from getting
what they want from the bidding process: chillers with equal annual energy
consumption at the best capital cost. The problem occurs when two measures
of chiller efficiency—the design-efficiency rating
and the Non-standard Part Load Value (NPLV)
rating—are both used in the specification.
This HVAC&R Engineering Update clarifies
the application of these ratings to help
designers write the most effective specification
for chiller efficiency.
Use only NPLV to specify
chiller efficiency
A chiller selection with a design-efficiency rating of 0.576 kW/TR could have
an NPLV rating between 0.55 and 0.35 kW/TR.
1. NPLV rating can be measured in kW/TR, COP, or EER. This Update uses kW/TR.
Test your NPLV IQ
First, it helps to clarify the meaning of the NPLV
rating1. To understand the rating’s applicability, there
are three presuppositions that must be challenged:
1) True or False: A chiller’s NPLV rating measures
only its off-design efficiency.
Answer: False. The NPLV rating includes both
off-design efficiency and design efficiency.
2) True or False: The IPLV rating is a subset
of the NPLV rating.
Answer: True. The NPLV rating allows for efficiency
measurements at a wide range of conditions. The
Integrated Part Load Value (IPLV) rating is targeted
to a very specific situation: when the project’s
design conditions are equal to the ARI standard
conditions. We will refer only to the NPLV rating
throughout this Update for brevity.
3) True or False: A chiller will automatically have a
good NPLV rating if it has good efficiency at design
conditions, which is the simultaneous occurrence
of both design load and design cooling-towerwater
temperature (or design outdoor-ambient
temperature, if air-cooled).
Answer: False. Chillers can have the same design
efficiency, but have NPLV ratings that vary widely,
depending on capital cost. For example, chillers
with design efficiencies of 0.576 kW/TR can have
NPLV ratings that vary anywhere between 0.55
and 0.35 kW/TR. That’s because chiller selections
can have different off-design efficiencies. Of
course, off-design performance is of paramount
importance because chillers, including those in
Designers may
be missing an
opportunity to
provide additional
value for their clients
hvac&r engineering update
