Air Conditioning/Handling Units

The heart of the mechanical air system, the air-conditioning or air-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 to generic ACs/AHUs, 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). It is, however, entirely likely that the connectors in this generic unit will not be in the correct location when the more detailed manufacturer's family is loaded. This can mean some rework is necessary in order to produce a more accurate model.

The majority of large ACs/AHUs are generally placed on the level where they are inserted, while fan coil units (FCUs) or heat pumps are generally placed in a ceiling void, although they can also be in a plant room. The placement of these units can depend on whether the unit is mounted on rails, a plinth, or anti-vibration mounts and whether these mounting devices are part of the AC/AHU family (see Figure 10.5).

VAV Boxes

Generally created as level-hosted 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 relative to the building level associated with the active view. 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 elevation (see Figure 10.6).

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 connector labels in this view, which allow easier identification of connections and their flow direction, where applicable. These symbols also double as a shortcut to start creating ducts from the connector; just click the icon to start.

Heating and Cooling Elements

Heating coils, chilled-water coils, or reheat coils can be shown as part of the equipment or provided separately. They can also be duct mounted. Generally for FCUs, heat pumps, and most ACs/AHUs, the coil connections would be built into the equipment, but for a built-up AHU or for a VAV that may or may not include reheat, providing a coil separately can make it easier to swap out without changing the entire family. Duct-mounted coils need to be made adjustable to duct size and coil width (determined by the number of rows) and will need to have the pressure drop provided from the manufacturer's data.

Electric duct heaters can also be required. With these you need to ensure that you allow for the fire rating required., Building this into the family ensures that you are not relying on the users to remember it every time.

Creating New Duct Types

Although it may be tempting to create types with names such as Exhaust and Supply, try to keep these names more specific, such as Stainless-2D Radius/Taps or Galvanized-Mitered/Tees. This way, the user only has to think about what material was used for the duct. Using a name that specifies Supply or Extract may be initially attractive, but 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 (as is typical for all Revit system families). To create a new type, click Duplicate. This copies an existing duct type and appends 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 need to be preloaded to use them, but you can also reuse those already in use in the project/template.

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 10.15) for default Duct Type, Offset, and Maximum Flex Duct Length values because these are used during the routing process. Also note that these settings can be set for the different system types.

Now you're ready to use automatic duct routing. Here's how:

1. Place your VAV box, ensuring that it is located at the correct height. You can change it later if needed, but this may lead to you changing your duct route. Sometimes it is worthwhile to place the unit at the correct level and create a short run of duct from each connector. This gives you the correct levels of ducts (and pipes) from which you can then adjust the default height (Offset), as described in step 8.
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 level-hosted families?
   • Are you going to create your own placeholder ceiling to host your families?
   • Should you use the 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 10.16 shows (1) a space where the upper limit has been defined as the level above; (2) that the Specified Supply Airflow value for the space has been entered manually; (3) Calculated Supply Airflow has been computed using the analysis tools; and (4) Actual Supply Airflow is calculated from the flow rates for the air terminals.

   

4. Creating a system is a relatively easy process in Revit MEP 2015. 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 (see Figure 10.17).

   

   After you click the Duct button, the Create Duct System dialog box appears (Figure 10.18).

   

   This allows you to edit the system name and, if necessary, open the new system in the System Editor to add/remove 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 10.19).

   

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 10.19.
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-line box. Without moving your cursor, left-click. This selects the system, as shown in Figure 10.20, and displays the system in the System Browser.

   

   With the system selected, you have several options; one of these is to create a layout. You can use either regular ducts or the placeholder option.

8. Select Generate Placeholder and, as shown in Figure 10.21, the sketch options for layout are displayed. For Solution Type, select Network, option 5. Remember that you can also edit the default settings for the duct offsets, as shown earlier in Figure 10.15, by clicking the Settings button on the Options Bar. Most errors that occur using Layout are associated with either settings that fail to deal with elevations and duct types well, or duct or equipment that have not been placed at the correct elevations to allow the duct run to work at all.

   

   You have several options for automatically generating your duct layout. These include Network, Perimeter, and Intersection options. Each 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 10.22 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. Using a plan and a 3D view side by side is a great way to ensure that a layout will work before clicking Finish Layout.

   

   Main duct runs are shown in blue, while branch runs are shown in green.

9. Once you have settled on your preferred layout, click Finish Layout, and the duct layout is created (see Figure 10.23). Notice that the placeholder ducts are shown only as a single line.

   

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 high enough above the air terminals and associated equipment to generate the layout; otherwise, the Layout tool cannot generate a layout, even if the layout is created with placeholder ducts. This can be achieved in two ways: either by selecting the Settings button on the Options Bar (shown earlier in Figure 10.21) or by selecting the actual sketch lines of the solution, which allows the user to manually choose the offset.

Using Manual Duct Routing

Using the automatic tools may seem too limiting or even pointless to an experienced design drafter who already knows the sizes and routing that must be used. These tools are great for a quick mock-up or presentation of a design that is more conceptual. However, with experience, 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 10.24.

   

   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, shown in Figure 10.25.

   

   You can also inherit the elevation and size of existing connectors, whether from ducts or families, 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 you can 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 10.26, 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.

   

3. Using tools such as the Trim command, you can complete the run, as shown in Figure 10.27. Bear in mind, sketching a duct in sections or elevations must be done from connector to connector. Otherwise, the work plane of the view will be where the duct is placed. In section and elevation views, this is where the view's cut plane is. You see some strange duct runs if you don't pay attention to that.

   

When creating ducts that set down/up or drop/rise, it can be much easier to model these in a section or elevation view, as shown in Figure 10.26 and Figure 10.27. 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. This section, which should be named for the individual, can then be used to show a quick 3D view of the area using the Orient To View option.

Adjusting Fittings and Extending the Design

Changing the justification of ducts is a relatively easy task, but it can take practice to actually master. Selecting a duct or a run of ducts/fittings gives the user the option to change justification. Select your duct run, and from the Modify tab, select Justify.

This opens the Justification Editor, shown in Figure 10.28. 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 significant effects on your layout, so it is best to use this tool in small, easy-to-view sections or 3D views.

You can also 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 select the duct run, as you can see in Figure 10.29. 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.

This is a logical step, because the ducts and duct fittings in the selection set are usually also defined within the duct type. Therefore, when you select the objects and click the Change Type button, 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 10.30).

In this next example, the design has gone through a few iterations, and changes have been made to various fittings and portions of the duct run. Since Revit MEP 2014, the Reapply Type option has been available. Selecting the entire run allows the user to reapply the original duct types to the objects selected, as shown in Figure 10.31.

The direction of flow and graphical warnings are really helpful for the designer. The direction of flow is a temporary display when you select any object that has a mechanical or piping connection. This is a great benefit for designers and drafters because they can visualize which way air (or gas or water) is flowing through an object. As you can see in Figure 10.32, the selected VAV box has two supply ducts: one entering from the right and the other exiting from the left. There is also a return-air and electrical connection.

Warnings related to MEP connections can be displayed as 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, as shown in Figure 10.33.

Since Revit MEP 2014 we have had the ability to cap open ends of a duct or pipe. The reason for doing this is that an open end in Revit (pipe or duct) results in incorrect calculations because Revit is expecting a contained system. To achieve this, either select a duct or create a selection set of ducts, and click the Cap Open Ends button displayed on the ribbon, as shown in Figure 10.34.

To extend an existing layout, as indicated in Figure 10.35, select one of the fittings (typically a bend or tee). Notice a small plus sign (+) adjacent to the side of the fitting that does not have a connector. Clicking this turns a bend into a tee and a tee into a cross, eliminating the need to delete a fitting and insert an appropriate one.

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 10.38 and Figure 10.39).

During the sizing process, you can also apply constraints to the branch parts of the run. These are defined as Calculated Size Only, Match Connector Size, and Larger Of Connector And Calculated (see Figure 10.40). Depending on the stage of the design or your actual role (such as consultant), you could choose 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″ (600 mm) of space above the ceiling, you could restrict the height of your duct to 20″ (500 mm).

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.

To add insulation and lining to a selection set that contains both ducts and duct fittings, simply create your selection set and choose the appropriate option, as shown in Figure 10.41.

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 10.42.

Although lining and insulation can be added only as instance parameters, the workflow for using this tool is much easier than in most previous releases. Even so, if the entire system requires lined or insulated ducts, you have to select all ducts for the system and then specify the insulation or lining required.

Duct sizing works 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.

Once a system is created, you can use the analytical tools to inspect the duct system for airflow, pressure, and pressure loss. From the System Browser, select the system you want to analyze (as shown in Figure 10.43). Remember, you can also Tab-select the system. With the system selected, click System Inspector on the ribbon. If the Inspector tool isn't available, it's a clue that something is wrong.

This activates the System Inspector tool, shown in Figure 10.44. Click System Inspector again and pass your cursor over the elements in the system; notice that airflow and pressure loss are defined.

As with all calculations, you may want to keep a record of this analysis as part of your project documentation. Introduced in Revit MEP 2014 was the ability to create a duct (or pipe) pressure-loss report as discussed in Chapter 8, “HVAC Cooling and Heating Load Analysis.” This tool is located on the Analyze tab or on the Modify | Duct Systems tab. Here you have a choice: either select the system first or specify the system you want to create the report on in the Analyze tab. Once the system is selected, the dialog box shown in Figure 10.45 is displayed. In this dialog, you can specify the fields to use and save the report format for later use. Clicking Generate creates an HTML web page of your results.

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/masteringrevitmep2015.
2. Open the floor plan view 1 – Mech, as shown in Figure 10.46. You can see that there are four air terminals (level hosted) mounted at 8′-0″ (2400 mm) above level and a VAV unit mounted at 11′-0″ (3300 mm). These have already been connected to a supply air system.

   

3. Hover your cursor over any one of the air terminals, and press the Tab key. This highlights the system, as indicated in Figure 10.46. 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: Rectangular Duct: Mitered Elbows/Tees
   • Offset: 11′-0″ (3,300 mm)
   • Branch duct: Rectangular Duct: Mitered Elbows/Tees
   • Offset: 11′-0″ (3,300 mm)
   • Flexible duct: Flex Duct Round: Flex – Round with a maximum length of 6′-0″ (1,800 mm)
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 10.47, 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, which needs to be rectified.

   

6. If the flex duct is too short to be displayed properly, select one of the rectangular-to-round transitions and move it 2′-0″ (600 mm) toward the center of the duct run. This stretches both the placeholder duct and the flex duct, as shown in Figure 10.48. 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.

   

7. The placeholder duct is ideal for concept and preliminary design, in which little detail is required for your drawings. Once this stage has 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 them individually or selecting everything and using the Filter tool, as shown in Figure 10.49. Then click the Convert Placeholder button on the ribbon. Check to see that your results match Figure 10.50.

   

   

8. With the duct in place, open the Section 10.1 view. 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, as shown in Figure 10.51.

   

9. Complete the duct as required to avoid conflict with the beam.