HVAC controls explained: sensors, controllers, sequences and how a zone is kept comfortable

A heating, ventilation and air conditioning system moves air and water around a building and heats or cools them. The controls are what tell each piece of that equipment what to do. They are built from three kinds of device. Sensors measure something, such as room temperature, duct air temperature, duct static pressure or carbon dioxide. Controllers compare the measurement with a target and decide what to do. Actuators carry out the decision by moving a damper, opening a valve or changing a fan speed.

Controls exist at several levels. At the bottom, a thermostat or a zone controller looks after one room or one air terminal. In the middle, a controller on an air handling unit coordinates its fans, coils and dampers. At the top, a building automation system front end lets people see every point, change setpoints, view alarms and trends, and set schedules. These levels together are what people mean when they say HVAC controls, building automation or a building management system.

Most HVAC control is the same feedback loop repeated many times. A sensor measures a variable, a controller compares it with the setpoint, and an actuator moves to reduce the difference. A supply air temperature sensor, a controller and a chilled water valve make one loop. A duct static pressure sensor, a controller and a supply fan drive make another. A room temperature sensor, a controller and a VAV damper make a third.

The controller usually runs a PID algorithm, the same family of control covered on the PID tuning guide. Too aggressive a tuning makes a valve or damper hunt, opening and closing in a cycle that wastes energy and wears the actuator. Too gentle a tuning leaves a room slow to respond. Many comfort complaints that look like faulty equipment are really loops that were never tuned for the system they ended up in.

A sequence of operations is the written description of how a system is meant to behave in each mode. For an air handling unit it states when the fan starts, how the outdoor air damper behaves, how heating and cooling are staged or modulated, what the setpoints are, which alarms exist and what happens on a fault such as a freeze alarm. It is the specification the control programmer writes to and the technician troubleshoots against.

When a system is misbehaving, the sequence of operations is the first thing to read. It tells you whether what you see is a fault or the system doing exactly what it was told. A technician who skips it can spend a day fixing something that was never broken.

Every sensor and actuator connects to a controller as a point. Knowing the point types lets you read a point list, which is the table of everything a controller reads and commands, and which you will use constantly.

The scaling from a signal to a real unit is the same skill used in process instrumentation. A 4-20 mA pressure sensor on a duct is scaled exactly the way a transmitter on a pipe is, and the 4-20 mA scaling calculator on this site works for either.

Older buildings used pneumatic controls, which move air-pressure signals through tubing to actuate dampers and valves. Later came analog electronic controls, and then direct digital control, or DDC, in which a programmable digital controller reads sensors, runs the control logic and drives the outputs. Almost every new installation is DDC, and many buildings have a mix of generations.

Controllers share data with each other and with the front end over a network. BACnet is the most common open protocol in buildings, and the BACnet basics page on this site explains it. Modbus is also widely used, particularly for plant such as boilers, meters and drives. Some older systems use LonWorks or a manufacturer protocol. A gateway can translate between protocols, at the cost of an extra thing that can fail.

Fans and centrifugal pumps follow the affinity laws. Flow changes in proportion to speed, pressure changes with the square of speed, and power changes with the cube of speed. These are ideal relationships, and real systems with static pressure and efficiency losses fall a little short of them, but the principle is why variable frequency drives are used on HVAC fans and pumps.

The cube law is dramatic. Running a fan at 80 percent speed cuts the flow to about 80 percent of full, and the power to about 0.8 x 0.8 x 0.8 = 0.512, so roughly 51 percent of full-speed power in the ideal case. A modest speed reduction gives a large energy saving. The VFD simulator on this site shows how a drive is commanded and what its parameters do.

• A sensor that has drifted or is badly placed, so the controller is working accurately on a wrong number. Compare it with a trusted reference reading.
• A stuck or disconnected actuator, where the controller commands a position and the damper or valve does not follow. Compare the commanded position with the feedback or with a visual check.
• Points left overridden in manual. A hand-off-auto switch or a software override left in hand quietly defeats the sequence for months.
• Simultaneous heating and cooling, which wastes energy and usually means a setpoint overlap, a stuck valve or a poorly tuned loop.
• A hunting loop, where an output cycles up and down, usually from over-aggressive tuning or a sensor that responds too slowly.
• A network problem, where a controller is working but the front end shows stale or missing values.

The troubleshooting method is the same as for any controls problem. Read the sequence, check what each device is commanded to do, check what it is actually doing, and find the first place the two disagree.

The building automation training page on this site offers PLC exercises that mirror building sequences: a VAV damper modulation scenario, a boiler startup sequence with purge and flame proving, and chiller lead-lag sequencing. They teach transferable control-sequence skills, including analog values, deadband, interlocks and fault behaviour.

They do not teach vendor front ends, commission a BACnet network or certify anyone. Real building automation work uses the manufacturer tools for the system in front of you, and supervised work on live equipment.