Before any HVAC loads can be run, spaces need to be created in the project file. Why? It seems redundant to create spaces, seeing as how the architectural model already has rooms created and defined, right? Not so.

In Revit MEP, spaces are not created from rooms. The same elements that define a room in the linked model define spaces in your MEP model. After you load your MEP project and link in the correct architectural model, you want to make sure that the elements that make up a room — walls, doors, ceilings, and so on — will define your MEP space accurately. Figure 9.1 shows a sample building model.

Figure 9.1. Sample building

By selecting the architectural link in your project, the Edit Type button in the Properties palette is enabled. Clicking this will allow you to enable the selected link to be Room Bounding. This option will force the boundaries of the designed rooms to define the boundaries of the MEP spaces to be modeled. Figure 9.2 shows (a) the Edit Type button and (b) the type parameter Room Bounding.

If the link is not set to be room bounding, Revit MEP will offer a warning that the placed space is not in an enclosed region, and subsequently, if HVAC analysis is attempted, Revit will not be able to calculate load data for that space.

Revit allows you to locate the spaces within the architectural model in two ways: placing spaces manually or letting Revit locate and place spaces automatically. If your design building is complex, allowing the program to automatically place the spaces for you will save time.

Figure 9.2. Defining boundaries of MEP spaces

Be sure that all the spaces created are spaces that you intend to model. Check that areas such as utility chases, furrowed columns, or air spaces in wall constructions were not included when spaces were placed. To remove any unwanted spaces from the project file, open the System Browser, which is located on the View tab under User Interface, and set the view to Zones by right-clicking the System titlebar and choosing Zones from the View drop-down menu.

Select the desired spaces to remove by highlighting the space name, right-clicking, and selecting Delete, as shown in Figure 9.3. Click OK in the pop-up window to permanently remove the spaces and any associated space tags.

If you were to just select a space within an open view and delete it without using the System Browser, the space tag and visible space marker will be deleted, but the space information will still be present in your project. This will affect any building analysis performed, as well as any HVAC systems that will be set up in the building. Systems are covered in depth in Chapter 8 and Chapter 10.

When simulating the heating and cooling loads of a building, correctly modeling and accounting for space usage can be time-consuming. Often, design loads have to be revisited because of inaccurate or incomplete accounting of space usage, components, internal loading, or design changes of the space use. Creating a working schedule of the building space properties in Revit MEP will help you account for and coordinate these factors.

Figure 9.3. Deleting unwanted spaces from the project file

Similar to other schedules in Revit, you are given a choice of several fields to display in the Space Properties schedule to help track and modify data pertaining to each space. Setting up a working schedule such as this one for space properties will be an essential tool that will enable you to track how changes in certain properties will affect the spaces. Note that the schedule shown in Figure 9.4 is not intended to be provided to your client on a project sheet but rather to be used as a personal design tool within Revit to help you, the designer, organize the space data to better fit your personal workflow. Create this schedule as you see fit, using the many fields, sorting, and other formatting options available to you within the program to readily organize the data you need.

Useful fields to display, during the initial space creation, include the following:

Figure 9.4. Sample Space Properties schedule

You will notice that the program defaults to a generic space naming and numbering convention when placing spaces. It also sets the Space Type and Construction Type values as <Building>, indicating that the spaces will be modeled generically, relying on default global building characteristics within the project.

How a space will be used is the overall factor driving the internal loads within a space. An enclosed office space is modeled differently than the conference room next door, and an office break room is modeled differently than a typical restaurant dining room. Lighting, population density, activity levels, equipment, and ventilation loads are all major factors that will vary with the space type.

If you open the Manage tab and then select MEP Settings

Figure 9.5. Building/Space Type Settings

Under the Building Type category, you are able to choose a global usage for the building. This can be a good option if you want to do a quick takeoff of a typical building type for the overall model. You are given similar parameters to model as in the Space Type Settings window, as well as options to set building operating times — Opening Time and Closing Time — and an unoccupied space cooling temperature set point.

Revit MEP has several predefined options to model how each space is utilized, and each option has preset parameter and schedule values for the internal loads. Parameters such as Area per Person, Lighting Load Density, Power Load Density, and Heat Gain per Person should all be familiar to you, the mechanical designer. You may use the preset values within the program, or you are allowed to input more accurate design load values based on typical code-driven or industry-standard values.

Note that you can access the Space Type Settings window from the Properties palette when a space is selected in your view, as well as from within your space schedule by locating the cell in the Space Type column (if used in your space schedule) for the space you would like to modify and then selecting the ellipsis next to the current type name. If you click the People or Electrical Loads buttons on the space Properties palette, the default values within these windows are the values listed in the space type settings (see Figures 9.6 and 9.7).

Figure 9.6. Default Occupancy and Heat Gain values

Figure 9.7. Space Type Settings window

Revit MEP also has default building and space schedule types that it uses to generate load data. You can modify the default schedules or create new schedules to fit your varying occupancy, power, and lighting needs. You can find Default Schedules, People Heat Gain, and Building and Space Type Data in the Revit MEP 2011 Help file, under Reference

Real World Scenario: Going Above and Beyond

Doug has an issue. He has received the preliminary building model from the architect but has not been able to set up each individual space to get an accurate building load takeoff prior to the next project design meeting. Knowing that Revit MEP automatically defaults to a global building type, he adjusts the building type to Office, adjusts the required parameter values, and tells the program to lay out the spaces automatically.

He is able to quickly generate initial heating, cooling, and ventilation loads, and with that data, the mechanical design team will be able to analyze different system possibilities to condition the building and present options to the project team.

The next step in generating HVAC loads for your design building is to group similar spaces into zones. The main purpose of zoning spaces in an HVAC system is to provide controllability of the space air quality or condition. A single terminal unit, heat pump, or air handling unit can control the temperature and air quality of the spaces within its defined zone.

Creating HVAC zones within your building allows the designer to control the airflow to given spaces, shutting off airflow to areas that are not occupied or increasing airflow to spaces when the space load increases, via a signal from a temperature sensor or other sensing devices within the zone to a central control panel. Zoning also allows certain spaces to be controlled via a different system than the rest of the building, such as having a dining area and kitchen served via a dedicated outdoor air system that is cycled on with increasing occupancy to the space, while the rest of the building is controlled by a packaged VAV rooftop system on a typical office's 8 a.m. to 5 p.m. schedule.

Zoning spaces in Revit MEP is easy. On the Analyze tab, click Zone. This activates the Zone tool, and you are automatically prompted to add the first space to the zone. Clicking a space adds it to the zone, and then you have the option of adding other spaces to that zone. Click the Finish Editing Zone button on the ribbon to end the editing session.

To edit a zone that has already been created, simply select the required zone in the open view, and the Modify HVAC Zones tab appears. Click the Edit Zone button, and the zone creation ribbon appears. Here you are able to modify the zone by adding or removing spaces.

A second way to create a zone is to first select all the spaces you want to group together, click the Analyze tab, and select the Zone tool. Creating a zone this way automatically groups the selected spaces, without having to select each space individually with the tool active.

Selecting a zone displayed in an open view, the zone data appears in the Properties palette. From there, you are able to view the calculated heating and cooling loads and zone airflow (once Heating and Cooling Analysis has been run), the physical data of the zone (area, volume, and perimeter), and its characteristic data, which include the following:

So, now all the spaces within your design building are placed, the internal data has been identified, and the spaces have been grouped into zones ready for the heating and cooling data to be analyzed. The next step is to investigate the physical construction of your building's exterior and interior walls, roofs, ceilings, glazing, doors, and floor slabs. Each of these elements controls how heat enters or leaves your design spaces to or from the surrounding outdoor environment. Each element has a specific coefficient of heat transfer, or U-value, that is dependent on the element's material composition and assembly, as well as its thickness.

But the architect who created the building elements has already defined the wall or roof construction. That data should already be loaded in with the link, right? Well, no. Although the architectural model will have accurate wall and roof constructions modeled in the linked building, the U-values will not transfer from the link. In the MEP model, the building construction parameters have to be defined by you. This will set U-values for the different elements that bound your design spaces, allowing for heat transfer into and out of the spaces to be calculated and accounted for.

To set the building construction U-values, open the Manage tab, and click Project Information.

From this window, click the Edit button for Energy Settings, and then click the ellipsis next to <Building> in Building Construction. This brings you to the Building Construction window (see Figure 9.8).

Once here, you will be able to define the default construction characteristics. Clicking the drop-down arrow at each category exposes a wide array of common material constructions and assemblies that you can choose from to represent the design building walls, slab, roof, and glazing, each with a unique U-value associated to it.

Unfortunately, as of this version of Revit MEP, the constructions offered for each category in Building Construction are the only constructions that are available to use. The option to edit or customize wall or roof assemblies or even to alter the thermal properties of the materials that make up the assemblies is not present in this version, nor can you enter specific U-values, such as ASHRAE Standard 90.1 building envelope values. As the designer, you must choose the construction option that is closest to the actual U-value that has been calculated through material thermal takeoffs of the architectural design.

Just as all spaces are not going to be conditioned alike, some spaces have a different physical construction than the rest of the design building. For example, a utility space such as a mechanical or electrical room typically can be seen with exposed block wall construction, bare floor slab, open roof trusses or beams, and little or no insulation on the exterior walls. In Revit MEP, just as the individual space types can be defined differently from the overall building type, each individual space has the option to define its own construction U-values.

To alter the space construction, select a space in an open view; or, if you have created a Space Properties schedule, select the ellipsis next to <Building> in the Construction Type column of the space that you want to edit. The Construction Type window will open, and here you will be able to create individual space construction types as needed (see Figure 9.9).

Figure 9.8. Building Construction window

The same construction categories and options exist here that are available in the Building Construction window. Create as many different constructions as needed, and if there exists, in your design building, several spaces that utilize a different construction from the main building, you will be able to easily select the appropriate construction from your created list. Note that you are not able to alter the default <Building> construction type in this window. This is accessed from the Building Construction window, mentioned earlier.

You now have a complete building modeled and ready to analyze. The next step is to verify and specify, if needed, the building energy parameters, as shown in Figure 9.11. On the Analyze tab, click the Heating And Cooling Loads button. The Heating and Cooling Loads window will appear, and on the General tab, you will be able to view the building energy analysis project information that directly affects the heating and cooling analysis. You can also access these parameters through the Manage tab's Project Settings

Figure 9.11. Heating and Cooling Loads window

In this window, you are able to set parameters such as the global building use type, building location, the global conditioning system, overall building construction and ground plane, the ground level reference, and building infiltration. You are also able to define the level of load analysis report detail you want to see.

We discussed earlier in the chapter building type, service type, and building construction, but another major component of energy analysis is the physical location of the building. The location determines environmental conditions such as outdoor air temperature and humidity and also ASHRAE climate zone data. To define the building location, click the ellipsis next to the city and state in the Location cell. You will be taken to the Location dialog box, as shown in Figure 9.12.

You have two ways to input the project design city: select the location from the default city list, or use the Internet mapping service, as shown in Figure 9.12. If you are connected to the Internet, this option allows you to locate your project using an interactive map through the Google Maps mapping service. The default city list contains major cities from which to select your project location. Select the major city from the list that is nearest to the project location address, or, if it is known, you can enter the latitude and longitude coordinates for a more exact location. Select Use Daylight Savings Time if it is appropriate for your project location.

The next step is to modify the cooling and heating design temperatures, if needed. You may use the default values that are associated with the closest weather station to your design city; or, by deselecting that option, you are able to modify the Dry Bulb, Wet Bulb, and Mean Daily Range temperatures as needed to fit your design location. You may specify the Heating Design Temperature value and set the Clearness Number value, which ranges from 0 to 2, with 1.0 being an average clearness. Clearness is defined in section 33.4 of the 2007 ASHRAE Handbook – HVAC Applications in the following way:

Figure 9.12. Location Weather and Site

The Building Infiltration class needs to be set next. You are given four choices to model the rate of outdoor air that enters the building, typically through leaks in the building envelope created at envelope openings such as windows, doors, and where perpendicular building surfaces join. In Revit, infiltration is defined with the following categories:

The next parameter to define is Sliver Space Tolerance. Sliver spaces in Revit are considered to be narrow areas that are bounded by parallel interior room-bounding components — parallel interior walls. These spaces include, but are not limited to, pipe chases, HVAC shafts, furrowed columns, and wall cavities. A sliver space is included in the heating and cooling load analysis only if identical parallel room-bounding elements enclose the space, the width of the sliver space is equal to or less than the Sliver Space Tolerance parameter, and if a space component has been placed in the tangent spaces on either side of the sliver space. If any one of these three requirements is not satisfied, Revit does not recognize any effects of the sliver space. If there are different geometries to the same sliver space, only the areas of the space that meets the previous criteria is counted in the load analysis. The sliver space volumes are added to the volume of the larger tangent analytical spaces.

Define the detail level of the heating and cooling analysis report. Three report detail levels are available: simple, which contains summary data for systems, zones, and spaces; standard, which expands the simple report to include psychrometric data as well as building level summaries and load summary data for each space; and detailed, which further expands the data displayed to include individual component contributions to zone and space loads.

Before you finish, define the ground plane, project phase, and whether to include heating or cooling load credits, which are negative load values that come from heating entering or leaving a space through a partition into another zone, for example.

Before you analyze the building performance, you are given the opportunity to go through your created zones and spaces to make sure your desired settings have not been compromised and to make sure there are no warnings that would produce undesired effects on your loads. Switching to the Details tab, you can view the space and zone data that directly affects the heating and cooling analysis, as in Figure 9.13.

Figure 9.13. Building model details

Selecting a zone, you are able to expand the tree to view the associated spaces and verify or modify the zone service type, heating, cooling, and outdoor air information as needed. You can see whether a space is occupied, unoccupied, or a Plenum space by the symbol preceding the space name in the tree.

If a space is selected, you can set the space and construction type, as well as internal load information. If, when expanding a zone, a warning symbol appears alongside a space, you are able to investigate by clicking the Show Related Warnings symbol and then correct or ignore the cause of the warning (see Figure 9.14).

Figure 9.14. Warning

The Space Warning shown in Figure 9.14 suggests that a space exists without an upper bounding element. Click the Save Settings button, and you will be directed back to your model. Locate the space in question, and verify that the upper limit of the space is correct through the space Properties palette. You can also inspect the space visually by creating a section view through the space and verifying that the upper limit extends past a bounding element in the linked model.

Opening the Heating and Cooling Loads window and selecting Analytical Surfaces allows you to view and isolate the physical elements that bound the spaces to be analyzed. You are able to isolate every individual bounding element that has been defined for the space — roofs, exterior and interior walls, ceilings, floors, and any air gaps or sliver spaces — and view them for any modeling errors, before the simulation is performed.

Go ahead, click the Calculate button.

After the simulation is completed, you are directed to the Heating and Cooling Loads report, as shown in Figure 9.15. Depending on the level of report detail you selected prior to running the simulation, the tabulated results will be shown, broken into an overall project summary, a building summary, building level summaries, individual zone data, and individual space data.

Figure 9.15. Sample Heating and Cooling Loads Report

The Project Summary area lists the project information (name, address, location), as well as calculated design date and time, Summer Dry Bulb and Wet Bulb temperatures, the Winter Dry Bulb temperatures, and the Mean Daily Range — values that should match the inputs you have entered with the weather data. The Building Summary area includes the global building type and its total analytical area and volume, as well as the overall calculated performance of the building — Peak Cooling Loads, Peak Heating Load, Airflows, and building checksums.

The Level Summary area includes the analytical area and volume of each level of the design building, if applicable, as well as each level's individual performance values, similar to the Building Summary. The Zone Summary lists each analyzed zone, listing its inputs, psychrometrics, and the calculated performance results. It also breaks down the various cooling and heating components and displays a list of the spaces that make up the zone along with a brief summary of the space performance. The Space Summary displays the space analytical areas and space volume, load inputs, and space type, as well as the calculated results for the space. It, too, contains a breakdown of the individual space components and how they contribute to the cooling and heating loads.

Revit MEP allows you to run a heating and cooling load analysis, make changes, and run subsequent analysis all while retaining the reports run for each analysis. Load reports are individually timestamped and can be accessed in the Project Browser under Reports. This enables you to easily flip to a previous report and quantify the changes in your design, without resorting to printing out each report as it is generated. Each report can grow to several hundred pages in length, depending on the size of your job and the detail level selected.

Now is a good time to set up and run a sample HVAC load analysis in Revit. Here's how: