HVAC

Case study: HVAC system conversion includes remote monitoring

A government facility’s HVAC system was overhauled to make it more efficient and cost-effective.
By Randy Schrecengost, PE, CEM, Stanley Consultants, Austin, Texas February 1, 2019
Photo 2: A multi-zone (2 zones) cooling-only air handling unit (AHU) that serves fan power boxes (FPB) variable air volume (VAV) boxes with hot water reheat coils. Courtesy: Stanley Consultants Inc.

An 80,000-sq-ft, 5-story government building with a basement had undergone one complete renovation, and several smaller upgrades since it was originally constructed in 1917. The early major renovation installed one multi-zone (MZ), constant volume air handling unit (AHU) on each floor, including the basement. Each individual AHU (six total) had two exterior and one interior zone per floor. Each of three zone duct runs had a hot water reheat coil installed and located within the corresponding mechanical rooms.

These six AHUs were identified as Texas multi-zone (MZ) units (see Figure 5 and “AHUs in HVAC system design configurations”). Zone occupant control design included a few retrofitted single-duct variable air volume (VAV) boxes (SDBs), but mostly parallel fan-powered variable air volume (FPVAV) boxes with no additional reheat.

Figure 5: The schematic illustrates multiple options for component positions within a multi-zone air handling unit (AHU) serving true multiple zones. Courtesy: Stanley Consultants Inc.

 

One problem with this system is that the entire zone was heated if the zone thermostat required it. Many of the interior spaces were heated no matter what. Photo 3 shows one of the old Texas MZ illustrating the damper actuators on the AHU’s three zones and some hot water piping routed to one reheat coil in the ductwork above (upper right). In this picture, you can see staining on the duct and pipe wrap due to leaks from years of service.

This building had three main concerns that are driving the need for replacement of the mechanical HVAC systems: system age, outside air (OA) distribution, and temperature control accuracy in all HVAC zones, particularly at the perimeter. The system was in constant simultaneously heating and cooling modes, not only at the AHUs due to leaks and controls issues, but also within the zoned spaces because most areas were common open areas mixed with some perimeter offices. In addition, even with yearly maintenance, the duct reheat coils were nearly plugged and exhibited reduced airflows from original design. The constant-volume AHUs had been modified to provide the maximum airflow possible due to the high pressure drops through the reheat coils.

Photo 3: An old Texas multi-zone (MZ) system illustrating the MZ damper actuators and hot water piping routed to one reheat coil in the ductwork above (upper right). Courtesy: Stanley Consultants Inc.

OA was drawn from louvers located on the one side of the building overlooking and alley, which allowed parking and contained a loading dock, and along a busy street in front of the building. The original system was designed with the intent to allow for 100% economizer mode for free cooling, under the proper OA conditions, with the excess air being relieved using return/relief fans out of a chase adjacent to the mechanical rooms.

Several of the relief fans were not working. Many of the system’s MZ dampers as well as the economizer controls had been disconnected and/or were not used for years. and needed to be replaced along with a complete modification to both the controls (direct digital controls, or DDC), and sequences of operation.

After completing a validation and feasibility study and cost estimate, a design was developed to address the identified deficiencies of the HVAC systems. The recommended HVAC systems included miscellaneous remedial upgrades to maintain appropriate ventilation along with the proper air conditioning requirements for each zone’s common spaces and offices.

Figure 7: Shown is a cooling-only, single-duct variable air volume (VAV) box that would be found in an HVAC system’s interior zones. Courtesy: Stanley Consultants Inc.

Figure 8: A schematic illustrates a parallel fan powered variable air volume (VAV) box that would be found in an HVAC system’s exterior zones. Courtesy: Stanley Consultants Inc.

The upgrades included replacing all the AHUs with new cooling only VAV AHUs serving two or three zones each and, supplemented with new VAV boxes, both single-duct and fan-powered VAV boxes with hot water reheat for occupant comfort. Along with new ductwork and a complete electrical and DDC upgrades to make the systems fully functional, everything began to be remotely monitored and controlled from the central utility energy management system. Figures 7 and 8 are schematics of single-duct variable air volume (VAV) boxes and fan power boxes (FPB) that would be connected to HVAC zones as indicated in Figure 9.

Figure 9: In this case, the concept of two branch main duct runs—one each of an interior and exterior zone—is illustrated, along with various variable air volume (VAV) boxes. Courtesy: Stanley Consultants Inc.

Combinations of several other items were also included in the HVAC system design for a demand-control ventilation (DCV) strategy that could be enhanced in future funding cycles. These items included occupancy schedules and lighting sensors (CO2 sensors were not used in this design), airflow measuring stations (AMS), and fan pressure optimization control to reduce OA to the spaces if they were not occupied (see Photo 2).

Photo 2: A multi-zone (2 zones) cooling-only air handling unit (AHU) that serves fan power boxes (FPB) variable air volume (VAV) boxes with hot water reheat coils. Courtesy: Stanley Consultants Inc.

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Randy Schrecengost, PE, CEM, Stanley Consultants, Austin, Texas
Author Bio: Randy Schrecengost is the Stanley Consultants Austin mechanical department manager and is a principal mechanical engineer. He is a member of the Consulting-Specifying Engineer editorial advisory board.