Chapter 10
Mechanical Systems and Ductwork
Ductwork, like pipework, is a system family through which Revit MEP can calculate airflow rates and pressure drops on any correctly defined system. Ductwork also provides the graphics for the traditional “drawn” documentation and the variety of ways this can be represented on a drawing sheet at different stages of a project, regardless of whether the duct is presented as a single line at concept design or as a fully coordinated double line for construction issue.
There are three main types of duct: rectangular, round, and oval. These three types are the basis for your documentation, while new to Revit MEP 2012 are placeholder ducts (and pipes). Placeholders allow the designer to rough in layouts in single line. Although at first glance this seems similar to a view displaying at a coarse level of detail, the placeholders display only a single line, regardless of the level of detail that is set. In addition, placeholders do not create fittings when a duct run is drawn, but they still have the use of all the design tools that an engineer requires, allowing the duct to be sized based on airflow and so on. All these duct types, whether placeholder or normal, connect to air terminals and a variety of mechanical equipment. They also host duct fittings and accessories. By using the fittings and accessories available with the standard installation, the user can create supply, return, and exhaust systems with very little additional thought.
During your implementation, however, you should also consider the benefits of creating additional duct types that suit the way you work and your company’s standards.
In this chapter, you will learn to do the following:
- Distinguish between hosted and nonhosted components
- Convert placeholder ducts into ducts with fittings
- Use the different duct routing options
- Adjust duct fittings
Air distribution components come in many shapes and sizes. Depending on the design, they can be mounted in a variety of ways, such as the following:
- Diffusers in a ceiling
- Duct-mounted sidewall diffusers
- Wall mounted
- Suspended
In each of these instances, the designer must decide whether to use hosted components (and if so, which type of hosting) or whether to host at all. There are different (and similar) ways of placing these objects, and some may be conflicting.
An example of this is an installation that has diffusers hosted in the ceiling as well as areas where the architect’s design is for suspended fittings (see Figure 10-1). In this project, assume there is an external architect and that the architectural model is being linked. This means straightaway that you cannot use ceiling-hosted air terminals. Although you can see the linked ceiling, Revit recognizes it only as a face. Because of this, the air terminals will need to be created either as face-hosted families or families that are hosted to the level—that is, no physical hosting.
Figure 10-1: Diffuser hosting methods
The most important point here is that, after you commit to one type of family (hosted, nonhosted, wall, and so on), you can exchange, for example, a ceiling-hosted air terminal only with a similarly-hosted air terminal, not just those that are of the same category (see Figure 10-2).
Figure 10-2: Hosting error message
The next problem is that the air terminals that are suspended are in fact the same type as those that are mounted within the ceiling tiles. The engineer wants to be able to schedule and filter them as one. How do you manage this?
The ability to copy/monitor air terminals (as well as lighting fixtures, mechanical equipment, and plumbing fixtures) means that the services engineers can monitor the locations of air terminals that the architect has placed because the architect is responsible for placing these objects. As with the other items that can be copy/monitored, the services designer can choose to copy the original family type from the linked file or “map” it to one of their own choosing. In Figure 10-3, you can see, however, that after the air terminals have been copied/monitored, regardless of the type of host association, this ceiling-hosted family has no host but is in the correct location specified by the architect.
Figure 10-3: Copy/monitor nonhosting
Mechanical Equipment Components
Mechanical equipment comprises the components that make up the majority of large- to medium-size plant objects for the mechanical designer. From air conditioners (ACs) and air-handling units (AHUs) to air curtains and heat pumps, these all provide the geometry and parameters associated with HVAC design. As with all components, choosing the hosting type is important. A level-hosted object cannot be exchanged for a face-hosted or ceiling-hosted one.
Air Conditioning/Handling Units
The heart of the mechanical air system, air conditioning/handling units, can start life as generic “boxes” with intake and exhaust. Although basic in construction and with no manufacturer data attached, generic ACs/AHUs can have the same number of parameters as more-detailed families. Similar in dimension and performance, the concept box can be swapped out during the detail design period for a more detailed manufacturer or “construction-issue” family, or even for a set of families if the AC/AHU unit has been constructed from its manufacturer’s component parts (see Figure 10-4).
Figure 10-4: Basic AHU and type parameters
The majority of ACs/AHUs are generally placed on the level where they are inserted, with no offset. This placement can depend on whether the unit is mounted on rails and whether those rails are part of the AC/AHU family (see Figure 10-5).
Figure 10-5: Floor-mounted and skid-mounted AC/AHU
Mounting Options for ACs/AHUs
There are two mounting options for ACs/AHUs.
Floor Mounted
In Figure 10-5, the mechanical plant on the right has been constructed to be mounted directly to the floor, which would probably be best created with a standard or default template (in other words, nonhosted).
Skid Mounted
For the example on the left side of Figure 10-5, the family can be created in a couple of ways:
- The family can be created in the same manner as the previous example, which allows for an offset to be applied. This allows for the structural engineer to locate elements in his or her file. Then subsequent placement of the mechanical plant is done in the MEP file.
- Alternatively, the Mechanical Equipment family can have the structural elements built in, allowing the MEP designer to place the unit—including the rails—directly to the slab.
VAV Boxes
Generally created as nonhosted families, variable air volume (VAV) terminal boxes are usually mounted somewhere within the ceiling void, suspended from the underside of the slab above. In terms of placement, these are given an offset. Using the Properties palette, the offset can be predefined prior to the VAV being placed in this case. This makes for an easier workflow than in previous releases, in which most objects were placed on the reference level and then subsequently moved to the correct invert level (see Figure 10-6).
Figure 10-6: Changing the invert level
Connections to both AC units and VAV boxes can include heated and chilled water services, and electrical for connecting to water and electrical systems (see Figure 10-7). Also note the new connector labels in this view, which allow easier identification of connections and their flow direction (where applicable).
Figure 10-7: Typical mechanical and electrical connections to mechanical equipment
Ducts can be displayed in a variety of ways, including rectangular, round, oval, or placeholder, as shown in Figure 10-8.
Figure 10-8: Ductwork
Placeholder ducts, as you can see in Figure 10-8, are shown only as a single line, although they retain nearly all the characteristics and properties of a regular duct. In Figure 10-9, you can see the differences. Placeholder ducts do not have values for Justification, Insulation, or Lining, because they are just not needed at the conceptual stage of the project when placeholders are being used.
Figure 10-9: Design criteria for placeholder ducts
The Duct tool is a system family, and it is the glue that holds systems together. However, it also relies on standard families to create a duct type, as described in the next section. Although ducts hold a huge amount of information, for the user there are several important considerations to note.
As you saw in Chapter 8, “HVAC Cooling and Heating Load Analysis,” Revit MEP 2012 provides significant enhancements to systems, including the ability to specify the type of system that a duct is associated with (Exhaust, Return, or Supply), without actually being connected to any equipment, as shown in Figure 10-10.
Figure 10-10: Ducts associated with systems
When highlighted in the System Browser, both ducts and placeholder ducts are highlighted in the drawing area.
If a duct is created in the wrong system type, simply select one section of the run and change it to the required system. All the connected ducts and fittings will also change.
Connecting a defined system of mechanical equipment into an existing duct run of the same system type will combine them together into one system, so the ducts will always be associated with a system, as shown in Figure 10-11.
Figure 10-11: Connecting ducts
Figure 10-12 shows what happens when you try to connect a defined system of one type to a run of ducts that are part of another system. This warning is a good heads-up for the user. If the operation was deliberate, however, all you need to do is select a duct and change it to the relevant system.
Figure 10-12: A warning about differing systems
In Revit MEP 2012, all systems are named. Even though they may be default names, it is easy to change them on the fly.
New to Revit MEP 2012 is that, when running an interference check, the duct insulation can be included as part of the interference (see Figure 10-13).
Figure 10-13: Interference check
Creating new duct types is a way of managing how your runs of duct work and connect into each other, such as whether bends are mitered or radiused. Although you can change any of the fittings inserted retrospectively, it is much easier to create a run in one go, using either the automatic or manual tools.
Creating New Duct Types
Although it may be tempting to create types such as Extract and Supply, try to keep these names more specific, such as Stainless-2D Radius/Taps or Galvanized-Mitered/Tees. This way, the user has to think about only what material the duct is constructed of. Although using a name that specifies Supply or Extract may be initially attractive, doing so can lead to ambiguity and misunderstandings among the modeler, designer, engineer, and potentially the client. Another downside to this is the need to create multiple types of bends, tees, crosses, and so on for all the different material types and system types you are likely to use.
To create additional duct types or to edit existing types, right-click one of the existing types in the Project Browser. To create a new type, click Duplicate. This will copy an existing duct type and append the number 2 to the end of the name (for example, Mitered Elbows/Tees 2). Right-click again to access the Type Properties. At this point, you can choose to rename the duct type and supply the necessary fittings. These fittings will need to be preloaded to use, but you can also reuse those already in use in the project/template.
Schematic Layouts
Mike, the office drafter, has been given the task of preparing a schematic ductwork layout for mechanical engineer Carla. Because the design architect has yet to supply a Building Information Model, Mike has some initial 2D AutoCAD plans and sections. Carla completes a rough design on paper along with calculations for duct sizes. When Mike starts the job, he links the AutoCAD section into a new section view. From here, he can create building levels based on the section. With this done, floor plans are created, and the AutoCAD plans are linked into the relevant levels. Mike can now create a single-line, schematic layout for the project. At this stage, floor offsets, downstand beams, and coordination between services can almost be ignored, because the duct is being shown only as a single line. The main benefit of this is that the bulk of modeling for the duct system can be achieved at an early stage of the project and retained or modified as the project progresses.
Using Automatic Duct Routing
When using the automatic routing tools, as a rule of thumb you should work on small sections—all the feeds to a VAV box is one good example. This means the computer has fewer objects to calculate, and the routing suggestions have less room for error. Before even starting this process, check the options under Mechanical Settings (see Figure 10-14) for default duct types, offsets, and length of flex duct because these are used during the routing process. Also note that these settings can be set for the different system types.
Figure 10-14: Mechanical settings
Now you’re ready to use automatic duct routing. Here’s how:
1. Place your VAV box, ensuring it is located at the correct height. You can change it later if needed, but this may lead to you changing your duct route.
2. Place your air terminals. At this point, it is a good idea to consider the following, not from a design point of view but from a Revit one:
- What type are the ceilings (if any)?
- If there are ceilings, should you use face, ceiling, or nonhosted families?
- Are you going to create your own placeholder ceiling to host your families?
- Should you use the new ability to copy/monitor the air terminals already placed by the architect?
- Choose the type of air terminal. Is it top, side, or even sidewall entry?
3. Once you’ve made all these decisions, it’s time to start laying out the equipment. Figure 10-15 shows (1) a space where the upper limit of the space has been defined as the level above; (2) the specified supply airflow has been entered manually; (3) calculated supply airflow is not computed because the analysis tools have not been used for this building; and (4) the flow rates for the air terminals have been adjusted to suit the calculation.
Figure 10-15: Space properties
4. Creating a system has become easier in Revit MEP 2012 than in previous releases. You can either select one air terminal, create the system, and then add others, or you can select all the air terminals you want to be in the system. On the ribbon, select the Duct option on the Create Systems panel (Figure 10-16).
Figure 10-16: Create Systems panel
After you click the Duct button, the Create Duct System dialog box displays, as shown in Figure 10-17.
Figure 10-17: Create Duct System dialog box
This allows you to edit the system name and, if necessary, open the new system in the system editor to add/remove additional objects and select equipment. Objects that are not part of the current system appear as halftone in the drawing area. When they are selected to be added to the system, their appearance changes to full weight (see Figure 10-18).
Figure 10-18: Add To System command
5. Once you have completed adding air terminals, click Select Equipment, and either select the equipment from the Options Bar drop-down or select the actual VAV box indicated on the plan, as in Figure 10-18.
6. Complete this task by clicking Finish Editing System. This is where the fun begins!
7. Hover over one of the items in the system (but do not select it) and press the Tab key. All the items in the system—air terminals and the VAV box—will now be enclosed in a dotted box. Without moving your cursor, left-click. This selects the system, as shown in Figure 10-19, and displays the system in the System Browser.
Figure 10-19: Tab-selecting the system
8. With the system selected, you now have several options; one of these is to create a duct layout. New to Revit MEP 2012 is the option to use placeholder ducts. Select Generate Placeholder and, as shown in Figure 10-20, the sketch options for layout are displayed. Select Network, option 4. Remember that you can also edit the default settings for the duct offsets, as shown earlier in Figure 10-14, by clicking the Settings button on the Options Bar.
Figure 10-20: Generating a layout
9. There are several options for automatically generating your duct layout. These include Network, Perimeter, and Intersection options. Each of these can give you several solutions that can vary depending on the predefined settings for the duct layout, which can also be accessed from the Options Bar. Figure 10-21 shows that you can click the Edit Layout button, which allows you to select the layout lines in order to make changes to the layout.
Figure 10-21: Ductwork routing solutions
10. Main duct runs are shown in blue, while branch runs are shown in green. Once you have settled on your preferred layout, click Finish Layout, and the duct layout is created (see Figure 10-22). Notice that the placeholder ducts are shown only as a single line
Figure 10-22: Completed duct layout
It’s worth pointing out here that the sizes used for this layout are based on the connection sizes and the settings for duct sizes located in the Mechanical Settings dialog box. There is another important consideration at this point. The designer/drafter must ensure that the default settings for the main and branch ducts are adequately high enough above the air terminals and associated equipment to generate the layout; otherwise, the layout tool will not be able to generate a layout, even if the layout is created with placeholder ducts.
Using Manual Duct Routing
For the experienced design drafter, using the automatic tools may seem too limiting. However, wih experience and use, most users eventually settle for a variety, sometimes for no better reason than “a change is as good as a rest.” Sometimes, however, the manual tool is much more efficient, especially when connecting different areas into a system or laying out runs back to a rooftop AHU.
To begin manual duct routing, do the following:
1. Click the Duct tool on the Home tab on the ribbon, and then choose the type and its various options from the Properties palette, as shown in Figure 10-23.
Figure 10-23: Duct types
Note the default settings in the Properties palette for constraints, including Justification, Reference Level, and Offset. The System type is undefined and, because the duct is unconnected, the airflow is 0.
You can adjust additional properties from the Options Bar and the Modify/Place Duct tab.
Additional functionality in Revit MEP 2012 allows you to inherit the elevation and size of existing ducts when connecting to them. For round or oval ducts, you can also choose the Ignore Slope To Connect option, meaning that the duct will connect regardless of slope, or use the specified angle.
2. The duct can then be created/drawn to whatever path you choose. For vertical offsets, type the new height into the offset box on the Options Bar. For an angled setup or set-down, it is best to work in an elevation or sectional view.
In Figure 10-24, the duct has been split and a section created. To create a duct set-down, you would hover over one end of the duct, right-click the connector, and select Draw Duct. The duct would then be drawn along the preferred route.
Figure 10-24: Editing duct in sections
3. Using tools such as the Trim command, you can complete the run as shown in Figure 10-25.
Figure 10-25: Completing the set-down
When creating ducts that set down/up or drop/rise, it is much easier to model these in a section or elevation view, as seen in Figures 10-24 and 10-25. To do this, first you need to create a suitable view. Quite often, individuals have a personal section that they move around the model. It can be opened when required, and then the duct? (or whatever service is being worked on) is modified and the view is closed.
Adjusting Fittings and Extending the Design
The duct justification tool has been given a big makeover in Revit MEP 2012. Selecting a duct or a run of ducts/fittings gives the user the option to change justification with greater ease than in previous releases. Select your duct run and from the Modify tab, select Justify.
This opens the Justification Editor, as shown in Figure 10-26. Here, the control point can be displayed, and the alignment along the selected route can be selected by using the Alignment Line button. You can choose justification by using one of the nine Justification control buttons. This can have an effect on your layout, so it is best to use this tool in small, easy-to-view sections or 3D views.
Figure 10-26: Duct Justification Editor
It is worth making the point here that duct justification is not available for duct placeholders.
Another new enhancement to the 2012 version of Revit MEP is the ability to change an entire duct run from one type of duct to another. In previous versions, this involved multiple selections and filters. Now all the user has to do is to select the duct run, as you can see in Figure 10-27. Here, the duct system has been selected, and the only proviso is that flexible ducts, air terminals, and mechanical equipment are not in the selection set.
Figure 10-27: Changing the duct type
This is a logical step forward, because the ducts and duct fittings in the selection set are usually also defined within the duct type. Therefore, when the objects are selected and the Change Type button is used, you are presented with a condensed Properties box that allows you to change the run type for others already in use in the project (see Figure 10-28).
Figure 10-28: The changed duct type
Direction of flow and graphical warnings have also been added to the MEP toolset in 2012. The direction of flow is a temporary display when you select any object that has a mechanical or piping connection. This should be a huge benefit for designers and drafters, as they no longer have to guess which way air (or gas or water) is flowing through an object. As you can see in Figure 10-29, the selected VAV box has two supply ducts: one entering from the right and the other exiting left. There is also a return-air and electrical connection.
Figure 10-29: Flow connectors
Warnings related to MEP connections have also been given a boost in this release, so you can have graphical notifications when there is a disconnect in the system. You can change the warnings to suit your current task from the Analyze tab. Select the Show Disconnects button and then select the options you require.
To extend an existing layout, as indicated in Figure 10-30, select one of the fittings (typically a bend or tee). You will notice a small plus sign (+) adjacent to the side of the fitting that does not have a connector. Clicking this will turn a bend into a tee, and a tee into a cross, eliminating the need to delete a fitting and insert an appropriate one.
Figure 10-30: Extending the design
The most important factors to consider when using the duct sizing tools are that the ducts form part of a system and that this system should have a nominated name that suits your design (not a default), which is created at the same time as the system being created. The system must also have a valid airflow, so either you specify the airflow of the air terminals or that flow is specified from the space and volume calculations.
Use the Tab key to select your system, as shown in Figure 10-31.
Figure 10-31: Tab-selecting the system
Figure 10-32 shows the Duct/Pipe Sizing tool located on the ribbon. This button becomes active when you select a single duct or duct run that is part of a fully enclosed system whose components are connected properly in regard to flow direction. Clicking the button opens the Duct Sizing dialog box. Figure 10-33 shows its available options.
Figure 10-32: Duct/Pipe Sizing tool
Figure 10-33: Duct Sizing dialog box
Choosing a Duct Sizing Method
Although the sizing method you choose can be applied to an entire system, this is not necessarily the most efficient way to do things. If there are 5,257 objects in your system, it’s probably not a great idea to ask your computer to process that amount of information—it may take a while! The logical choice is to split the task of duct sizing into manageable sections, such as a floor plan, a zone, or a group of air terminals fed from the same VAV box.
The various methods for sizing are as follows:
- Friction
- Velocity
- Equal Friction
- Static Regain
Friction and Velocity can be used independently of each other when using the Only option. Alternatively, they can be used in conjunction with each other with the And and Or functions.
These options allow the designer to force the sizing ducts to meet the parameters specified for both Velocity and Friction. With the Or method, the least restrictive of either of the parameters is used.
The Equal Friction and Static Regain methods use the ASHRAE Duct Fitting Database.
Air properties are set in the Mechanical Settings dialog box, as are the available sizes of ducts used in the actual sizing calculations (see Figure 10-34 and Figure 10-35).
Figure 10-34: Mechanical Settings dialog box, duct settings
Figure 10-35: Mechanical Settings dialog box, duct sizes
During the sizing process, you can also apply constraints to the branch parts of the run. These are defined as follows: Calculated Size Only, Match Connector Size, and Larger Of Connector And Calculated (see Figure 10-36). Depending on the stage of the design or your actual role (such as consultant), you could choose to select Calculated Size Only, because the design is still in its early stages and the equipment is Generic. These settings allow you to coordinate with other model components in areas where space is limited. For example, if you have only 2′-0″ of space above the ceiling, you could restrict the height of your duct to 20″.
Figure 10-36: Duct sizing constraints
However, later in the project when the equipment has been specified, as a contractor you may want to select Match Connector Size and Larger Of Connector And Calculated to reduce the number of duct sizes used on the project, thereby reducing your manufacturing costs.
This dialog box also allows you to specify a limit on the size of the ducts, which can further reduce costs or give you the ability to specify, say, a continuous duct height in places where you know access is a potential issue.
A new addition to the tools in Revit MEP 2012 is the ability to add insulation and lining to a selection set that contains both ducts and duct fittings, as shown in Figure 10-37.
Figure 10-37: Insulation and lining tools
With this new addition comes more functionality for the user. Lining and insulation can have overrides or filters applied so that they display in the required manner, even to the extent of showing insulation as transparent with a hidden line style, as indicated in Figure 10-38.
Figure 10-38: Insulation and lining graphic overrides
Although lining and insulation can be added only as instance parameters, with this new functionality, the process is much easier than in previous releases. Even so, if the entire system requires lined or insulated duct, the user will have to select all ducts for the system and then specify the insulation or lining required.
Duct sizing will work only where air terminals, ducts, duct fittings, and mechanical equipment are seamlessly connected with no gaps, and where the equipment has a defined airflow. This airflow could be entered as part of the initial analysis or subsequent manipulation of the objects.
Using the Duct Routing Tools
Now that you have reviewed the process of using the duct routing tools, you will learn how to apply them in a simple exercise:
1. Open the Chapter10_Dataset.rvt file found at www.sybex.com/go/masteringrevitmep2012.
2. Open the floor plan view 1 - Mech, as shown in Figure 10-39. You will see that there are four air terminals (nonhosted) mounted at 8′-0″ above level and a VAV unit mounted at 11′-0″. These have already been connected to a supply air system.
Figure 10-39: Plan view
3. Hover over any one of the air terminals, and press the Tab key. This highlights the system, as indicated in Figure 10-36. Left-click, and the Modify | Duct Systems tab is activated. Click the Generate Placeholder button.
4. Click the Settings button on the Options Bar, and check that the Duct conversion settings are as follows:
- Main duct type: Rectangular Duct: Mitered Elbows/Tees
- Offset 11′-0″
- Branch duct: Rectangular Duct: Mitered Elbows/Tees
- Offset 11′-0″
- Flexible duct: Flex Duct Round: Flex - Round with a maximum length of 6′-0″
5. Use Solution Type: Network, 1 of 6, and click the Finish Layout button. Activate the default {3D} view and notice that, as shown in Figure 10-40, the main run of duct is shown as a single line. If the flexible duct is also shown as a single line, this is not by design; it indicates that the flexible duct is not connected correctly and needs to be rectified.
Figure 10-40: Default 3D view with flex errors
6. If the flex duct is too short to display properly, select one of the rectangular-to-round transitions and move it 2′-0″ toward the center of the duct run. This will stretch both the placeholder duct and the flex duct, as shown in Figure 10-41. Repeat this process for each transition. You may find it easier to tile the open view windows and select the transition in the 3D view, but then actually move it in the plan view. Clicking the plan view should retain the selection, but if you lose it, just right-click and choose Select Previous.
Figure 10-41: Moving transitions
7. The placeholder duct is ideal for concept and preliminary design in which little detail is required for your drawings. Once this stage is passed, you can change the placeholder ducts to actual ducts with fittings. To do this, simply select the placeholder ducts in the 3D view, either by selecting individually or selecting everything and using the Filter tool, as shown in Figure 10-42. Then click the Convert Placeholder button on the ribbon.
Figure 10-42: Duct placeholders
8. With the duct in place, open the view Section 1. Here you can see there is a clash where you need to edit your duct to go under the beam. Using the Split tool, split the duct and delete the inner section and duct union fittings (or use the Split With Gap option), as shown in Figure 10-43.
Figure 10-43: Split duct
9. Complete the duct as required to avoid conflict with the beam.
Distinguish between hosted and nonhosted components. Deciding whether hosted or nonhosted components are used is crucial for the success of your project. This decision will play a large factor in performance and coordination with other companies.
Master It Should you choose hosted or nonhosted components for your project?
Use the different duct routing options. When using Revit MEP 2012 for your duct layouts, the user must understand the functions of automatic duct routing and manual duct routing. Once these functions are mastered, the user can lay out any type of ductwork system.
Master It When asked to submit a design proposal for a multifloor office building, the HVAC designer needs to show a typical open plan office and the supply and extract ductwork. How should the designer start this process?
Adjust duct fittings. Duct fittings are needed in systems to make the systems function properly and to produce documentation for construction. Being able to add or modify fittings can increase productivity.
Master It You have just finished your modeled layout and given it to your employer for review. Your boss asks you to remove a couple of elbows and replace them with tees for future expansion. What method would you use to accomplish this quickly?