HVAC Control Diagrams

HVAC control diagrams are essential tools for designing, installing, and maintaining HVAC systems. They provide a visual representation of the system’s components, how they are interconnected, and how they are controlled. This information can be used to troubleshoot problems, optimize performance, and make informed decisions about system upgrades.

This blog post will introduce you to the most common HVAC control diagrams and explain what they mean. Here are a few of the benefits of using HVAC control diagrams:

Improved troubleshooting: Control diagrams can help you to quickly identify the source of a problem in your HVAC system. By looking at the diagram, you can see how the different components are interconnected and how they are supposed to work together. This can help you to narrow down the possible causes of the problem and to identify the most likely solution.
Optimized performance: Control diagrams can also help you to optimize the performance of your HVAC system. By understanding how the different components work together, you can make adjustments to the system to improve its efficiency and effectiveness. For example, you can adjust the settings of the VAV dampers to ensure that each zone is receiving the correct amount of air.
Informed decision-making: Control diagrams can also help you to make informed decisions about system upgrades. When you are considering upgrading your HVAC system, it is important to understand how the existing system works. Control diagrams can provide you with this information, which can help you to choose the right upgrade for your needs.

Table of Contents

VAV Terminal Unit with Reheat

A VAV terminal unit with reheat is a type of terminal unit that uses a variable air volume (VAV) damper to control the flow of primary air into a space. The primary air is typically conditioned by a central air handling unit (AHU). If the space requires additional heating, a reheat coil is used to heat the primary air.

Airflow: The unit receives conditioned air from a central Air Handling Unit (AHU) through a main duct.
Damper: A damper in the unit modulates to control the amount of air from the main duct based on the heating or cooling needs of the space.
Reheat Coil: If additional heating is needed, the unit has a reheat coil. When the damper is nearly or fully closed and the space requires additional heat, air from the main duct passes over this coil before being supplied to the space.
Supply: The conditioned air is then supplied to the space to maintain the desired temperature.

This type of terminal unit allows for energy-efficient, precise control of indoor temperatures, but it can be quite complex and requires proper design, installation, and maintenance to operate effectively.

Parallel Fan-Powered Terminal Unit

A parallel fan-powered terminal unit is a type of terminal unit that uses a fan to draw air from the plenum space and mix it with primary air from the AHU. This allows the terminal unit to provide heating and cooling to the space without increasing the static pressure of the AHU fan.

Parallel fan-powered terminal units can be configured with either a constant volume fan or a variable volume fan. A constant volume fan will always run at the same speed, regardless of the space’s heating or cooling load. A variable volume fan will adjust its speed to meet the space’s load, which can save energy.

Parallel Fan-Powered Terminal Unit, Constant Volume Fan

Airflow: The unit receives conditioned air from a central Air Handling Unit (AHU) through a main duct.
Damper: A damper in the unit modulates to control the amount of air from the main duct based on the heating or cooling needs of the space.
Fan: The unit has an internal fan that operates continuously when the system is on. This fan draws in air from the plenum (the space between the drop ceiling and the structural ceiling), which is typically at room temperature.
Heating Coil: If additional heating is needed, the unit may have an optional heating coil. Air from the fan passes over this coil before being supplied to the space.
Supply: The mixed and conditioned air is then supplied to the space to maintain the desired temperature.

Parallel Fan-Powered Terminal Unit, Variable Volume Fan

Fan: The unit has an internal fan that operates when the damper is nearly or fully closed and the space requires additional air for ventilation or to meet the heating load. This fan draws in air from the plenum (the space between the drop ceiling and the structural ceiling), which is typically at room temperature.

This type of terminal unit provides better temperature control and can be more energy-efficient compared to other types of terminal units. However, it can be slightly noisier due to the operation of the fan.

Series Fan-Powered Terminal Unit

A series fan-powered terminal unit is a type of terminal unit that uses a fan to boost the pressure of the primary air from the AHU. This allows the terminal unit to deliver air to the space more efficiently, especially in systems with long ducts.

Airflow: The unit receives conditioned air from a central Air Handling Unit (AHU) through a main duct.
Fan: The unit has an internal fan that operates continuously when the system is on. This fan draws in a mix of air from the main duct and the room (return air).
Damper: A damper in the unit modulates to control the amount of air from the main duct based on the heating or cooling needs of the space.
Heating Coil: If additional heating is needed, the unit may have an optional heating coil. Air from the fan passes over this coil before being supplied to the space.
Supply: The mixed and conditioned air is then supplied to the space to maintain the desired temperature.

Dual Duct Terminal Unit

A dual duct terminal unit is a type of terminal unit that receives two separate air streams from the AHU: a hot air stream and a cold air stream. The terminal unit uses a blend damper to mix the two air streams to achieve the desired temperature in the space.

Dual duct terminal units can be configured with either inlet sensors or discharge sensors. Inlet sensors measure the temperature of the air entering the terminal unit, while discharge sensors measure the temperature of the air leaving the terminal unit.

Dual Duct Terminal Unit with Inlet Sensors

Dual Duct Terminal Unit: This is a device that controls the amount of heated and cooled air delivered to a space. It has two ducts: one for heated air and one for cooled air. The unit mixes the air from these two ducts to achieve the desired temperature.
Inlet Sensors: These are sensors located at the inlets of the terminal unit. They measure the temperature of the air entering the unit from both the hot and cold ducts. The sensors send this information to the control system, which adjusts the mix of heated and cooled air to maintain the desired temperature.

A Dual Duct Terminal Unit with Discharge Sensor is a type of HVAC system that provides precise temperature control in a building. Here’s a simplified explanation of how it works:

Discharge Sensor: This is a sensor located at the discharge of the terminal unit. It measures the temperature of the air being supplied to the space. The sensor sends this information to the control system, which adjusts the mix of heated and cooled air to maintain the desired temperature.

This type of system allows for precise, energy-efficient control of indoor temperatures, but it can be quite complex and requires proper design, installation, and maintenance to operate effectively.

Multiple Zone VAV Air Handling Unit with Return Fan and OA Measurement Station

A multiple zone VAV air handling unit with a return fan and OA measurement station is a type of AHU that serves multiple zones in a building. The AHU uses a VAV damper to control the flow of air to each zone. The return fan recirculates air from the zones back to the AHU, and the OA measurement station measures the amount of outdoor air that is mixed with the return air.

A Multiple Zone Variable Air Volume (VAV) Air Handling Unit with a Return Fan and Outdoor Air (OA) Measurement Station is a type of HVAC system that is designed to provide precise temperature control in multiple zones within a building. Here’s a simplified explanation of how it works:

Air Handling Unit (AHU): The AHU conditions the air and distributes it throughout the building. It typically includes components for heating, cooling, and filtering the air.
Multiple Zone VAV: The conditioned air from the AHU is distributed to different zones in the building through a network of ducts. Each zone has a VAV box that controls the amount of air delivered to that zone based on its heating or cooling needs.
Return Fan: The return fan is part of the ventilation system. It helps circulate air back to the AHU from the building for reconditioning and redistribution.
OA Measurement Station: This is a device that measures the amount of outdoor air being brought into the system. It helps ensure that the system is bringing in an adequate amount of fresh air for ventilation.

This type of system allows for energy-efficient, precise control of indoor temperatures, but it can be quite complex and requires proper design, installation, and maintenance to operate effectively.

Multiple Zone VAV Air Handling Unit with Relief Fan and Differential Pressure OA Measurement

A multiple zone VAV air handling unit with a relief fan and differential pressure OA measurement is a type of AHU that is similar to the previous type of AHU, but with a few key differences. The relief fan helps to maintain the static pressure in the ductwork, and the differential pressure OA measurement station measures the pressure difference between the outdoor air and the return air. This information is used to control the flow of outdoor air into the AHU.

A Multiple Zone Variable Air Volume (VAV) Air Handling Unit with a Relief Fan and Differential Pressure Outdoor Air (OA) Measurement is a complex HVAC system designed to provide precise temperature control in multiple zones within a building. Here’s a simplified explanation of how it works:

Air Handling Unit (AHU): The AHU conditions the air and distributes it throughout the building. It typically includes components for heating, cooling, and filtering the air.
Multiple Zone VAV: The conditioned air from the AHU is distributed to different zones in the building through a network of ducts. Each zone has a VAV box that controls the amount of air delivered to that zone based on its heating or cooling needs.
Relief Fan: The relief fan is part of the building’s ventilation system. It helps maintain proper building pressure by expelling excess air from the building.
Differential Pressure OA Measurement: This is a method of measuring the amount of outdoor air being brought into the system. It works by comparing the pressure difference between the outdoor air and the air inside the duct system.

This type of system allows for energy-efficient, precise control of indoor temperatures, but it can be quite complex and requires proper design, installation, and maintenance to operate effectively.

Conclusion

HVAC control diagrams are essential tools for anyone who is responsible for the design, installation, or maintenance of an HVAC system. They can help to improve troubleshooting, optimize performance, and make informed decisions about system upgrades.

Even a basic understanding of HVAC control diagrams can be very helpful. By understanding how the different components of an HVAC system work together, you can better troubleshoot problems and keep your system running efficiently.

FREQUENTLY ASKED QUESTIONS

What are HVAC control diagrams, and why are they important?

HVAC control diagrams are visual representations of HVAC system components, their interconnections, and control strategies. They are essential tools for designing, installing, and maintaining HVAC systems, as they provide a clear understanding of system operation, facilitate troubleshooting, and enable optimization and informed decision-making for system upgrades. By using control diagrams, engineers and technicians can quickly identify components, understand system behavior, and make data-driven decisions.

What types of HVAC control diagrams are commonly used?

There are several types of HVAC control diagrams, including ladder diagrams, wiring diagrams, pneumatic diagrams, and electronic diagrams. Ladder diagrams are used to represent electrical circuits, while wiring diagrams show the physical connections between components. Pneumatic diagrams illustrate pneumatic control systems, and electronic diagrams represent electronic control systems. Each type of diagram serves a specific purpose and is used in different contexts.

How do HVAC control diagrams improve troubleshooting?

HVAC control diagrams improve troubleshooting by providing a clear understanding of system operation and component interactions. By analyzing the diagram, technicians can quickly identify faulty components, incorrect wiring, or misconfigured control sequences, allowing them to focus their troubleshooting efforts and reduce downtime. Additionally, control diagrams enable technicians to simulate system behavior, predict potential issues, and develop targeted solutions.

Can HVAC control diagrams be used for system optimization?

Yes, HVAC control diagrams can be used for system optimization. By analyzing the diagram, engineers and technicians can identify opportunities to improve system performance, reduce energy consumption, and enhance overall efficiency. For example, control diagrams can help identify areas where sensors can be added or modified to improve system feedback, or where control sequences can be optimized to reduce energy waste.

How do I read and interpret an HVAC control diagram?

To read and interpret an HVAC control diagram, start by identifying the different components and their symbols. Understand the flow of the diagram, following the sequence of operations and control signals. Pay attention to the relationships between components, such as sensor inputs, controller outputs, and actuator responses. Finally, analyze the diagram in the context of the specific HVAC system, considering factors like system type, size, and application.

Can HVAC control diagrams be used for training and education?

Yes, HVAC control diagrams are valuable tools for training and education. They provide a visual representation of complex systems, making it easier for students and trainees to understand system operation and component interactions. By using control diagrams, instructors can illustrate key concepts, demonstrate system behavior, and facilitate hands-on learning experiences. Additionally, control diagrams can be used to develop interactive simulations and virtual labs, enhancing the learning experience.

What software is commonly used to create HVAC control diagrams?

Several software tools are commonly used to create HVAC control diagrams, including AutoCAD, Revit, and specialized HVAC design software like Carrier’s Hourly Analysis Program (HAP) or Trane’s TRACE 700. These tools provide a range of features and functionalities, such as symbol libraries, drag-and-drop interfaces, and automated drafting capabilities, making it easier to create accurate and detailed control diagrams.