HVAC Automatic Controls

• Automatic controls in HVAC use control systems to regulate temperature, humidity, airflow and other environmental conditions without user interaction.
• These controls should maintain stable conditions, protect equipment and people, and self-regulate within design boundaries.

Normally Closed Controls

• Current passes through the control at rest.

Normally Open Controls

• Control is open, and current cannot pass through when unpowered.

Close on Rise

• When temperature or pressure increases, the control closes and allows current to pass through.

Open on Rise:

• The control opens and stops current flow when pressure or temperature rises

Direct Acting Control (DAC)

• A control strategy in which an increase in the measured variable (temperature, pressure, humidity) results in a direct increase in controlled output.
• The controlled device responds in the same direction as the change in the sensed condition.
• An increase in output equals an increase in output from the valve/actuator being controlled.
• With 0 VDC output to the actuator, the valve is closed; then the signal changes to 5 VDC and the actuator opens.
• When Inverter output increases to the compressor, cooling also increases.

Reverse Acting Control (RAC)

• A control strategy where an increase in the measured variable (temperature, pressure, humidity) results in a decrease in the controlled output.
• In other words, the controlled device responds in the opposite direction to the change in the sensed condition.
• The output decreases when the input signal increases.
• When duct pressure increases, the fan speed decreases.

Analog Signals in HVAC

• Continuous signals vary over a range of values (e.g., 0-10V or 4-20mA).
• Offer smooth and proportional control.
• Temperature sensors send real-time temperature variations.
• Variable Frequency Drives (VFDs) adjust motor speed for fans and pumps.
• Pressure sensors monitor air or fluid pressure with continuous feedback.
• Humidity sensors provide varying signal based on moisture level.
• Smooth control adjusts equipment gradually, reducing wear and energy spikes and is ideal for variable-speed fans, dampers, and modulating valves.
• Susceptible to noise, as the signal can degrade over long distances.
• Harder to interface with digital systems, requiring Analog-to-Digital Conversion (ADC).

Digital Signals in HVAC

• Binary signals have only two states: ON (1) or OFF (0).
• Used for simple control commands.
• Thermostats turn heating/cooling ON or OFF.
• Fans start or stop airflow.
• Relays and contactors control power to compressors and motors.
• Limit switches detect open/closed positions of dampers and valves.
• Offer reliable and precise operation, with less signal degradation over long distances.
• Easier integration, working seamlessly with microcontrollers and smart HVAC systems.
• Limited control, and provide only gradual adjustments (only ON/OFF).
• Less energy efficient for some applications, since cycling between ON/OFF can cause wear.

Analog vs. Digital Signals in HVAC Comparison

• Analog signals are continuous, while digital signals are binary (ON/OFF).
• Analog signals offer high precision, while digital signals offer lower precision (only two states).
• Analog: modulating controls (variable speed fans, dampers).
• Digital: Simple ON/OFF controls (thermostats, relays).
• Analog: More efficient for gradual changes.
• Digital: Can cause energy spikes when switching.
• Analog signals are more susceptible to interference.
• Digital signals are low (resistant to interference).
• Analog systems require ADC for digital systems.
• Digital systems work directly with smart systems.

Automatic Electrical Controls

• Electrically operated.
• Normally control electrical devices.
• Relays, contactors, motor starters and thermostats.

Automatic Mechanical Controls

• Pressure and Temperature operated
• Control fluid flow.
• Mechanical defrost clocks & mechanical timers.
• Water regulating valve.

Automatic Electromechanical Controls

• Driven by pressure/temperature to provide electrical functions.
• Driven by electricity to control fluid flow.
• Pressure switches.
• Fluid filled temperature bulbs.
• Mechanical thermostat (mercury bulb type).

Automatic Electronic Controls

• It uses electronic circuits to perform the same functions that electrical and electromechanical controls perform such as Controllers, Temperature sensors, and Programmable communicating thermostats

Open Loop Control Systems

• No feedback to adjust systems behavior.
• Control action is independent of the actual output of the system.
• Used when precision is not critical.

Closed Loop Control Systems

• Output is constantly monitored and feedback to the input is provided.
• Allows operation to be adjusted in real time.
• Adjusts for environmental factors, ex. Outdoor air temperature

Direct Digital Control (DDC)

• When digital electronic controls are networked together into a central computer system
• Most popular is BACnet

Pneumatic Controls

• Uses compressed air as the signal.
• Example is 3-15 PSI versus 0-10VDC in an electrical control.
• Is a type of analog control that can be direct or reverse acting.

Temperature Sensors

• Used to sense temperatures.
• Most common type is a thermistor.
• Thermocouples – voltage is created when heated.
• Overloads, Hi-Limits and Thermostats.

NTC thermistor (Negative Temperature Coefficient thermistor)

• A type of temperature sensor whose resistance decreases as temperature increases
• Commonly used in HVAC systems for temperature measurement and control due to its high sensitivity and fast response time.

PTC thermistor (Positive Temperature Coefficient thermistor)

• Is a temperature-sensitive resistor whose resistance increases as temperature increases.
• Commonly used in HVAC systems for overcurrent protection, heating elements, and temperature sensing.

Humidity Sensors

• Measure relative humidity and dew point temperature.
• HRVs use relative humidity sensors and economizers use humidity sensors for dew point.

Sensor/Switch

• Transducer type converts pressure to electrical signal, allowing controller to give a pressure reading.
• High pressure switch opens for high pressure protection.
• Low pressure switch opens for fall of pressure for low pressure protection.
• Air pressure switch can open or close depending on air pressure.

Flow Switch

• Paddle type measures water flow
• Differential flow switch senses differential in air flow
• Piston type airflow moves a piston, which makes or breaks the switch.

Liquid Level Switch

• Float switch, used for water levels in cooling towers and reservoirs.
• Used to determine liquid level in some larger receivers.
• Prevents condensate from breaking cooling if water is not draining.
• Used to determine mix level in food equipment.

Gas Detection Sensor

• Combustible gas detectors for boiler rooms and commercial applications.
• CO detectors for homes and parking garages. They either alert or start ventilation.
• Refrigerant detectors for chiller/mechanical rooms. They alert occupants or shut down equipment.