ASHRAE Journal - September 2022 - 45

TECHNICAL FEATURE
converted from non-DDC to DDC controls, or when the
existing DDC controls are replaced.
Life-cycle cost analysis was conducted for an incremental
(i.e., limited scope " add on " ) retrofi t of the demonstration
units using mechanical cost data for equipment
and installation costs and local energy and labor
rates. Maintenance impact was deemed negligible by
the operations & maintenance personnel, based on the
number of issues that arose during the demonstration
period and their ability to understand how the system
operates.
Although signifi cant energy savings percentages were
seen across all air handlers for both variable volume
operating modes (24% to 60% in Table 3), not all projects
successfully reduced life-cycle costs (LCC), i.e., had a
net present value, NPV > 0, or had a saving to investment
ratio, SIR ≥ 1, which are typical economic objectives
for government projects (Table 5); LCC analysis
showed attractive returns for the conventional units and
adequate returns for two of the three neutral deck units.
The DCV option showed merit as it added approximately
5% to the retrofi t cost and delivered an average 50%
increase in NPV. The economic indicators of the neutral
deck units at Fort Bragg are not as strong as those of the
conventional units at CERL. This is because the neutral
zone units, by design, have already eliminated the
simultaneous heating and cooling of the conventional
unit and because the baseline energy consumption of
these units is uncharacteristically low since Fort Bragg
adhered to the Army's strict energy conservation policies
for zone thermostat setpoint temperatures.
The percent energy cost reductions were not strongly
correlated to air handler size (in cfm) and corresponding
energy use, technology model (conventional vs.
neutral deck) or baseline consumption (Figure 8). The
baseline utility costs preretrofi t is the prime indicator
of adequate payback. This is amplifi ed by the fact that
retrofi t costs do not increase linearly with AHU size for
the basic variable volume retrofi t (Mode 1). The baseline
utility bills of the AHUs at Fort Bragg (at ~$1,000/yr) were
signifi cantly lower than the CERL units, which had preretrofi
ts utility costs of about $6,000/yr. The non-representative
nature of the low annual consumption totals of
all the demonstration units was addressed in the development
of the technology transfer tools (see " Technology
Transfer " sidebar), which reveal stronger economics for
typical applications.
Larger utility costs will typically be seen on larger units
or those with higher unit energy costs. Two contractor
estimates for a full system replacement (to VAV) were
received for a couple of similar CERL AHUs and both
were over $500,000. These estimates are cited only to
provide an indication of the signifi cant cost of a full system
replacement compared to the MZ-VV retrofi t.
Comfort
To assess the impact of the controls retrofi t on comfort
conditions, each zone temperature (provided by
the zone temperature sensing module) was compared
every 15 minutes to its setpoint in each of the three
demonstration modes. All systems, in all modes maintained
zone setpoints, on average, within 0.5°F (0.3°C).
Additionally, comfort conditions based on zone temperature
and relative humidity and the corresponding
ASHRAE Standard 55-2010 comfort zone (with constant
values used for offi ce clothing, sedentary activity level,
still air speed and an assumed mean radiant temperature)
were compared between the operating modes.
The CERL systems showed little difference in ASHRAE
comfort between the different modes. Two of the three
Fort Bragg systems had slightly worse ASHRAE comfort
performance where Mode 0 (constant volume mode)
comfort was maintained about 5% to 10% more of the
time than when in Mode 1 or Mode 2 (variable volume
modes). This was subjectively attributed to lower airfl ow
rates and slow fan speed response in Mode 1 and Mode
2. Overall, retrofi t thermal comfort conditions did not
vary signifi cantly from base case conditions and were
deemed acceptable by the project team and site O&M
staff.
Discussion and Conclusions
Retrofi t of an existing constant volume multizone airhandling
unit control system to convert it to variable volume
can be a simpler and less costly means of increasing
energy effi ciency compared to completely changing
out the system. This variable volume retrofi t technique
minimally impacts the physical system by focusing on
instrumentation and controls rather than a full system
change out requiring demolition and replacement of the
central unit and installation of new ductwork and terminal
units in the spaces. The retrofi t includes adding a
VFD and AFMA plus controls programming for the basic
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