Is it time to upgrade your commercial air conditioner to an advanced rooftop unit (RTU)?
The Department of Energy (DOE) recently
field-tested one of just two commercial air
conditioning units that met the specifications of the RTU Challenge, which urged
manufacturers to create 10- to 20-ton units
that could meet the DOE's high performance requirements.
Both advanced rooftop units boasted an
integrated energy efficiency ratio of more
than 18, but the field test was needed to
understand the seasonal energy efficiency
of the units rather than relying on the units'
performance under standard lab conditions.
What's in a Commercial
According to the DOE, most standard
commercial HVAC units typically have a
rated energy efficiency ratio (EER) of 8-12.
Standard rooftop units also typically fit this
Fans: Constant-speed supply, regardless
of whether it's in heating, cooling and/or
Compressors: Also constant-speed, which
means it can't modulate depending on the
specific load in a space. Instead, it cycles on
and off, which is inefficient.
Stages: RTUs with capacities over 7. 5
tons usually have two or more stages of
cooling featuring two or more constant-speed compressors of equal size.
Heating: Most conventional RTUs use a
gas furnace for heating, though some use
Economizers: RTUs that are 7. 5 tons or
more are usually required to have air-side
economizers, unless they're located in hot
and humid climates.
The reliance on cycling fans and compres-
sors on and off rather than modulating to
meet a space's needs leads to expensive
inefficiency. Advanced RTUs like the one
tested by the DOE modulate the supply fan
speed when the unit is in ventilation mode
and may do so during heating or cooling as
well. A variable- or multi-speed fan and vari-
able-speed compressors are better suited to
meet variable space loads and can make a
considerable dent in your energy costs.
Before field-testing the unit, advanced
energy modeling was used to predict the
tested unit's performance in Houston, Los
Angeles and Chicago vs. three different standard units: existing RTUs already in the field,
models that meet the 2013 federal regulations for commercial equipment, and units
that meet the ASHRAE 90.1-2010
requirements, which require two-speed or variable
fan control for cooling capacities over 11,000
The modeling exercise predicted that the
advanced unit would reduce HVAC electricity
and gas costs by 27-45 percent in Houston,
18-45 percent in Los Angeles and 15-33 percent in Chicago, depending on the type of
rooftop HVAC unit it was compared to.
In all three simulations, the tested unit
that met the RTU Challenge specifications
would significantly reduce energy use, costs
and greenhouse gas emissions if it was
widely adopted. The DOE research team
didn't estimate a payback period, because
the cost of the tested units wasn't widely
available when the study was conducted.
How Advanced RTUs
The advanced RTU tested by the DOE
was a 5-ton unit instead of the 10-ton model
that was entered into the challenge. Smaller
units are typically less efficient, but the
5-ton unit was chosen because it had the
same capacity as the existing Rheem unit
that was already installed at the test site, an
office building in Fort Worth owned by the
General Services Administration.
The standard unit had one compressor for
staged cooling, a constant-speed fan and
no economizer, while the tested unit had a
variable-speed inverter-driven compressor,
composite condenser fans with a variable-speed motor and an economizer that introduced outdoor air to the space.
The Pacific Northwest National
Laboratory, which conducted the data gathering and analysis on the RTU retrofit for
the DOE, measured an average daily EER of
8-17 Btu/Wh for the standard unit and 12-17
for the advanced unit, with the EER increasing for both as the outdoor temperature air
decreased. The advanced unit's daily EER
averaged about 18 percent higher than that
of the standard unit.
However, when the research team
accounted for the energy consumed by
the advanced RTU's ventilation mode, they
discovered that the EER difference between
the advanced and standard units jumped
to 26 percent in the advanced RTU's favor.
This is likely because the advanced unit
always introduces at least some outdoor air,
compared to the standard, which generally
recirculates return air, the study notes.
When to Upgrade
Investments in commercial HVAC equip-
ment can be substantial, so it may be best
to wait until it's almost time to replace
your existing RTU before you invest in an
advanced model. The savings can be sub-
stantial, but depending on your area and
what type of unit the advanced RTU would
replace, it may not be cost-effective to
remove a unit that still has some life left in it
even if the new model is more efficient.
The advanced model used in this test was
also heavier and had a larger footprint than
the standard Rheem unit it replaced, which
required structural changes to the building
to complete the installation.
Make sure the HVAC vendors you work
with are aware of any disparities that could
crop up with respect to weight, physical
footprint or other hurdles that might make it
tough to replace an older unit.
Janelle Penny firstname.lastname@example.org
is a senior writer for BUILDINGS.
Advanced Rooftop HVAC Field Tests
Reveal Substantial Energy Savings
INSIDE THE RESULTS OF THE ROOFTOP UNIT CHALLENGE
ADVANCED ROOFTOP UNITS delivered
high performance in a real-world field
test by the Department of Energy.