English version machine-translated from the Vietnamese original — under review.
Sizing ducts and VAV boxes aims to balance First Cost and Annual Costs, especially energy costs, in order to minimise Life-Cycle Costs (LCCs).
Below are the factors and specific design recommendations affecting the sizing and configuration of ducts/VAV boxes to optimise LCCs, based on the reference sources:
I. VAV Duct Main Sizing
Although using LCC analysis for duct design is impractical because of the difficulty of calculating first cost (materials, labour) and energy cost (pressure losses due to fittings), the sources still offer an approach that helps minimise LCC:
- Optimal Friction Loss for VAV: If the LCC-minimum friction loss for a CAV (Constant Air Volume) system is about 0.1 in.w.c./100 ft, then the LCC-minimum friction loss for a VAV system will be higher, about 0.25” to 0.3”/100’. This is because the first cost of a VAV system is the same, but its annual energy cost is much lower than that of CAV.
- Friction Rate Reduction Method: This method is recommended for main ducts upstream of the VAV boxes:
- "Start fast; end slow": Start at the fan outlet with the maximum friction rate (e.g. 0.3”/100’).
- Gradually reduce the friction rate at transitions along the longest hydraulic path (index run) until the minimum friction rate (usually 0.10” to 0.15”/100’) is reached at the end of the duct system.
- This method achieves an equivalent pressure loss but at lower cost and with less space required.
II. VAV Box Sizing
VAV box sizing is a trade-off between first cost (larger boxes are more expensive) and operating cost (pressure loss and reheat energy):
- Oversized Box: Has lower pressure loss and less noise, but costs more and can lead to a higher box minimum airflow, wasting reheat energy.
- Undersized Box: Has higher pressure loss and noise, but a lower first cost and may have a lower minimum airflow, reducing reheat energy.
- LCC recommendation: Based on LCC analysis, the recommendation is to select standard VAV reheat boxes with a total pressure loss (ΔTP) of 0.5 inch w.g..
- Use an Oversized Reheat Coil: This is a standard factory option and brings large life-cycle cost benefits:
- Reduces air pressure loss.
- Allows a larger hot water temperature difference (HW ΔT) (~10°F), reducing pump and piping sizes.
- Improves the efficiency of condensing boilers.
III. Duct Configuration at the VAV Box
Factors at the inlet and outlet of the VAV box significantly affect pressure loss and operating cost:
- Flexible Duct at the Inlet: Avoid using flexible duct at the VAV box inlet, because it causes high pressure loss (especially if kinked).
- Inlet Duct Sizing: Oversized inlet ducts should be designed for obviously critical zones to reduce pressure loss. However, applying oversized ducts to every zone can lead to very long payback periods (e.g. 75 years), so this should be limited to no more than 20% of candidate zones.
- Discharge Plenum: Use a rectangular discharge plenum followed by all-round ducts. Using an oversized reheat coil significantly reduces the total discharge pressure loss of the VAV box.
IV. Other Factors That Reduce Life-Cycle Cost
The following design and control strategies also directly reduce energy and maintenance costs, thereby reducing LCC:
- Use ASHRAE Guideline 36 Sequences of Operation (G36 Sequences): This is considered the most important tip. G36 delivers lower first cost, lower maintenance cost and lower energy cost thanks to advanced sequences that have been pre-programmed and pre-tested.
- Set the VAV Box Minimum Airflow Equal to the Minimum Ventilation Rate: This is recommended to avoid the energy waste that occurs when the minimum airflow is high. ASHRAE Standard 90.1-2019 requires the minimum primary airflow to equal the minimum ventilation airflow per the Simplified Procedure.
- Designers should ignore VAV box manufacturers' data on the minimum controllable airflow, because it is usually way too conservative.
- The control system should be specified with a minimum velocity pressure (VP) ≤ 0.004” and a VAV airflow probe amplification factor (F) ≥ 2.
- Use Fan Arrays: This has no downside and allows the use of very large AHUs (Mega AHUs).
- Benefits include lower first cost (for large AHUs), lower fan energy (if silencers are eliminated), lower sound power and increased redundancy.
- Control Minimum Outdoor Airflow by Differential Pressure (DP) across a Fixed Orifice: This method uses a differential pressure transmitter across a fixed orifice (e.g. a grille or mesh screen) that produces ~0.06” DP at design velocity.
- This is a reliable outdoor airflow measurement method (AFMS) with low maintenance cost, and it is little affected by low velocity, dirt, rain or wind.
