Constructed in 1997/98 the HVAC system serves
the Kitchen and Dining building for the entire WAATS site as
well as approximately 100 Dorm rooms used to house the flight
training students. Although relatively old, the facility utilizes
many energy conservation capabilities available at that time:
variable pumping and airflow schemes, air and water-side economizers,
and a four-pipe hot water/chilled water distribution system
should have made for an efficient HVAC system, but the system
has been plagued by poor performance and excessive utility costs.
This poor performance manifested itself in unsatisfactory space
temperature control and fashioned unhappy tenants.
The HVAC system consists of:
- The central plant includes two water chillers with associated
pumps and cooling towers, and is primary-secondary pumped
and piped. The secondary pumps and the cooling tower fans
are all equipped with variable frequency drives.
- There is a hydronic plate/frame heat exchanger to provide
“free” cooling during the appropriate ambient conditions.
- The hot water system is comprised of one boiler and associated
pumps and provides space heating used for zone heating.
- 2 central station air handling units, one of which is VAV
and serves the VAV boxes in the dining areas.
- Room fan coil units in each Dorm room.
- Kitchen exhaust/makeup air units and associated hoods.
- Complete direct digitally controlled EMS.
- Severe space pressure variations
- Inability to utilize the existing plate/frame heat exchanger
- Numerous maintenance related operational deficiencies
- Questionable design practices
- Excessive utility costs
As is often the case, the facility’s HVAC system
was substantially oversized and as a result required exacting
controls capabilities for efficient operation, yet a lack of
HVAC system commissioning, EMS deficiencies, inadequate maintenance
practices and questionable design criteria has exacerbated the
problems. In most cases, a “bigger is better” approach to HVAC
systems is counterproductive and results in the system’s using
excessive energy and creates poor space conditions. The combined
efforts of the OptimissioningSM team rectified this situation
and in the process carried out numerous repairs to inoperable
or poorly performing system components
a detailed engineering study and report of the HVAC System’s
current (existing) condition and operation including: HVAC system
design and DDC system deficiencies, potential energy conservation
opportunities with budgeted project costs and economic analysis,
and maintenance related operational and efficiency analysis.
The result of this study became an approximately $250,000 Energy
Conservation Project including Retro-Commissioning and OptimissioningSM
- a combination of HVAC system commissioning and optimization
- of the facility. The project included the engineering required
for the modernization and automation of the chilled water central
plant and the air distribution systems as well as the kitchen
systems. Also, project coordination services were provided in
concert with the commissioning and optimization services provided.
Results of the project reflected a 35% reduction in HVAC related
energy costs and improved space conditions.
Considering the numerous deficiencies discovered
during the commissioning process, the absence of satisfied tenants
is not surprising. Many fan coil units serving individual Dorm
Rooms had inoperable or disconnected control valves, all of
the makeup air units intended to provide “fresh” (ventilation
air) to the Dorm Building were inoperable, and some air filters
had never been replaced since construction of the facility.
The chilled water control valve serving air handling unit #3
(Dining Room) was broken and as such continuously delivering
approximately 55-degree air that required reheating during most
ambient conditions. The outside air damper actuators for air
handling units #3, 4 were broken, thereby introducing excessive
outside air into the facility. The condenser water isolation
valves serving both chillers were inoperable and as such flowed
condenser water thru both chillers at all times, causing higher
than normal pressures in the operating chiller and possible
reducing life expectancy. Obviously, these issues have created
unsatisfactory space conditions and have contributed to the
excessive utility costs. The lack of maintenance on the DFac
was problematic at best and appeared to be ubiquitous.
Nearly all of the exhaust fans serving the
Dining Building had airflow problems, due either to quite aged
fan belts or non-existent fan belts. The main exhaust fan serving
the Kitchen hood was found to be operating backwards and thus
delivering approximately 40% of design airflow. The lack of
airflow precludes the required velocity to properly entrain
cooking grease in the air and exhaust it to the roof, where
it is intended to be collected and disposed of, and is a serious
fire hazard. Also, the makeup air unit serving the main Kitchen
hood had been inoperable for many months that created a negative
pressure condition in the Kitchen, leading to excessive conditioned
air being exhausted and wasting considerable amounts of energy.
These and many other maintenance related issues
were rectified, but there were also several original design
decisions made that could be considered questionable. The following
verbiage will attempt to clarify the findings and actions taken
made during the OptimissioningSM
HVAC System Design Considerations:
- The condenser water system incorporated a bypass, possibly
for head pressure control, which was allowing condenser water
to bypass the chillers during mechanical refrigeration mode
and causing the chillers to shut down. This bypass valve has
been permanently closed.
- The cooling towers were not equipped with automatic isolation
valves, and as such each flowed approximately ½ of the design
water flow during one-cooling tower operation. Reduced flow
rates across the cooling tower will significantly accelerate
scaling, as is evidenced by the current condition of the cooling
towers. Also, reduced flows thru condenser barrels – an ongoing
condition due to the failed condenser water isolation valve
actuators - will cause head pressure control issues, and will
enhance the possibility of scaling of the condenser tubes.
The cooling tower fill has been replaced, and the heavy scaling
that has been reported in the condenser tubes is being removed.
- The outside air quantities scheduled for air handling units
#3 and #4 and for the makeup air units serving the Dorm fan
coil units are extremely high and, coupled with the lack of
temperature controls, would have had a significant contribution
to overall energy costs. Of course the lack of maintenance
and condition of the Dorm makeup air units (i.e. inoperable)
mitigated this from occurring, albeit at a cost of providing
inadequate “fresh” air to the Dorm rooms. The units are back
in operation and airflow quantities have been reduced as much
as possible using current motors and drives, but are still
moving approximately 200% too much outside air. Air handling
unit #3 has been equipped with a space CO2 control, which
will prevent the introduction of unnecessary outside air
- The chilled water and condenser water pumps appear to be
egregiously oversized, as do the hot water pumps. The secondary
chilled water pumps and the hot water pumps are served by
variable frequency drives, making this a lesser issue (except
for maintaining minimum flow rates thru the hot water boilers).
The constant flow condenser water pumps’ manual isolation
valves were utilized to prevent over-pumping and overflowing
of the cooling towers, an adequate but not particularly efficient
means of control.
The chilled water/condenser water system’s
OptimissioningSM effort included the replacement of the cooling
tower fill, the replacement of the two chiller’s condenser water
isolation valves, the addition of two cooling tower isolation
valves, the addition of variable frequency drives for the cooling
tower fans, and the addition of a flow measuring station in
the chilled water system decoupler. The central plant was also
re-balanced, and new temperature sensors were installed throughout
the system. Controls hardware was added as required, and numerous
modifications to the sequences of operations and setpoints were
An innovative solution to the lack of chilled
water temperature control was also devised using existing control
wiring, so the chillers are now capable of resetting their leaving
chilled water temperature as required based upon system demand.
The reset schedule is currently based upon wet-bulb (WB) ambient
temperature, and is indexed to the heat exchanger operation
in order to minimize radical temperature swings in the supply
chilled water. Having the ability to control and monitor the
leaving water temperature setpoint also allowed the system “fail-overs”
to be based upon specific criteria and all but eliminated the
possibility of operating two chillers (one on partial capacity
due to a refrigerant circuit malfunction) when one was capable
of meeting the demand.
The addition of the flow meter in the decoupler
allowed flow rates to be monitored, and was a valuable tool
in the water balance process and establishing the differential
pressure setpoint for secondary pump control. This setpoint,
originally at 14 PSI, was adjusted down to 8 PSI during OptimissioningSM.
Also, the (controls sequence) timing of the lead/lag of the
secondary pumps, an issue that prevented one of the pumps from
ever running, was corrected and now allows the run times to
be equalized and provides redundancy in the event of a pump
Also, as predicted, it appears that one chiller
will be adequate to provide cooling for the entire facility
on a design cooling day as opposed to both systems being in
operation. A major factor in achieving the goal was the reduction
of outside air quantities throughout the entire facility, including
the Kitchen air handling units and the Dorm makeup air units.
This will significantly reduce operating costs and was made
possible by changes made throughout the HVAC System, including
the air distribution and controls systems as well as the central
Inoperable air distribution system components
were identified and replaced with superior technology. Air handling
unit #3’s inoperable chilled water control valve was replaced
as was the valve at air handling unit #4, using pressure independent
type valves. Use of a pressure independent type control valve
will improve the chilled water system’s temperature differential
(DT), reducing pumping costs and chiller run-times. Our experience
with these types of valves has been excellent control capability,
industrial quality, and substantially reduced a system differential
pressure (DP) requirement that in turn allows other system chilled
water valves to control more effectively. The higher system
DT also allows the chillers to load more fully, eliminating
the need to run multiple chillers at reduced loads, and saving
the energy associated with the unnecessary chiller’s operation
(cooling tower fans, condenser water pumps and primary chilled
The outside air/mixed air/return air damper
actuators at both air handling units were replaced, and demand
controlled ventilation (CO2 monitoring) was installed for air
handling unit #3. Air handling unit #3 is equipped with variable
frequency drives on the supply and return fans. Previously,
the return fan tracked the supply fan as a percentage of speed.
This method is common, but due to differing fan types, their
respective operating characteristics and much different operating
duties it is never an effective method of control. It was determined
that the return air fan was unnecessary except at high (above
50%) outside air quantities introduced during outside air economizer
operations, and as such is now disabled below 50% damper position.
It was also apparent that the supply static
pressure setpoint was be too high, as evidenced by the supply
fan operating at 58 Hz even though the deck temperature was
54 degrees. At a 54-degree deck temperature, you would expect
that at least some of the VAV boxes would be satisfied, requiring
reduced airflows and that the supply fan in turn would unload.
However, the VAV boxes are not currently visible on the Alerton
DDC system, so the automatic static pressure reset control scheme
is not capable of being used. A new, much lower setpoint of
.5” was selected to control supply fan speed. Also, a supply
air temperature reset schedule, based upon a linear range of
outside air temperatures, was established for the air handling
unit and will result in reduced energy costs and more comfortable
No space pressure controls existed in the Dining/Kitchen
Facility even though it is desirable to ensure that the Kitchen
remain negative in relation to the Dining area at all times
for odor control. The minimum outside air damper position on
air handling unit #3 was 15%, and we discovered at this condition
that excess “fresh” air was being delivered at this condition.
The addition of ventilation controls (i.e. CO2 sensor) made
it possible to allow air handling unit #3’s outside air damper
to close (0%) completely and avoid delivering too much (and
wasting energy) or too little outside air to the space.
The Test and Balance process determined minimal
impact on space pressure relationships with widely varying outside
air quantities delivered by air handling unit #3, and the same
was discovered of air handling unit #4. Exhaust fans #10, 11,
13 are electrically interlocked with air handling unit #3, and
it was determined that a minimum outside air damper position
(allowable) of 0% for air handling unit #3 still has sufficient
leakage to not effect space pressures. As such, the minimum
damper position was set to 0%, reducing outside air loads and
conserving energy. The same was true of air handling unit #4.
The balance of the exhaust fans and the makeup air unit was
excellent, essentially eliminating the need for additional makeup
Exhaust hoods #11, 13 are served (provided
with) makeup air directly from VAV boxes, an unusual and wasteful
energy practice. It is unusual in the sense that VAV boxes by
definition vary the amount of airflow delivered to a space,
but the exhaust fan serving the hoods are constant volume. It
is wasteful because the makeup air is conditioned and then exhausted.
Because hood #13 is never used, the supply grilles serving the
adjacent spaces were closed, the exhaust fan was disabled, and
the airflow to the hood was set to provide space cooling. If
the hood is placed back into operation, the current supply air
quantity will match the exhaust air quantity.
The outside air quantities for the Dorm makeup
air units were also adjusted down as far as possible. The new
quantities are still unnecessarily high and consideration should
be given to changing the fan sheaves/motors to allow the quantities
to be reduced further. However, the current conditions of the
Dorm makeup air units, combined with the changes made to the
Kitchen outside air quantities, and the changes made to the
EMS system, have already proven extremely effective in reducing
the load on the chilled water system.
Numerous changes to the operating sequences
of the central plant, makeup air units serving the Dorm, and
the air handling units serving the Dining facility were made
during OptimissioningSM. Notably, an outside air wet-bulb (WB)
temperature based supply chilled water temperature reset was
developed. Using the supply chilled water reset schedule, the
lag chiller is now called only when a deviation from setpoint
(from the outside air WB reset schedule) of 15-degrees for 10
minutes is detected. Deviation from setpoint for a period of
time is also used to enable the lag cooling tower and the lag
secondary chilled water pump, 5-degrees for 20 minutes and 5
PSI for 15 minutes respectively. All of these setpoints and
times are user adjustable.
The 100% outside air makeup air units serving
the room fan coil units in the Dorm were previously being controlled
identically to the room fan coil units: maintaining a “room”
(duct) temperature of approximately 75-degrees. Working within
the original Honeywell LonWorks database – the makeup air units
are not currently controlled by the Alerton system – their control
was changed to maintain a setpoint of 60-degrees below 60-degrees
outside, 80-degrees above 80-degrees outside, and provide no
heating or cooling between 60-80 degrees outside.
The VAV boxes served by air handling unit #3
and serving the Dining area are not currently controlled by
the Alerton system. However, the supply air static pressure
and the supply air temperature were in “Auto-Reset” mode, implying
that the VAV boxes actual demand was controlling air handling
unit #3’s operation. As this was impossible, the static pressure
control was changed to manual, and a setpoint of .5” established
during OptimissioningSM and airside balance. The supply air
temperature is now controlled by an outside air temperature
reset schedule from 68-degrees to 55-degrees.
The return air fan of air handling unit #3
was controlled as a percentage of supply fan speed (76%). Typically,
return air fans are unnecessary devices that are improperly
controlled, and this was the case for air handling unit #3.
the return air fan has been disabled except when the economizer
is allowed (outside air temperature below 70-degrees) and the
outside air dampers have opened to at least 50%, at which time
the return air fan is operated at 30% of the supply fan speed
in order to provide pressure relief from the space. The minimum
outside air damper position was changed to 0% when it was determined
that sufficient outside air leaked thru closed dampers to maintain
acceptable indoor air quality. A new CO2 sensor modulates the
dampers to maintain an adjustable indoor CO2 level of 1000 ppm.
Cumulatively, all aspects of the project may
have actually increased the actual load served by the central
plant; many necessary electrical loads were repaired and placed
back into operation, outside air is now being supplied to the
Dorm rooms, and many inoperable control valves were repaired
so as to satisfy the loads. But when considered comprehensively
the OptimissioningSM of the facility has more than offset those
additional loads and even lowered the entire facility’s energy
consumption by approximately 20+% and demand by approximately
Considering that only the HVAC system – a single component of
the facility’s energy consuming systems – was optimized these
are very impressive results. The following charts were created
from actual utility data provided by Mr. Seaton, and generated
by his Utility Manager software. The project was completed in