English version machine-translated from the Vietnamese original — under review.
The VAV system is presented through design methods and tips aimed at significantly optimising the first cost, the operating (energy) cost and the operating performance of the system.
The main optimisation methods covered include four Design Tips and guidance on VAV duct design.
1. Use ASHRAE Guideline 36 Sequences of Operation (Tip #1)
Using ASHRAE Guideline 36 Sequences (G36) (High-Performance Sequences of Operation for HVAC Systems) is considered the most important tip.
Cost Optimisation:
- Lower first cost: G36 combines the best of the configured and programmed worlds ("Plug and play"). This results in lower first cost for the entire design/construction team.
- Saves design and programming time: Engineers only need to state: "The control sequences shall be fully implemented and comply with ASHRAE Guideline 36". These sequences have already been pre-programmed, tested and debugged by building automation system (BAS) manufacturers.
- Lower maintenance cost: Helps reduce maintenance costs, and the system is easier to maintain.
- Energy savings: Advanced sequences such as "Dual Max" VAV Control reduce fan energy, heating energy and reheat energy. This sequence avoids high minimum airflow pushing the zone into heating mode.
- Detection and handling of faulty zones (Rogue Zones): G36 includes logic to identify "Rogue Zones" (zones that always demand more resources, such as higher static pressure or lower/higher temperature). Detecting and repairing or locking out these zones (by setting the Importance Multiplier to 0) prevents the control loop reset from being driven to extremes, thereby preventing energy waste.
Performance Optimisation:
- Advanced sequences: Provides the best sequences of operation, already tested and debugged.
- Improved comfort: Dual Max VAV logic helps improve comfort.
- Better control: Using Trim & Respond logic to reset the static pressure setpoint (Static Pressure Setpoint Reset) based on zone demand (e.g. valve or damper position) reduces fan energy and provides better control.
2. Set VAV Box Minimum Airflow Setpoints Equal to the Minimum Ventilation Rate (Tip #2)
This method focuses on setting the minimum airflow (Vmin) of the VAV box equal to the minimum ventilation rate required by the standard.
Cost and Performance Optimisation:
- Maximum energy savings: According to research (RP-1515), setting minimum airflow higher than necessary wastes energy. ASHRAE 90.1-2019 requires the minimum airflow to equal the minimum ventilation airflow according to the Simplified Procedure.
- Ignore manufacturer data: Designers do not need to consider the minimum controllable airflow recommended by VAV box manufacturers, because this data is often wrong and overly conservative (e.g. equivalent to ~30% of design CFM).
- Accurate control at low airflow: Modern VAV controllers are highly accurate down to about 0.003 inch of velocity pressure (VP). It is advisable to specify a minimum VP of the BAS VAV controller ≤ 0.004 inch.
- Time-averaged ventilation (TAV): If the minimum ventilation airflow is below the controllable range of the box, G36 automatically uses TAV, pulsing the minimum airflow from 0 up to the minimum controllable level over a 15-minute period, ensuring the required average ventilation level is maintained while still saving energy.
- Dynamic adjustment: Guideline 36-2021 can automatically calculate Vmin dynamically, allowing the minimum to be set to "AUTO".
3. Use Fan Arrays (Tip #3)
Fan Arrays are arrays of small, direct-drive, single-width, single-inlet fans placed in a plenum (plenum fans).
Cost Optimisation:
- Lower first cost: Fan Arrays usually have the same or lower total net installed cost compared with conventional fan systems.
- Enables large AHUs (Mega AHUs): Fan Arrays make it possible to build very large air handling units (AHUs) (up to 200,000 cfm or more). For large buildings, Mega AHUs have a much lower first cost than using many small AHUs.
- Lower energy cost: Helps reduce fan energy if silencers or system effects can be eliminated.
- Reduced space: Helps reduce AHU length and requires less space when Mega AHUs are used.
- Easy installation and maintenance: Motors and fans are easy to replace, and installation is easier in retrofit applications (retrofit).
Performance Optimisation:
- Higher reliability (Redundancy): Improves redundancy, especially if multiple Variable Frequency Drives (VFDs) or Electronically Commutated Motors (ECMs) are provided.
- Reduced noise: Reduces sound power, especially on the discharge side.
- Higher energy efficiency: Mega AHUs are more energy efficient.
4. Control Minimum Outdoor Airflow Using DP (Differential Pressure) Across a Fixed Orifice (Tip #4)
This method addresses the problem of outdoor air (OA) airflow measurement, which is often inaccurate.
Cost and Performance Optimisation:
- Higher accuracy and reliability: Most Airflow Measurement Systems (AFMS) perform poorly because of low air velocity, turbulence and sensors that foul easily. Using DP across a fixed orifice (e.g. a louver or screen) as the signal minimises the effects of non-uniform velocity and wind.
- Stronger signal: This method provides a signal 2 to 3 times higher than conventional velocity sensors, allowing accurate measurement down to about 150 fpm. This ensures accurate control of the minimum outdoor airflow.
- Low maintenance cost: Less prone to fouling or damage from dust, rain or snow.
- No additional AHU length: Does not require additional AHU length.
- Avoids field calibration: Avoids field calibration if the DP of the fixed orifice is accurately known.
5. Optimising Duct and VAV Box Design (Life-Cycle Cost - LCC)
VAV duct design methods are presented to balance First Cost and Annual Costs (mainly energy) in order to achieve the optimal Life-Cycle Cost (LCC).
Cost and Performance Optimisation of Main Duct Design:
- Friction Rate Reduction Method: To optimise LCC, this method should be used instead of the traditional Equal Friction method.
- "Start fast; end slow": Start at the beginning of the system (near the fan) with a higher friction rate (~0.3 inch w.c./100 ft) and gradually reduce the friction rate at each transition, ending at a lower level (e.g. 0.10” to 0.15”/100’).
- Benefits: This method delivers lower cost and less space required compared with the Equal Friction method, at an equivalent pressure drop.
VAV Box Size Optimisation:
- Standard size selection: Select standard VAV reheat boxes based on a Total pressure drop (ΔTP) of 0.5 inch w.g..
- Use an oversized reheat coil (Oversized Reheat Coil): This is an optimisation option available from the factory.
- Lower operating cost: Results in lower air pressure drop.
- Improved water system performance: Allows a larger hot water temperature difference (HW ΔT) (~10°F), reducing pump and pipe sizes, and improving condensing boiler efficiency.
Return Air System Optimisation:
- Use Return Air Plenums: Use architectural plenums and unducted return air shafts (unducted) where permitted.
- Significant cost savings: This method reduces HVAC system cost (estimated at $3 to $5/ft²), reduces fan energy cost by about 20%, and requires little or no balancing cost (balancing costs).
- Maintains balance: For VAV systems, balance is better maintained as the supply airflow rate varies with load.
