HVAC controls

HVAC control systems, particularly modern Building Automation Systems (BAS), operate through a feedback loop involving three key components:

1. Sensors: These are devices placed throughout a building to collect real-time data about the environment. They measure critical parameters like:
   • Temperature and humidity.
   • Occupancy, often using motion or CO2 sensors to determine if a space is in use.
   • Air quality by measuring CO2, volatile organic compounds (VOCs), or other pollutants.
2. Controllers: This is the heart of the system, typically a computer or a specialized microcontroller. Controllers receive the data from the sensors and process it based on a pre-programmed set of instructions or "control sequences."
   • For example, a controller might be programmed to turn on the air conditioning when a sensor reports the temperature is above 75∘F and the occupancy sensor shows the room is in use.
   • In more advanced systems, controllers can implement complex strategies like demand-controlled ventilation (reducing fresh air when a room is empty) or supply air temperature reset (adjusting the temperature of the air leaving the unit based on outdoor conditions).
3. Actuators: These are the mechanical devices that physically carry out the commands from the controllers. They translate electrical signals into physical actions to control the HVAC equipment.
   • Examples include dampers that open or close to regulate airflow, and valves that control the flow of hot or chilled water to a coil.

In essence, building HVAC controls create a continuous, automated cycle: Sensors detect a change, controllers decide on a response, and actuators execute that response, ensuring the building environment remains optimal and efficient.

Control types

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  HVAC control types
  Standby refers to a system being in an idle or inactive state, where equipment is not actively running but is ready to be turned on when needed.
• Demand control means that the equipment operation is adjusted based on the demand for cooling or heating. When VSD are used on pumps, the pump speed is controlled by differential pressure. In this case, the differential pressure in the system is used as a proxy for demand: when the pressure changes, the system adjusts the pump speed to match the cooling demand of the building. As the demand for chilled water changes, the pump speed increases or decreases to maintain the required pressure difference. Example: Demand Controlled Ventilation (DCV) adjusts ventilation rates based on CO₂ levels or occupancy, reducing ventilation when spaces are underutilized and saving energy.
• Scheduled control operates based on predefined times rather than real-time system conditions.
• Temperature-enabled control would adjust the system based on temperature setpoints or conditions.

Temperature, Humidification & De-Humidification Controls

• Setpoint ranges and schedules
• Thermostatic control
• Humidistatic controls

Typical Energy Saving Strategies

• Temperature setbacks during unoccupied hours
• Optimum start controls
• Expand the zone throttling range
• Reduce setpoint in winter and increase it in the summer

Demand control ventilation

Systems that modulate ventilation based on occupancy or air quality (e.g., CO₂ levels) reduce unnecessary ventilation, saving energy while maintaining indoor air quality. DCV can link to occupancy schedules and outside air (OA) ventilation requirements (minimum setting is usually the floor area OA requirement).

Airside economizer

Economizer operation typically increases the intake of outdoor air when conditions are favorable (i.e., cooler outdoor temperatures) to reduce mechanical cooling during the day.

Types of economizers:

• Fixed Dry Bulb: the outdoor air dry bulb is less than the specified economizer dry bulb limit
• Fixed Enthalpy: the outdoor air enthalpy is less than the specified economizer enthalpy limit
• Differential Dry Bulb: the outdoor air dry bulb is less than the return air dry bulb temperature
• Differential Enthalpy: the outdoor air enthalpy is less than the return air enthalpy

High-limit shutoff temperature is the setpoint when the economizer will reset itself to the minimum position to meet ASHRAE 62.1 for minimum ventilation.

Night setback

Night setback is a common energy-saving strategy where the thermostat setpoints are adjusted to allow the temperature to drift during unoccupied hours (typically allowing the space to get warmer in summer or cooler in winter).

Night ventilation

Night ventilation involves running the fans at night to bring in cooler outside air to cool the space. Night ventilation typically requires higher fan speeds during unoccupied hours to purge heat from the building.

Night flushing is a passive cooling technique that uses natural ventilation in the evening.