This simulator models a pressure-independent variable-air-volume (VAV) terminal — airflow pickup, modulating damper, electric actuator, reheat coil, diffuser and a controlled zone with its own thermostat — as a PI-controlled single-zone system. Set the zone's sensible load and setpoint, the primary supply-air temperature and available duct pressure, and watch the damper, delivered airflow, reheat power and zone temperature respond.
• Equipment laboratory tab: a real-time 3D cutaway of the terminal with Home view, Focus selected part, Show full enclosure / cutaway toggle, Exploded view, Auto rotate, Expand and Hide/show labels camera controls, a clickable, numbered component list (primary-air inlet collar, differential-pressure airflow pickup, modulating damper blade, electric damper actuator, zone controller, reheat coil, supply diffuser, controlled room, room temperature sensor) with a callout describing each part, live stat readouts, a running commentary of what is happening, and an experiment-controls panel to pause/step the simulation (0.1 s or 1 s advances, four playback speeds from 10× slow motion to 1 minute per second), enable/stop the terminal, jam or clear the actuator fault, and adjust zone sensible heat input (−2 to 12 kW), zone temperature setpoint (20–26°C), primary supply-air temperature (10–18°C), available inlet static pressure (60–450 Pa), minimum ventilation flow (0.05–0.2 m³/s), toggle automatic thermostat control on/off with a manual damper-opening slider (0–1 fraction) when automatic control is off, force a stuck damper actuator, and enable/disable reheat availability. • Curves & measurements tab: a zone-and-supply-temperatures chart, a requested-and-delivered-airflow chart, the model's governing equations, a model-boundaries note, and snapshot measurements for zone temperature, actual and requested airflow, actual damper position, reheat power and terminal leaving air temperature. • Experiments tab: guided experiments (room-load response, stuck damper, low duct pressure, heating at minimum airflow) that apply a preset and predict what you should observe, a Run model checks button that runs the built-in verification suite against independent fresh models without disturbing your current trial, and a timestamped event log with a Prepare trial report button that assembles a copyable text report of settings, measurements and event times. • Learn & assess tab: lesson content on reading temperature error, regulating airflow within limits, allowing actuator travel, and reheating at minimum flow, a knowledge-check quiz with reset, and a scope/references note linking to Trane's VAV glossary entry.
A VAV box modulates the volume of already-conditioned primary air delivered to a zone to match a changing sensible load, rather than varying the air's temperature. A differential-pressure airflow pickup measures actual delivered flow; a zone controller compares room temperature against setpoint using a proportional-integral (PI) control law and requests more or less cooling airflow, subject to a configured minimum ventilation floor.
In this model, actual flow follows V̇ = V̇max·damper·√(Pduct/250 Pa), so the damper position alone cannot guarantee a requested flow if available duct static pressure is too low. The PI controller output is effort = 0.25·(Tzone − Tset) + 0.02·∫error dt, bounded to prevent unbounded integral windup, and the electric actuator moves the damper at a finite rate of 0.15 opening-fraction per second — a stuck actuator freezes position regardless of controller demand. The zone itself is a lumped 600 kJ/K thermal capacitance responding to load, supply cooling and reheat: Czone·dTzone/dt = Qload − ρ·cp·V̇·(Tzone−Tsupply) + Qreheat. Below 0.03 m³/s of flow, reheat is inhibited even if enabled, since there isn't enough airflow to carry heat usefully.
Watch how the requested-vs-delivered airflow chart can diverge: a fully open damper can still under-deliver if available duct pressure is too low, since opening the blade cannot manufacture fan pressure that isn't there — that's the point of the low-duct-pressure guided experiment. In the stuck-damper experiment, actual damper position stays fixed at its jammed value even as the requested value (and zone temperature) drifts away, since the actuator cannot execute new commands. In the heating experiment, negative zone load (a cooling load turns off) eventually drives the controller to hold minimum ventilation flow and add reheat at the diffuser, and terminal leaving-air temperature rises above supply-air temperature by an amount set by the heat balance Tleaving = Tsupply + Qreheat/(ρ·cp·V̇).
The Run model checks button exercises the eight built-in checks from the shared verification suite for this simulator: increasing zone load opens the damper, manual damper commands are obeyed when automatic control is off, a stuck actuator holds its initial position, less duct pressure reduces flow at a fixed damper opening, negative zone load eventually requests reheat, disabling reheat produces zero heater power, heating energy matches the modeled air-temperature rise, and stopping the system zeroes both airflow and reheat. These confirm the implemented control logic and airflow/thermal relationships are self-consistent rather than certifying a specific manufacturer's terminal sizing.
It models a pressure-independent single-zone VAV terminal with a differential-pressure airflow pickup, a bounded PI zone-temperature controller, a finite-rate electric damper actuator, a reheat coil inhibited below minimum flow, and a lumped 600 kJ/K zone thermal capacitance with a 0.6 m³/s design flow and 6 kW maximum reheat. It excludes moisture, building envelope, acoustics, sensor delay and certified ventilation sizing.
Actual flow depends on both damper position and available duct static pressure through V̇ = V̇max·damper·√(Pduct/250 Pa). Opening the damper fully cannot create additional fan pressure, so if the available inlet static pressure is too low — as in the low-duct-pressure guided experiment — delivered airflow stays below what the zone requests.
Selecting the stuck-damper fault (or the corresponding guided experiment) freezes the actual damper position at whatever it was when the fault occurred. The zone controller keeps computing a new requested position based on temperature error, but the actuator cannot execute it, so zone temperature drifts away from setpoint while the built-in check confirms damper position stays fixed.
The Run model checks button verifies independent invariants in the implemented terminal model — such as the damper opening with load, the stuck actuator holding position, reheat only firing above the minimum-flow inhibit threshold, and the heating-energy balance closing — using fresh, independent model instances that do not disturb your current trial.