Modeling HVAC Systems

Modeling HVAC Systems


In this lesson, students will learn how to model mechanical systems that provide ventilation and conditioned air to the spaces inside a building. These systems are typically referred to as heating, ventilation, and air-conditioning (HVAC) systems. Students will place components to model the essential elements of an HVAC system—air handling units and diffusers—and link these elements using ductwork to create systems that supply to the spaces and return air to the handlers. The endpoint of the lesson will be a mechanical model that can be used for mechanical system analysis and detailed mechanical design, as well as shared with the other members of the design team and disciplines affected by the mechanical design decisions.

Copying Shared Elements into a Mechanical Model

Link the preliminary architectural model to an Autodesk® Revit® MEP host project and use the Copy/Monitor tool to copy shared levels and grids as the starting point for the mechanical design tasks.You will place components to model the HVAC equipment and devices later in this lesson, so you do not need to copy any fixtures from the linked model.

Modeling Exposed HVAC Systems

Model exposed HVAC systems (where the ductwork is visible and not concealed by a ceiling) using tools in the HVAC Panel of the Home tab. You can create two types of systems:

  • Supply systems that move conditioned air from air handling units to supply diffusers
  • Return systems that move air from return diffusers back to the air handling units

The essential steps include:

  • Place air handling unit components.
  • Place supply diffusers and return diffuser components at locations for these terminals.
  • Connect the diffusers to the air handling units with ducts.

To effectively move fresh air to where it is most needed, supply diffusers are typically placed along the perimeter of a space near doors and windows (and away from the return air intake). Return air diffusers are strategically placed to draw the conditioned air through the space.You can place ducts individually, or use Revit software’s auto-routing tools to generate recommended ducts layouts based on the equipment connections, duct sizes, and connectors required.

Modeling Closed Plenum HVAC Systems

You can also model closed plenum HVAC systems (where the enclosed ceiling space is used as a return air plenum) using tools in the HVAC Panel of the Home tab. In these systems:

  • Supply air is moved to the supply diffusers through ducts that are typically concealed in the ceiling space.
  • Return air is moved from return diffusers mounted on the ceiling through the return air plenum.

The guidelines for locating diffusers in exposed HVAC systems also apply here. Place diffusers strategically to draw conditioned air through the space.As with exposed systems, you can place ducts individually, or use Revit software’s auto-routing tools to generate recommended layouts. Since the ductwork is typically concealed by a ceiling, you can use rectangular ducts, which are typically less expensive than exposed round ducts.

Learning Objectives

After completing this lesson, you will be able to:

  • Understand the importance of copying in model elements that will drive the mechanical design.
  • Model some typical HVAC system, both closed plenum and exposed, within the space constraints of the building.
  • Understand the options available and tradeoffs for modeling ductwork for HVAC systems.
  • Investigate the register of HVAC systems and see how they are organized by type (supply or return).


Copying Shared Elements into a Mechanical Model

In this exercise, you will learn how to:

  • Copy shared levels and grids into a Revit MEP host project.
  • Create working views for mechanical design in Revit MEP.

Figure 4.5.1 - Applying view templates to customize views for mechanical design

Video Tutorial
Student Exercise
  • Create a new customized and filtered 3D view using the section box to assist with coordinating the lighting fixtures and the mechanical equipment to be placed.

Figure 4.5.2 - 3D view displaying lighting and HVAC disciplines

Modeling Exposed HVAC Systems

In this exercise, you will learn how to:

  • Place air handling units and terminals.
  • Create a return system.
  • View HVAC systems in the System Browser.

Figure 4.5.3 - Connecting diffusers to an air handling unit

Video Tutorial
Student Exercise
  • Place six supply diffusers for an exposed HVAC system in the retail space at the north end of level 1 around the front façade.
  • Create a supply system for the diffusers placed in the previous step:
    • For the main ducts, use oval ducts with gored elbows/tapsand an offset of 13'-0" (3.96 m).
    • For the branch ducts, use round ducts with tees, an offset of 13'-0"(3.96 m), and round flex ductswithmaximum flex duct length of 6'-0"(1.83 m).

Figure 4.5.4 -  HVAC supply system in the retail space

Modeling Closed Plenum HVAC Systems

In this exercise, you will learn how to:

  • Measure the plenum space available.
  • Place air handling units and terminals.
  • Create a supply system.

Figure 4.5.5 - Using auto-routing to place supply side ducts in closed plenum HVAC system

Video Tutorial
Student Exercise
  • Open the model for this exercise in Revit.
  • Place two supply diffusers for a closed plenum HVAC system in the office space at the northeast corner of level 2.
  • Place a variable air volume (VAV) unit above the corridor outside the office.
  • Create a supply system for the diffusers and VAV unit placed in the previous steps using the Generate Layout tool:
    • For the main ducts, use rectangular ducts with mitered elbows/teeswith an offset: 11'-0" (3.35 m).
    • For the branch ducts, use rectangular ducts with mitered elbows/teeswith an offset: 11'-0" (3.35 m) and no flex duct.
  • Open the 3D HVAC Lighting Coordination view to confirm that there are no conflicts between the HVAC elements and the lighting fixtures.

Figure 4.5.6 -  Duct layout for supply side of closed plenum HVAC system


Copying Shared Elements into a Mechanical Model

  • What elements and features of a model would you want to see to assist with mechanical design tasks?

You would want to see the location of the room boundary elements―including the walls, doors, windows, and ceilings―to understand the thermal loads and requirements. You would also want to see the structural members to avoid their locations while placing elements of the mechanical system.

  • What other types of equipment and systems should be considered when placing HVAC equipment and ducts?

You should consider the location of any equipment that will contribute to the heating or cooling. You should also consider the locations of other systems that could interfere when placing elements of the mechanical system.

  • Which views are best for placing HVAC equipment and ducts in a ceiling space?

Typically, ceiling plan views, sections, and 3D views will provide the most useful information when placing ductwork.

Modeling Exposed HVAC Systems

  • In what types of spaces are exposed HVAC system typically used?

Exposed HVAC systems are typically used in retail spaces, restaurants, lofts, and spaces where the architectural intent is to reveal and not hide the building systems. This is often associated with more modern and dynamic architectural styles.

  • What factors determine the size requirements for an air handling unit?

Air handling units are sized in accordance with the amount of air flow required to handle the supply and return diffusers.

  • Could a central air handling unit be used rather than a local unit in the space?

Yes, central air handling units are an efficient strategy for conditioning air for a large number of spaces.

  • Where could it be located? How would you modify the design to route air to and from the central unit?

Central air handling units are typically located on rooftops, below grade, or in dedicated utility spaces. When central air handling is used, mechanical chases and shafts must be provided to accommodate the ductwork required to distribute the air.

  • How does the shape of the ducts and number bends in ductwork affect the efficiency of the system?

Increasing the number of bends decreases the efficiency of the system because of air drag. When bends are used, 45-degree bends are more efficient than 90-degree bends. Similarly, round and oval are more efficient than rectangular ducts.

Modeling Closed Plenum HVAC Systems

  • Why are return ducts not needed in a closed plenum HVAC system?

The plenum between the ceiling and the floor above provides a closed channel that is used for drawing the return air back to the air handling units.

  • What types of HVAC systems are typically used for:
    • Residences?
    • Offices?
    • Hotel rooms?

For residences in the United States, central forced air systems with closed supply and return ductwork are used. In other parts of the world, split system air handlers, which control the heating and cooling locally, are more common.

In large offices and public spaces, central air handling systems are typically used to condition the air for the open spaces

Hotel rooms typically use split systems that allow occupants to control the comfort settings in the individual rooms.

  • What factors determine the best placement for supply diffusers in a room?

Supply diffusers are often placed near the windows or doors, as the largest temperature differences occur there.

Key Terms

Key Term
Plenum Space
In a structure, this is the space that exists in the middle of the actual ceiling and the dropped ceiling, which is frequently made use of as an air duct for heating and cooling purposes. This space also consists of electric, telephone, and network cables and wires.
Air Terminal
A device located in an opening provided at the boundaries of the treated space to ensure a predetermined motion of air in this space. These can include supply and return diffusers.
Variable Air Volume
A technique for controlling the capacity of a heating, ventilating, or air-conditioning (HVAC) system. The simplest VAV system incorporates one supply duct that, when in cooling mode, distributes cool supply air. Because the supply air temperature is constant, the air flow rate must vary to meet the rising and falling heat gains or losses within the thermal zone served.