Confiscated elevator shafts are your first hint that this isn’t the typical hospital retrofit. Just as the large building spans a Boston traffic artery, so the work spans a variety of areas, from the serious air distribution upgrade to better VAV boxes to new sensors allowing more flexibility. See how this facility’s setup went from above-average consumption to simply above average.
Located in the South End of Boston, Boston Medical Center (BMC) is composed of over 2.5 million sq ft of inpatient, ambulatory care, and medical office space. BMC has recently completed a significant portion of a phased HVAC project at the Yawkey Building that will result in a total energy reduction of 4.6 million kWh and 35,000 Mlbs of steam per year, with a total annual cost savings of $1,345,700.
The Yawkey Building at Boston Medical Center is a 260,000-sq-ft healthcare facility that was designed in the late 1960s and built in the early 1970s. The building consists of five floors of above-grade patient care space and a basement for support services. The building is unique in that it spans over a four lane city street on Massachusetts Avenue. Building columns on both sides of the street and between east and westbound Massachusetts Ave. traffic support the building’s weight. Horizontal building trusses located at each floor level transfer building weight to the building columns. These trusses, located below each floor slab, are located in interstitial ceiling spaces between floors so as not to disrupt the large open floor plate of each occupied floor. These interstitial spaces range from 6 ft to 9 ft clear overhead.
In addition to building trusses, the interstitial spaces contain supply and return air ductwork, electrical conduits, plumbing piping, and other MEP components. Building operators access this equipment via interstitial space cat walks accessed from building stairwells. These interstitial spaces are utilized as HVAC return air plenums. Room return air flows through ceiling light fixtures and transfer grilles into the interstitial space, where it is drawn into the return air duct system by original constant volume return air terminal boxes.
The building’s HVAC systems consists of a variable volume all air system equipped with pressure-dependent variable- and constant-volume terminal boxes on supply and return air duct systems. The low-pressure sides of the return air terminal boxes are open to the interstitial plenum space. The high-pressure sides of the supply and return terminal boxes are connected to high pressure horizontal duct mains located in the interstitial spaces. The supply boxes are equipped with hot water reheat coils for zone heating. The majority of the building terminal boxes are controlled with local pneumatic instrumentation.
Ten large AHUs with a total supply air capacity of 450,000 CFM serve the building. Each of these mixed air air?handling units are equipped with dual return air fans, prefilters, dual supply fans, chilled water cooling coils, hot water heating coils, and final filters. The air handlers were retrofitted with DDC in the late 1980s. To minimize the amount of duct shaft space in the building, the HVAC duct risers and horizontal mains (supply and return) were originally designed with significantly high air velocity and pressure. In some portions of the system, the duct velocity was designed as high as 6,500 FPM. To develop the necessary pressure to overcome this high pressure ductwork, the supply fans were originally rated for 10 in w.c. of total pressure. This resulted in a very high fan power per CFM ratio. This design approach was considered reasonable at the time as it was applied prior to the 1973 OPEC oil embargo and national energy crisis.
As part of Boston Medical Center’s 2011 commitment to reduce its energy consumption by 25% by 2020, BMC completed a detailed energy audit of their sprawling Boston campus. This audit included evaluation of current energy usage of all major campus buildings. The evaluation identified the Yawkey Building as one of the highest users of energy per square foot. This fact was made more concerning as the building is occupied primarily during normal business hours. The Site Energy Utilization Index (SEUI) of the Yawkey Building was found to be 162 kBTU/sq-ft /yr which is 71% greater than the average outpatient hospital building in the United States per the 2003 CBECS national average of 94.6 kBTU/sq-ft/yr, and it is 92% greater than the 84.5 kBTU/sf/yr CBECS average for outpatient hospital buildings in the New England and Middle Atlantic regions. The SEUI is defined as the amount of energy (in kBTU) used by a facility divided by the gross area of the building per year.
The cost of utilities for the Yawkey Building was also calculated to be $9.67/sq-ft/yr. This also is significantly high for an outpatient building, even when considering the high cost of energy in the Boston area. Building audits and interviews with hospital HVAC staff revealed that the HVAC systems operated continuously even during unoccupied hours and that the return and supply fans were almost all operating at 68 hertz, regardless of outdoor air conditions, in an attempt to maintain the necessary duct static pressure setpoint.
During heating months, the air handlers needed to operate during unoccupied hours to heat the perimeter via conventional terminal boxes equipped with hot water reheat. During the cooling season, the air handlers operated during unoccupied hours so that the spaces wouldn’t be too warm upon the start of the occupied period. In addition, operators found it difficult to supply enough air to different areas of the building. To alleviate this problem, three packaged rooftop units were added to the building for space cooling and heating of several building clinics. Further evaluation of the system revealed that all building of the AHU vane axial supply fans were replaced in the late ’80s or early ’90s with fans that could only generate 6 in w.c. of total static pressure. This realization quickly explained why the supply fans continuously operated at 68 hertz and areas of the occupied spaces still could not be satisfied due to the resistance in the high velocity ductwork.
Working closely with BMC’s engineering and facilities staff, ESI identified the following measures to improve the functionality of the system and to significantly reduce the energy consumption of the building.
Air Distribution System Upgrade
To reduce the amount of fan power needed to supply and return air to and from the building, a new return air duct system would be constructed so that the existing return air duct risers and horizontal mains could be converted to supply air. This would reduce the velocity of the supply air duct system by 50% and reduce fan shaft brake horsepower by as much as 75%. Normally, completing a project of this sort in an occupied outpatient care building would be impossible. Reducing occupied floor area to create new duct shafts is typically never an option in an occupied building and this type of construction work is very invasive and disruptive. However, the owner was willing to remove two of the building’s six elevators so that a new return air duct shaft could be created (Figure 1). This was enough shaft area to return half of the building air to eight of the building’s AHUs located in the penthouse.
To handle the other half of the building’s return air, new duct risers were installed on the exterior of the building in an area that can later be enclosed during a future expansion of the building (Figure 2). All new return air risers were then connected to a new common return air ring duct located on the roof above the eight penthouse large AHUs (Figure 3). All eight of these air handlers were then connected to the new return air ring duct. To make this connection, eight large roof penetrations were made so that a new return air duct could be installed to connect the air handler mixing box to the new ring duct (Figure 4).
Once the AHUs were connected to the new return air ring duct system, the existing return air risers and horizontal mains were thoroughly cleaned and connected to the supply side of each air handling unit. New horizontal duct mains off the new return air risers were then terminated at each interstitial space between occupied floors with a combination smoke/fire damper and wire mesh screen.
If this building did not have interstitial spaces that could function as return air plenums, it would have been impossible to complete this project in an occupied building. The cutover from the existing return air duct system to the new would have been too invasive for a conventional occupied clinical building to complete the work. Moreover, the existing return air terminal boxes that separated the high pressure return air duct system from the interstitial return air plenum were eliminated from the system. Now that the new return air system is low velocity, the need for return air terminal boxes is eliminated. Supply and return air fan airflow stations were also installed for proper airflow tracking to ensure accurate volume control of outside and return air at each AHU.
All of the new return air risers, rooftop ring duct, and floor takeoffs were designed for a maximum velocity of 1,250 FPM to keep return air fan power low. Under a phased plan over the next five years, the hospital is planning on renovating the entire building. When this occurs, new low pressure return air horizontal duct distribution will be installed and the return air plenums will be eliminated to meet the latest code requirements for outpatient healthcare facilities, which prohibit return air plenums. The three packaged rooftop air handling systems will also be eliminated as the problem with delivering enough air to different areas of the building is eliminated with the reduction of duct velocity and pressure.
Given the owner’s master plan to renovate the entire building, and the fact that the existing supply fans could not generate enough pressure to overcome the pressure drop of the high velocity ductwork, either the fans would have had to be replaced with fans of higher pressure capabilities or the new return air duct system would have to be installed as described above. The decision was quickly made by the owner to move ahead with the air distribution upgrade rather than replace the fans, because not only was the cost to replace the fans a substantial capital investment, but it also increased the cost to operate the fan systems. In other words, upgrading the air distribution system would cost more but the fan operating cost savings would quickly outweigh this first-cost difference.
Improve Unoccupied Setback
The building’s central AHUs are equipped with DDC, but there were no DDC space temperature sensors to provide temperature feedback to the control system. This prevented the AHUs from being shut down in a controlled manner during unoccupied hours. Under this project, eight new DDC temperature sensors per floor were installed and the AHU sequences of operation were modified. Now the air handlers will remain off during unoccupied hours unless one of the zone temperatures rises above 80ºF or drops below 60ºF. These new temperature sensors also allowed for the implementation of an optimal AHU start/stop sequence so the air handlers will be started with just enough time prior to the occupied period to bring the zone temperatures within an acceptable occupied temperature range.
Install New Pressure Independent VAV Boxes
As the building was renovated to meet different hospital programming needs over the past 40 years, the original constant volume and variable volume terminal boxes remained in service. These original boxes are pressure dependent, meaning they don’t have the capability of maintaining a specific airflow regardless of duct pressure. The existing boxes are also in various states of disrepair and are beyond their useful life. Outpatient hospital buildings require a minimum of six air changes per hour in most exam and treatment spaces, so new pressure independent boxes can be set to maintain this minimum air change requirement. Also, new boxes will improve comfort and allow for airflow throttling when zone cooling loads are light. The owner is planning to replace these terminal boxes as part of the phased renovation of the building over the next five years and to install direct digital instrumentation to efficiently control the boxes.
Perimeter Hot Water Heating Finned Tube
The building perimeter is currently heated with forced air that is controlled by original pneumatic terminal reheat boxes. Given the floor to ceiling glass around the entire perimeter of the building, a substantial amount of heat is needed to heat the perimeter spaces. Also, since the perimeter is heated with forced air, the large central AHUs must operate to heat the building during nights and weekends when the building is unoccupied.
ESI recommended that a hot water finned tube be installed beneath the windows in the same location as the current floor air registers. This retrofit will permit building heating without the operation of the large AHUs during unoccupied hours. Finned tube heat under the windows will also provide better comfort than forced hot air for occupants who are sitting near the glass. Finned tube radiation will be added to the building as it is renovated over the next five years and has already been installed on the second floor.
Energy Savings Analysis
Engineered Solutions Inc (ESI) developed an eQuest model to determine the estimated annual energy savings for the energy conservation measures (ECMs) proposed above. This model was commissioned and funded equally by the owner and NSTAR Electric. TMY3 weather data for the Boston area was used in the eQuest analysis.
The base case model was calibrated using 2009 electric utility submetered data for the building. The eQuest model did not include a number of electric end?uses such as exterior lights and elevators that have minimal impact on HVAC energy use. Therefore, and as expected, the calibration model slightly under predicted electricity demand and consumption. Since the chilled water plant energy is not on the buildings meter this energy was not included in the eQuest model calibration, but was included in the analysis model for the ECM savings calculations.
The planned and partially completed HVAC upgrades will produce significant building efficiency improvements and operating cost reduction. Based on electricity sub?meter data and estimated steam usage and using 2010 utility rates, the Yawkey building costs $9.67/sq ft/yr in steam and electricity to operate annually. Based on similar energy projects performed by ESI, typical outpatient buildings of this type and usage in the Boston area have cost between $5 and $6/sq-ft/yr. With the implementation of the ECMs described above, the cost to operate the building will drop to a more reasonable level and will reduce annual electric operating cost by approximately $701,896. Table 1 summarize the estimated costs, energy consumption, and reduction that could be achieved by implementing the ECMs.