ASHRAE Journal - March 2014 - 38

Technical FeaTure

the standard (pre-retrofit mode) and the
advanced control modes on a daily basis.
The remainder of the article summarizes the
results from the field study. More details on
the field study can be found in Wang, et al.8

Standard Controls and Advanced RTU
Controller Description

Figure 2 Schematic diagram showing sensors used to measure critical parameters in the field test.

kW
Exhaust Air

Total RTU Power
CO2

Exhaust Fan
Return Air Damper

Space Temperature
(Applicable if the Controller Works as BMS)
Return Air
Sensor Labels
Temperature Sensor
CO2 CO Sensor
2
kW Power Consumption

Before the RTU controller retrofits, all 66
Outdoor Air
Discharge Air
units in the field used "standard" convenSupply Fan
tional control strategies: the supply fan ran
Outdoor Air
Air
Heating DX Cooling
continuously at full speed when the building
Damper
Filter
Coil
Coil
was occupied; the economizer used a fixed
dry-bulb high-limit of 55°F (12.8°C), and it
a site, storing it locally, and streaming the data in real
was not integrated with mechanical cooling; and DCV
time to the cloud for analysis.
was not used.
The advanced RTU controller had a "soft" service
In contrast, the advanced off-the-shelf controller
switch to change the RTU control logic between the stantested in the field had these features: integrated differential dry-bulb economizer, supply fan speed control dard (conventional) control and the advanced energy
saving control. During the field tests, the standard conand DCV. It was possible to modulate the supply fan
trol and the advanced control were alternated daily for
speed because as part of the controller installation, a
more than 12 months for most units. The standard convariable-frequency drive (VFD) was also installed. The
supply fan speed was modulated based on the operation trol was intended to emulate the RTU operation before
retrofitting the controller.
mode. In ventilation mode, it was set at 40%, while it
was either 75% or 90% during cooling and heating. For
Energy Savings Estimation Methodology
DCV, the advanced controller used the return air CO2
The methodology used to estimate the energy savconcentration as the trigger to regulate the outdoor air
damper and fan speed controls to ensure that the maxi- ings was similar to that defined in ASHRAE Guideline
14-2002, Measurement of Energy and Demand Savings. First,
mum allowable CO2 level (high CO2 setpoint = 1,000
ppm) was not exceeded.
based on the measured energy consumption data during the pre-retrofit period (standard controls) and the
Field Measurement, Metering and Monitoring
post-retrofit period (advanced controls), regression
The advanced controller was tested on 66 RTUs using
models were developed to correlate the daily RTU energy
the same metering and monitoring plan on all units
consumption with the average outdoor air temperature.
to verify the operations of the advanced controller,
Then, the pre-retrofit regression model was used under
estimate the energy savings resulting from retrofitting
the post-retrofit conditions (post-retrofit outdoor air
the RTUs with the advanced controller and estimate of
conditions) to estimate the projected energy consumpsimple payback periods.
tion in the "standard" control mode. The actual energy
A thermistor-type temperature sensor was used to
savings was computed as the differences between the
measure outdoor-, return-, mixed-, and discharge-air
projected energy use and the measured actual energy
temperatures, as shown in Figure 2. The total true power
use over the same post-retrofit period. In addition to
consumption of the RTU was measured using a power
the actual savings, normalized annual savings were also
transducer. The CO2 concentration in the return air duct calculated. For this purpose, the pre- and post-retrofit
was monitored using a CO2 sensor.
models were used to estimate the pre- and post-retrofit
Control signals were also monitored (damper, cooling
energy consumption using typical meteorological year
status, heating status, fan speed, etc.). The monitoring
(TMY) weather data. The difference between the estiplan consisted of data collection at each RTU at onemated pre- and post-retrofit energy consumption is the
minute intervals, aggregating the data from all RTUs on
normalized annual energy savings.
38

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ASHRAE Journal - March 2014

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