VAV System Principles and Guest Room Applicability Analysis
The variable air volume (VAV) system adapts to indoor load variations by changing the supply air volume rather than the supply air temperature, offering significant advantages in energy savings and comfort compared with the constant air volume (CAV) system. According to Article 7.5.5 of GB 50736 "Code for Design of Heating Ventilation and Air Conditioning of Civil Buildings," air conditioning systems should adopt reasonable regulation methods based on load variations; when the load of each room varies considerably and the system scale is large, a VAV system should be adopted. Working principle of the VAV system: when the room temperature sensor detects that the indoor temperature deviates from the setpoint, the opening of the air volume control damper in the VAV terminal box changes, thereby changing the air volume supplied to the room. When the indoor cooling load increases (e.g., enhanced solar radiation), the VAV box increases its opening to increase the supply air volume; when the cooling load decreases, the VAV box reduces its opening. Meanwhile, the AHU variable-frequency fan automatically adjusts its speed based on the static pressure variation in the supply air duct, matching the supply air volume to the total demand of all terminals and achieving supply-demand balance. Applicability analysis of VAV systems in guest rooms: (1) Guest room load characteristics: guest room loads vary greatly throughout the day, with solar radiation, occupancy, and occupant activity all affecting room loads. When unoccupied, the load is only the base load (approximately 30%-40% of design load); when occupied, it may reach 70%-100% of design load. The VAV system can precisely regulate supply air volume according to actual load, making it highly suitable for scenarios with large load fluctuations. (2) Comfort advantages: the VAV system can achieve temperature control accuracy of ±0.5°C, superior to the ±1.0°C of fan coil systems. Additionally, the VAV system maintains a constant supply air temperature (typically 12-14°C) and regulates room temperature through air volume variation, avoiding the problem of excessive supply air temperature differential when fan coils operate at low speed. (3) Energy-saving advantages: since the AHU fan uses variable-frequency control, when the total air volume of all guest rooms is less than the design total air volume, the fan automatically reduces speed, achieving significant energy savings. The annual fan energy consumption of a VAV system is only 40%-60% of that of a CAV system. (4) Noise control: the VAV box has a small opening under low load, reducing air velocity in the duct and consequently reducing noise, which is particularly suitable for the nighttime quiet requirements of guest rooms. However, under small opening conditions of the VAV box, attention must be paid to avoiding airflow noise generated by the damper (air velocity should not exceed 5 m/s). Combined scheme of VAV system with dedicated outdoor air system (DOAS): outdoor air is separately processed by the PAU to the supply air state (14-16°C) and then directly supplied to each room, handling the ventilation and outdoor air load required by occupants. The recirculated air handled by the VAV box bears the remaining sensible heat load of the room. This scheme separates latent load and sensible load treatment, improving the system's dehumidification capacity and control accuracy. Article 7.5.6 of GB 50736 recommends that VAV systems should be used in conjunction with dedicated outdoor air systems. Marriott MIDS standard recommended indicators for guest room VAV systems: room temperature control accuracy ±0.5°C, supply air temperature setpoint 12-14°C, VAV box minimum air volume set at 20%-30% of maximum air volume.
VAV Terminal Box Selection and Air Distribution Design
The VAV terminal box is the core component of the VAV system, and the rationality of its selection and the quality of air distribution design directly affect system performance, comfort, and noise level. According to Article 7.2.5 of GB 50243 "Code for Acceptance of Construction Quality of Ventilation and Air Conditioning Works" and the requirements of ASHRAE Standard 62.1, the selection of VAV terminal boxes should be comprehensively determined based on factors such as room design air volume, minimum air volume, pressure drop requirements, noise level, and installation space. Type selection of VAV terminal boxes: guest room VAV terminal boxes are mainly divided into two types: single-duct (Pressure Independent) and fan-powered. The single-duct VAV box controls supply air volume through a regulating damper, featuring simple structure, low cost, and easy maintenance, and is suitable for typical guest rooms (area 30-60 m²). The fan-powered VAV box has a built-in small fan that enables room recirculation air volume regulation, and is suitable for larger suites or rooms with special air distribution requirements. For five-star hotel guest rooms, pressure-independent single-duct VAV boxes are recommended, as their built-in airflow sensors measure air volume in real time and provide feedback to control damper opening, unaffected by upstream static pressure fluctuations. Selection technical parameters: (1) Air volume range: standard guest room air conditioning design supply air volume is 350-600 CMH (depending on room area and cooling load). The rated air volume of the VAV box should be no less than 1.2 times the design maximum air volume, and the minimum air volume should be 10%-30% of the rated air volume. (2) Noise level: the noise of guest room VAV boxes at maximum air volume should be ≤NC-25 (below 25 dB(A)), and ≤NC-20 at minimum air volume. Low-noise VAV boxes with built-in acoustic lining and outer sound insulation layer should be selected. (3) Pressure drop: the pressure drop of the VAV box at full opening should not exceed 30 Pa to reduce fan energy consumption. (4) Interface dimensions: select the VAV box interface diameter based on duct dimensions (typically Φ200-250 mm). Key points of air distribution design: (1) Supply air outlet selection: swirl diffusers or linear slot diffusers are recommended for guest rooms, offering good induction ratio and air mixing performance. The face velocity of the air outlet should be controlled at 2.0-3.0 m/s, and the supply air throw should cover the main room area but should not blow directly toward the bed. (2) Return air inlet arrangement: the return air inlet should be located in the ceiling above the entryway, with return air velocity controlled at 1.0-1.5 m/s. (3) Outdoor air inlet: the outdoor air supply outlet of the DOAS system should be installed independently and supply air directly to the interior zone of the room (rather than the corridor) to ensure effective utilization of outdoor air. The outdoor air inlet velocity should be controlled at 2.0-3.0 m/s. (4) Air distribution: the air distribution design of the guest room VAV system should avoid supply air blowing directly toward the bed and rest areas. Supply air outlets should preferably be arranged on the window side or the interior wall side of the room, so that the supply air attaches to the ceiling and descends naturally. Installation space requirements: VAV boxes are typically installed in the ceiling above the guest room entry foyer, requiring sufficient maintenance space. An access panel (not less than 450×450 mm) should be provided above the VAV box for maintenance personnel to service the actuator, controller, and airflow sensor. Article 7.2.6 of GB 50243 stipulates that VAV boxes should be installed level, with correct inlet and outlet connection orientation, and electrical wiring in compliance with code requirements.
VAV System Control Strategy and BAS Integration
The control strategy of the VAV system is key to achieving energy savings and comfort targets. It includes two levels: room-level control (VAV box level) and system-level control (AHU level), as well as the coordination between the two. According to Section 8.4 of GB 50736 "Code for Design of Heating, Ventilation and Air Conditioning of Civil Buildings," VAV system control should adopt a distributed intelligent control system to achieve unified independent control of each terminal and overall system optimization. Room-level control strategy: (1) Temperature control mode: The thermostat detects the actual room temperature and compares it with the setpoint (24°C in summer, 22°C in winter). A PID algorithm calculates the required airflow and outputs a control signal to adjust the VAV box damper opening. When the room temperature deviates from the setpoint by more than 1°C, the VAV box opens fully (100%) for rapid response; when the temperature approaches the setpoint, the opening gradually decreases to prevent overshoot. (2) CO₂ control mode: A CO₂ sensor is installed in the room (range 0–2,000 ppm, accuracy ±50 ppm). When the CO₂ concentration exceeds the setpoint (800 ppm), the VAV box adds a fresh air demand signal on top of maintaining temperature control, increasing the AHU fresh air damper opening. (3) Occupancy status control: Interlocked with the guest room RCU system, when the guest checks out and the room is unoccupied, the VAV box minimum airflow drops to the set lower limit (10%), and the temperature setpoint is adjusted to energy-saving mode (26–28°C in summer, 18–20°C in winter); when the guest checks in, it returns to comfort mode. System-level control strategy: (1) Static pressure control method: A static pressure sensor is installed at 2/3 of the distance along the main supply air duct (or at the most unfavorable terminal), with a static pressure setpoint (typically 250–400 Pa). The AHU variable-frequency fan adjusts its speed based on the deviation between the measured static pressure and the setpoint. When most VAV boxes have openings greater than 90%, the static pressure setpoint is gradually increased; when most VAV boxes have openings less than 40%, the static pressure setpoint is reduced (variable static pressure control). (2) Airflow tracking control method: An airflow measuring device is installed at the AHU fan outlet. The controller adjusts the fan speed based on the sum of the airflow demands from all VAV boxes, so that the total supply airflow matches the demanded airflow. The airflow tracking method has higher control accuracy and better energy savings than the static pressure method (additional 5%–10% energy savings), but places high demands on sensor accuracy. (3) Fresh air volume control: The AHU is equipped with CO₂ sensors and air quality sensors, and adjusts the fresh air damper opening based on the average or maximum CO₂ concentration of each room to ensure indoor air quality. BAS system integration: The VAV system should be networked with the hotel BAS system via BACnet/MS/TP or BACnet/IP protocol. The BAS system performs the following functions: monitoring the operating status of all VAV boxes (room temperature, airflow, damper position), setting operating modes (scheduled switching between comfort/energy-saving/night modes), recording energy consumption data (AHU power, VAV box operating hours), and abnormal alarms (temperature out of range, airflow deviation, equipment failure). The BAS system should also perform predictive control based on weather forecast information: when high temperatures are forecast, lower the supply air temperature in advance to improve system response speed.
VAV System Commissioning Methods and Acceptance Standards
Commissioning of the VAV system is a key step in ensuring the system achieves its design performance and energy-saving targets, and its workload and technical difficulty are far higher than those of traditional constant-air-volume systems. According to Section 11.1 and Section 11.2 of GB 50243 Code for Acceptance of Construction Quality of Ventilation and Air Conditioning Works, VAV system commissioning should include three parts: air volume balance commissioning, control function commissioning, and system integrated commissioning. After commissioning is completed, all performance indicators of the system should meet the design requirements and acceptance standards. Air volume balance commissioning: (1) VAV box air volume calibration: use an anemometer or air flow hood to calibrate the air volume of all VAV boxes, and correct the K factor of the air velocity sensor so that the measured air volume is consistent with the air volume reported by the VAV box (deviation ≤±5%). (2) Minimum/maximum air volume setting: set the minimum and maximum air volume limits of each VAV box according to the design values, and verify whether the terminal box can reach the maximum air volume under the AHU full-pressure condition and whether it can stably maintain the minimum air volume under the low-pressure condition. (3) AHU fan performance test: test the air volume-static pressure characteristic curve of the fan at different frequencies, and verify its match with the design values. Control function commissioning: (1) Temperature control response test: by changing the room setpoint temperature (±2°C), test the response time and steady-state deviation of the VAV box air damper. Damper response time ≤30 seconds (full stroke from 0% to 100%), temperature stabilization time ≤15 minutes. (2) Static pressure control test: simulate different load scenarios (such as 1/3 of rooms at high load and 2/3 of rooms at low load), and verify the static pressure control stability of the AHU variable-frequency fan, with static pressure fluctuation ≤±10Pa. (3) CO₂ control test: simulate a scenario of rising CO₂ concentration, and verify that the fresh air damper linkage operates normally. (4) Interlock function test: test that functions such as the fire linkage signal (VAV box closes and AHU shuts down upon fire alarm) and the RCU linkage signal (check-out/check-in mode switching) operate normally. System integrated commissioning: (1) Full-load condition test: operate the system for more than 4 hours under summer design conditions, test the temperature in each room and the distribution of VAV box opening, and verify the system's supply-demand balance capability. (2) Partial-load condition test: simulate low-load scenarios such as nighttime, and test the system's low-load operation stability and minimum air volume maintenance capability. (3) Transition condition test: test the response speed and temperature control stability of the VAV system under sudden load changes (such as rapid cooling caused by an afternoon shower). Acceptance standards: Clause 11.2.4 of GB 50243 specifies that the qualification standard for VAV system commissioning is: the deviation between the actual temperature and the set temperature in each room is not greater than ±1.0°C (five-star hotels require ±0.5°C); the air volume deviation of the VAV box is not greater than ±10%; the AHU variable-frequency fan operates smoothly without surge; the system noise meets the design standard. The Marriott MIDS acceptance standard is stricter: room temperature deviation ≤±0.8°C, VAV box air volume deviation ≤±5%, CO₂ concentration ≤800ppm.