9.1 HVAC Control Fundamentals, Signals, and Test States
Key Takeaways
- A control loop consists of a sensor, controller logic, controlled device, process, and feedback; each element can create a misleading symptom.
- Normal position describes the defined unpowered or control-reference state and must be read from the approved schedule or submittal.
- TAB overrides establish documented test conditions temporarily and must be authorized, time-limited, recorded, and released.
- A BAS display is a control-system value, not independent proof of physical flow, pressure, temperature, or position.
- Technicians verify measurements and sequences within scope; design-logic changes and final acceptance decisions belong to authorized parties.
HVAC Control Fundamentals, Signals, and Test States
The control loop
The current NEBB technician seminar includes a dedicated HVAC controls chapter. TAB results depend on control state: a stable fan speed, a fully commanded coil valve, a VAV maximum or minimum mode, an economizer position, or a pressure setpoint. Without understanding the loop, a technician may balance a moving target.
A feedback loop has five parts:
- Sensor measures the process variable.
- Controller compares the value with a setpoint and applies logic.
- Output signal commands an actuator or drive.
- Controlled device changes the process.
- Process feedback returns to the sensor.
For duct static pressure, the transmitter senses pressure, the controller compares it with setpoint, the VFD changes fan speed, the duct pressure responds, and the transmitter reports the new value. Hunting can arise from a poor sensor location, tubing leak, unstable system, aggressive tuning, actuator problem, or changing terminal demand.
Signals and scaling
Common analog signals include 0–10 VDC and 4–20 mA. Pneumatic systems may use a pressure signal such as 3–15 psi. Digital networks communicate values and commands over protocols, but the displayed engineering unit depends on correct sensor range and controller scaling.
Example: a 0–10 VDC pressure transmitter scaled for 0–5 in. w.g. should produce 4 VDC at 2 in. w.g. If the BAS interprets the same signal as a 0–10 in. w.g. range, it will display 4 in. w.g. even though the transmitter and wiring are functioning. Compare the physical independent measurement, raw signal, configured range, and displayed value.
| Observation | Possible layer to check |
|---|---|
| Command changes but actuator does not move | Power, linkage, actuator, safety interlock |
| Physical value is correct but BAS value is wrong | Sensor, tubing, scaling, address, calibration |
| BAS value changes but physical process does not | Wrong point, simulated value, disconnected device |
| Loop hunts | Sensor location, tuning, mechanical instability, rapid load change |
Direct and reverse action
In a direct-acting relation, controller output rises as the input rises; in a reverse-acting relation, output falls. The correct action depends on the process and device. A cooling valve may open as space temperature rises. A heating valve may also open as temperature falls, which requires the opposite relationship between measured error and output. Rather than memorize one universal action, trace what a rising sensor value should make the physical device do.
Normal, fail, and commanded positions
Normally open and normally closed must be interpreted using the approved control schedule and device submittal. They usually describe the device's defined state with control power or signal removed, but terminology can vary with valve body, actuator, pneumatic pressure, and sequence convention. Verify the actual fail action—spring return, fail in place, capacitor return, or another strategy—rather than infer it from heating or cooling service.
Limit switches or position feedback report actuator travel, not necessarily airflow or water flow. A damper can indicate 100% open while a disconnected linkage leaves blades shut. A valve stem can travel while the wrong port arrangement or closed isolation valve prevents coil flow.
Overrides for TAB
An override creates a repeatable test state. It should identify the point, commanded value, reason, person authorizing it, start time, and release. Examples include commanding a VAV box to cooling maximum, opening the coil flow path, fixing an economizer position, or holding fan speed for a traverse.
Before measuring, allow the physical system to stabilize and verify the command at the device. After testing, remove overrides and confirm normal control. An abandoned override can create comfort, freeze, pressure, or energy problems.
TAB boundaries
The technician may compare a calibrated independent measurement with the BAS value and provide a correction factor or field result under the approved procedure. The controls contractor or authorized party usually enters programming changes. Rewriting reset logic, changing life-safety interlocks, or altering design sequences is not an incidental TAB adjustment.
When a control problem blocks balancing, document the point name, command, feedback, physical position, independent measurement, time, and system mode. That evidence lets the CP and controls team isolate the fault without guesswork.
A control verification sequence
- Read the approved sequence and point list.
- Identify normal mode, test mode, interlocks, and safety limits.
- Command the authorized test state.
- Verify actuator motion and actual device position.
- Measure the physical controlled quantity independently.
- Compare command, feedback, BAS value, and field value.
- Record discrepancies and restore normal operation.
Controls do not replace measurement; they establish and report a state that independent measurement must confirm.
A BAS shows a damper at 100% open, but airflow is near zero. What should be checked before accepting the displayed position?
A 0–10 VDC transmitter represents 0–5 in. w.g. What pressure corresponds to 4 VDC under linear scaling?
What is the proper final step after an authorized BAS override used for TAB?