English version machine-translated from the Vietnamese original — under review.
PART 1 - GENERAL
1.1 Summary
Establishing clear and comprehensive standards from the outset of a project is a strategic factor that determines the success of a VAV (Variable Air Volume) system. This part sets out the basic requirements, the applicable standards and the core sequences of operation to ensure that the VAV system is designed, installed and operated to achieve the highest energy efficiency and performance. The provisions below form a solid foundation for the equipment selection and detailed construction in the following parts.
Scope of work
This specification applies to the design, supply and installation of the VAV system, including but not limited to the following components:
- Control sequences of operation: Based on ASHRAE Guideline 36 to optimise performance and save energy.
- VAV terminal units: Including VAV boxes with or without reheat.
- Supply fan system: Fan arrays are preferred to increase efficiency and reliability.
- Supply ductwork system: Including main ducts and connections to terminal units.
- Return air system: Including the option of using the ceiling space as a return air plenum.
- Minimum outdoor air control system: Ensures accurate measurement and control to maintain indoor air quality (IAQ).
Applicable standards
All work shall comply with the latest editions of the following standards and guidelines:
- ASHRAE Guideline 36: High-Performance Sequences of Operation for HVAC Systems. This is the foundational standard for system control, providing proven and optimised sequences to reduce energy costs and improve stability.
- ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality. This standard specifies ventilation requirements, in particular the method for determining the minimum outdoor airflow required for each zone.
- ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings. This standard sets out energy conservation requirements that directly affect the setting of VAV box minimum airflow setpoints.
Sequences of Operation (SOO)
Basic requirement: The control sequences shall comply with and be fully implemented in accordance with ASHRAE Guideline 36-2021.
Applying ASHRAE Guideline 36 (G36) benefits all stakeholders. Instead of engineers writing their own sequences, contractors doing their own programming and operators doing their own debugging, G36 provides a set of advanced sequences developed and verified by leading ASHRAE experts. This allows control system manufacturers to pre-program, test and debug these sequences for their dealers. The result is a streamlined process that lowers first cost for the entire design and construction team, lowers maintenance cost, lowers energy cost and improves the overall performance of the system.
Requirements for Building Automation System (BAS) manufacturers
To qualify for this project, the main BAS manufacturer shall agree to fully program and test all of the ASHRAE Guideline 36 sequences referenced herein for its dealers bidding on this project. The BAS manufacturer shall provide the programming and testing in accordance with ASHRAE Guideline 36 as part of its standard support package for its dealers, and this cost shall not be recorded as a direct cost item in the bid documents or payments for this project.
This part has established the general provisions and the operating philosophy. The next part goes into detail on the product and equipment requirements needed to achieve these performance objectives.
PART 2 - PRODUCTS
2.1 Overview
The quality and performance of a VAV system depend directly on the correct selection and specification of the main components. The specifications in this part are based on best practices and practical analysis to optimise the life-cycle cost (Life-Cycle Cost - LCC), including both the first investment cost and the long-term operating cost. Selecting suitable equipment is the first step to ensuring that the system operates stably, saves energy and delivers comfort to occupants.
2.2 Air Handling Units (AHU)
Fan Arrays
The supply fan shall be a fan array consisting of multiple direct-drive, single-width, single-inlet plenum fans.
Using fan arrays brings many technical and operational benefits compared with conventional centrifugal fans:
- Reduced AHU length: Saves valuable installation space.
- Reduced noise: Particularly on the discharge side of the fan.
- Increased redundancy: If one fan motor fails, the remaining fans can continue to operate to maintain part of the airflow, ensuring that the system is not completely interrupted.
- Reduced fan energy consumption: By reducing noise at the source, fan arrays can eliminate or reduce the need for silencers. Eliminating these components reduces the pressure loss of the whole system and saves fan operating energy.
- Easy maintenance and replacement: The smaller motors and fans are lightweight and easier to handle and replace than a single large fan.
Minimum Outdoor Air Measurement and Control
Accurately measuring outdoor airflow, especially at low velocities, is a major challenge for many types of sensors. Factors such as low air velocity, non-uniform flow, turbulence and the influence of outside wind can cause large errors.
To solve this problem, minimum outdoor airflow control shall be performed using a differential pressure (DP) sensor measuring across a fixed orifice (e.g. a louver or a filter screen). The system shall maintain the outdoor airflow at the minimum level by means of a continuous control loop. This method provides a much more stable and accurate signal than direct velocity measurement methods.
Equipment specifications:
- Differential pressure sensor (DP transmitter): Shall have an accuracy of ±0.25% over the measuring range from 0 to 0.1” w.g. (inches of water gauge).
- Fixed orifice (Fixed orifice): Shall be designed to produce a differential pressure of about 0.06” w.g at the design velocity, ensuring a sufficiently large signal for the sensor to measure accurately.
2.3 VAV Terminal Units (VAV Boxes)
Overview and Selection
Selecting the VAV box size is an important balance between first cost, operating cost (fan and reheat energy), noise level and controllability. A box that is too small will increase pressure loss, leading to higher fan energy consumption. Based on life-cycle cost (LCC) analysis, the optimal selection criterion has been determined as follows:
Selection criterion: All reheat VAV boxes (reheat) shall be selected to have a total pressure drop (ΔTP) of 0.5” w.g. at the design airflow.
Controllers and Sensors
Accurate control at low airflow is key to saving energy. Manufacturer data on the minimum controllable airflow is usually overly conservative and does not reflect actual capability. Therefore, the following requirements shall be met:
- The BAS VAV controller shall be capable of stable measurement and control at a minimum velocity pressure (Velocity Pressure) of ≤ 0.004” w.g.
- The VAV box airflow probe (e.g. cross type) shall have an amplification factor (amplification factor F) of ≥ 2.
Reheat Coils
All reheat VAV boxes shall be provided with the oversized reheat coil option. Specifically, the box casing and coil shall be the standard size of the next larger VAV box size, while the damper and velocity pressure sensor shall correspond to the inlet size indicated on the drawings.
Technical rationale:
- Reduced air pressure drop: The air velocity through the larger coil is lower, significantly reducing pressure drop and thereby reducing fan energy consumption.
- Improved hot water system performance: The hot water temperature difference (ΔT) across the larger coil is higher, reducing the required hot water flow and allowing smaller pumps and piping to be used.
- Allows less expensive discharge plenums: Supports more cost-effective discharge ductwork installation configurations.
VAV Box Schedule
The VAV box schedule shall include the following parameters. For the minimum airflow columns, the value shall be set to "AUTO" to allow the G36-compliant control system to automatically calculate and adjust the setpoint dynamically.
| TAG | INLET SIZE (in) | DESIGN CFM | COOL MIN CFM | HEAT MIN CFM |
| VR-101 | 8 | 540 | AUTO | AUTO |
| VR-102 | 10 | 700 | AUTO | AUTO |
| VR-103 | 12 | 1200 | AUTO | AUTO |
Guidance: Setting the value to "AUTO" allows the G36-compliant controller to automatically calculate and adjust the minimum airflow setpoint dynamically based on actual ventilation requirements according to ASHRAE 62.1, instead of using a fixed percentage value. If the required minimum airflow is below the controllable range of the box, the G36 sequence will automatically use the Time-Averaged Ventilation (TAV) method to ensure compliance.
Careful selection of products and equipment is only half of the story. To fully realise their performance potential, correct installation and construction are the decisive factor, which will be presented in Part 3.
PART 3 - EXECUTION
3.1 Overview
Correct construction and installation are an extremely important stage in ensuring that the VAV system operates as designed. Even the best equipment will perform poorly if the installation causes unnecessary pressure loss or affects sensor accuracy. The requirements in this part shall be strictly followed to minimise pressure loss, prevent noise problems and ensure accurate control.
3.2 Supply Ductwork Installation
Main Duct Design
The use of the traditional equal friction (equal friction) design method is strictly prohibited. VAV main duct design shall use the Friction Rate Reduction Method.
This method requires starting with a higher friction rate (e.g. 0.3”/100’) in the duct sections near the fan, where velocities are higher, and gradually reducing to a lower friction rate (e.g. 0.10” to 0.15”/100’) in the most remote duct sections. This "start fast, end slow" philosophy results in a system with a total pressure loss equivalent to that of the traditional method, but with lower material and installation costs, while also saving space.
Connections to VAV Boxes
The use of flexible duct for VAV box inlet connections is strictly prohibited. Flexible duct has high pressure loss, and its rough surface can cause errors in the airflow sensor.
The following connection types are required:
- Standard connection: Use rigid duct with a conical connector (Conical, Hard) for most VAV boxes.
- Connection for critical zones: For zones identified as "clearly critical" (likely to determine the static pressure setpoint of the whole system), rigid duct with an oversized conical connector (Oversized Conical, Hard) shall be used.
Applying oversized connections shall be a deliberate strategy, not a general standard. Life-cycle cost analysis shows that applying this option to a single critical zone has a very good payback period. Conversely, applying it to all zones in the system is not cost-effective, with a payback period that can be as long as 75 years. This strategy aims to minimise pressure loss at the most critical points, thereby reducing the static pressure required for the whole system and saving fan energy in the most economical way.
Ductwork Downstream of VAV Boxes
The discharge ductwork from the VAV box shall consist of an unlined rectangular plenum, followed by round ducts leading to the air outlets. This configuration, especially when combined with the oversized reheat coil option (section 2.3.3), delivers the best performance by reducing pressure loss at the VAV box discharge.
3.3 Return Air System Installation
Recommendation to Use a Ceiling Plenum
To the extent permitted by building and fire protection codes, the return air system shall be designed to use the ceiling space as a plenum and return air shafts that do not require ductwork.
This method brings many significant benefits:
- Reduced HVAC cost: Eliminates a large part of the return ductwork.
- Reduced cost for other systems: Reduces congestion in the ceiling space, making installation of other systems easier.
- Reduced fan energy: The ceiling plenum has very low pressure loss.
- Self-balancing: The system tends to self-balance the return airflow from the rooms.
- Reduced noise transmission between rooms: The large plenum acts as a natural silencer.
Critical Design Requirements for Plenums
To prevent condensation and indoor air quality (IAQ) problems, the design shall ensure that all return air plenums with enclosing walls exposed to the outdoor environment maintain continuous positive pressure relative to the outside in all operating modes. The contractor shall demonstrate this through design calculations and field verification. This prevents humid outdoor air from infiltrating the ceiling space and condensing on cold surfaces.
3.4 Conclusion
This specification provides a comprehensive framework for the design, equipment selection and construction of high-performance VAV systems. The objective is not merely to meet minimum requirements, but to create a system optimised from start to finish. Strict adherence to the guidance on sequences of operation according to ASHRAE G36, equipment selection based on life-cycle cost, and the application of best construction practices will ensure a system that operates energy-efficiently, delivers superior comfort and has low operating costs throughout the life of the building.
