Controls Tools

Controls laboratory · VAV v0.1

VAV Controls Simulator

Follow the airflow. Tune the loop. See the room respond.

Single duct · Hot-water reheat
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System overview

Ready
AHU / PRIMARY AIR—Entering VAV
VAV DAMPER—FLOW_CO —
AIRFLOW SENSOR—SP —
REHEAT COIL—Valve command
DISCHARGE AIR—Room demand control
ROOM—SP —

Enter settings, then start the simulation.

Operating conditions

Override airflow or control reheat to a SAT target independently, then Apply settings. Room Auto continues controlling the outputs you have not overridden.

Cascade uses room heating demand to reset SAT SP, then SAT feedback controls reheat.

FLOW_CO

Reverse acting · 0–100%

Actual airflow → FLOW_CO → damper command. The actuator takes time to follow.

How these CO settings work

PB and deadband together determine the proportional response. P correction is limited to ±50 percentage points and added to Bias. Inside the full-width deadband, the previous CO output holds.

Integral rate changes Bias by percentage points per simulated minute, rather than multiplying by error size. Reset band reduces this rate near setpoint; 0 disables it. Integral holds at 0% or 100%. With pure I, leaving a limit may require changing Bias or resetting.

Live tuning preserves accumulated Bias unless Initial Bias changes. No nonzero derivative action or firmware parity is claimed.

Advanced model settings

Available flow is the model's supply capacity, separate from Maximum airflow setpoint. A negative room heat load represents net heat loss.

Initial values are teaching examples.

Trend log

One timeline. Left: airflow · Right: °C · Outer right: %. Dashed lines show setpoints.

Visible tags
Current FLOW_CO diagnostics

Values from the last controller evaluation, before the physical response of that step.

Sampled airflow error (SP − actual)
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P correction
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Accumulated Bias
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Effective integral rate
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CO state
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Event log

Latest 100 events. Times refer to simulated time.

  1. No events yet.
Sequence & model assumptions

Without overrides, Room Auto uses a separate signed PI room controller. Positive demand maps Minimum to Maximum airflow with reheat off. Negative demand holds Minimum airflow and opens the reheat valve. A retained integral output can balance the room load at setpoint. Manual airflow override replaces only the airflow target; room-driven reheat remains available. Flow SP Test suspends the room controller and uses manual airflow, with reheat off unless SAT setpoint override is selected.

Reheat control offers Direct, Cascade and Manual SAT setpoint override. Cascade maps room heating demand to a SAT target between AHU supply temperature and Cascade SAT maximum, then a separate SAT PI loop controls the reheat valve. Cooling still uses airflow; this VAV has no cooling coil. Cascade operates in Room Auto, including with manual airflow, and is inactive in Flow SP Test.

Manual SAT setpoint override uses the same conventional PI loop with a user target. It overrides room heating demand and can therefore warm the room above Room SP. Select Direct or Cascade to restore room control. The coil can add heat only: targets below AHU supply temperature leave reheat off. Insufficient coil capacity leaves the SAT target unmet. In Cascade and Manual SAT control, reheat is inhibited below 10 CFM; this is an educational low-flow controller limit, not a manufacturer requirement or equipment protection specification.

FLOW_CO compares actual airflow with its setpoint. The damper is rate limited by full travel time. Flow follows available flow × (opening / 100)1.5, with a 2-second lag under fixed supply conditions. This is not a duct pressure network model.

The coil uses 55°C constant hot water and 160 W/K maximum conductance. Post-reheat SAT depends on primary temperature, actual flow and valve command. At zero flow SAT is undefined and air heat transfer is zero. Room heat capacity is 3 MJ/K and volumetric air heat capacity is 1206 J/(m³·K). Room energy includes supplied air and room heat load once, without duplicate reheat energy.

Numerical steps are one simulated second at every playback speed. Initial temperature and run duration lock after Start. Apply during a run queues settings for the next step; paused changes wait for Resume. Reset restores the run-start configuration and clears history. Returning from Flow SP Test resets the room integrator but preserves FLOW_CO Bias. Playback pauses when the simulator page becomes hidden.

All defaults and input limits are educational choices. The simplified physical model is not physically validated. It does not select products, implement equipment safeguards or reproduce manufacturer firmware. Nonzero D is unsupported.

Engineering disclaimer