Case study: Art center HVAC upgrade design
This example shows how upgrades were made to a deficient HVAC system in a step-by-step process.
An arts building for a confidential university client was a one-story section of approximately 20,000 sq ft supporting varying art/design functions such as photography, fabric design and weaving, ceramics and sculpture, printmaking, life drawing and painting, woodworking, and metal castings. Many of the functions of the rooms had been modified over the years with very little changes made to the original HVAC equipment.
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 and power upgrades included remedial upgrades to maintain appropriate ventilation along with the proper air conditioning requirements for each room to function safely in the removal of varying levels of accumulated particulates (sawdust, sanding, and dust) and noticeable odors (paints, solvents, and other chemicals) in eight different areas. The upgrades included replacing all the older HVAC equipment (air handling units or AHUs, exhaust and relief fans) and ductwork, along with all the electrical and controls upgrades required to make the systems fully functional. Everything would be remotely monitored and controlled from the campus utility energy management system.
The HVAC system was modified with new AHUs and variable air volume (VAV) terminal units—some fan powered—where required. The VAV units served classrooms or related office and other spaces within the building and were controlled by a thermostat for the cooling and heating needs of the space it served. Based upon the space use and ASHRAE Standard 62.1 requirements, the ventilation rates for the spaces were added together to get the total required outdoor air (OA) necessary for the AHU serving its associated VAVs. One key to these systems upgrades was the controls required to ensure the ventilation air was provided where and when it is needed. In most spaces, and particularly in educational facilities, the occupancy rates fluctuate during the day so the ventilation air requirements change and create opportunities for improved energy use. The additional design consideration for these systems was the requirement for operating various exhaust systems for contaminate removal.
Combinations of items were included in the HVAC system design for demand control ventilation (DCV) strategies. These items included occupancy schedules and lighting sensors (CO2 sensors were not used), airflow measuring stations (AMS), and fan pressure optimization control to reduce OA to the spaces if they were not occupied.
To reduce the impact of increasing OA, the design for the building’s DDC system’s controls sequence started with a building occupancy schedule, which sets the overall times when the building will generally be occupied (e.g., 6 a.m. to 8 p.m.). Design for full DDC control of OA and return air dampers with feedback from the OA AMS is helpful so the facility energy management system (FEMS) will know and can adjust the amount of OA coming into the building. Including occupied/unoccupied signal inputs from space or lighting occupancy sensors so the system is more aware of room occupancies will ensure the VAV boxes maintain minimum ventilation airflow to the spaces. The FEMS should “poll” all the VAV boxes and reset the static pressure (SP) setpoint within the ductwork to ensure that no VAV damper is more than 90% (adjustable) open. The AHU is provided with a variable frequency drive (VFD) so its fan can be modulated to meet this duct SP set point reset strategy.
The engineering team did not use CO2 sensors, which are sometimes used in spaces where large occupancies may occur, such as auditoriums. These devices provide input signals indicating the levels of CO2 and will override temperature controls and allow for more supply air into the spaces. This control strategy can also be added to the AHU to reset the OA damper to a more open position so additional OA is drawn into the entire system if needed. The use of CO2 sensors can enhance overall system control and provide additional energy efficiency, but using these sensors correctly can be difficult and usually adds costs to the system that many owners, and particularly K-12 schools and universities, are sometimes reluctant to spend.
Randy Schrecengost is a project manager/senior mechanical engineer with Stanley Consultants. He has extensive experience in design and project and program management at all levels of engineering, energy consulting, and facilities engineering. He is a member of the Consulting-Specifying Engineer editorial advisory board. Gayle Davis is a mechanical engineer with Stanley Consultants. He has experience in the design of HVAC systems, boiler plants, compressed air systems, plumbing systems, steam distribution systems, central heating, and cooling plant design. He is also experienced in commissioning and retro-commissioning.