Three Temperatures Model

Reading Time: 10 minutes

Created with software version: 2025.1

Introduction

This tutorial details the steps needed to:

Run multiple seasonal conditions with Scenarios in Ventus
Apply zone temperature schedules in Ventus
Run a transient simulation
Quickly export key results and streamline reports.
Iterate and re-run the model to define Pressure Differential Systems (PDS) and achieve regulatory compliance.

Necessary files are in this zip folder: condo-three-temperatures.zip. For information related to the Ventus starting model, as in Figure 1, see Ventus Validation #3 tutorial.

Requirements

In this example, you are asked to design an appropriate PDS to meet fictional mid-rise building requirements:

Pressure differentials on the doors connected to the stairwell must be between +0.10 and +0.45 inches water column (inWC). A positive differential is defined from the stairwell to surrounding volumes.
Air flow leakages must be greater than +6.56 ft/s (2 m/s) for these doors, with the same sign convention.
Air flow leakages must also be greater than +6.56 ft/s (2 m/s) for each door between condo units and the hall, on all levels. A positive direction is defined from the hallway to the condo unit.

You may assume total symmetry of the building, and only examine Unit 1 and Stair 1. In our model, the x-coordinate is measured along the east–west axis, the positive y-axis points north, and the z-coordinate measures height. Therefore, Unit 1 and Stair 1 are on the West side of the building.

Modify the file for a three temperature simulation

Chose either US Customary (EN) or Metric (SI) units for your model. Conditions are found in Table 1. The starting model and this tutorial were made in US Customary units.

Table 1. Starting File Conditions.

Zone	Temperature ( F)	Temperature ( C)
Ambient	-4	-20
Building	73	22
Stairwells	8	-13

Open the file and confirm the following:

The design flow rate for each AHS Supply (on Level 9.0 ft) is 2500 SCFM per supply.
In the Simulation Parameters menu, the Default Zone and Junction Temperature is set to 73 F as in Table 1.
In the Weather and Wind menu, the Ambient Temperature is set to -4F and the Wind Speed is set to 0 ft/s.

Tag flow elements needed for the pressurization study

Tags can be used to quickly select objects on different levels. Use the control key to select multiple objects.

In this case, it is useful to tag the flow paths needed for meeting requirements.

Select all the relevant flow paths on the West side of the building, as in Figure 2 and tag them by entering study and pressing Enter. You can enter a space-separated list of tags, prompting Ventus to create the tags in the object tree.
To review the selection, in the object tree expand tags and double-click on study. If you select select all, tagged objects will be selected and highlighted in the 3D view window.

This will be useful later to isolate important model results.

Figure 2. Tagged group of flow paths on the West side of the condo building, needed for the PDS study.

Create temperature scenarios

The weather changes as shown in Table 2.

To enter two more scenarios:

In the Weather and Wind menu, use the edit command for Ambient Temperature.
Enter additional values: 73 and 105 in Farenheight (22 and 41 in Celcius) and select OK. This will create multiple scenarios in Ventus and three associated results folders to organize results data.

Table 2. Temperature Scenarios for Ambient Weather.

Model Condition	Temperature ( F)	Temperature ( C)
Low	-4	-20
Isothermal	73	22
High	105	41

Create Zone temperature schedules

Often, when modelling seasonal variation, unheated spaces like the stairwells are not controlled. It is necessary to simulate the stairwell temperatures to accurately predict stack effect and airflow.

Table 3. Stairwell Temperatures for Each Scenario.

Model Condition	Temperature ( F)	Temperature ( C)
Low	8	-13
Isothermal	73	22
High	89	32

Create a temperature schedule for stairwell zones:

Select all stairwell zones. Make sure to avoid selecting the stair zones on Level 7, since the stair zones on that level are ceiling type areas above stairwell shafts. It may help to hide flowpaths and AHS zone points.
In the properties panel, toggle the temperature setting box. Turning temperature setting ON imposes a set condition onto the zone so that the zone will no longer be defined by the model’s default temperature. The zone definition should change as in Figure 3 so you can select schedule in the drop down menu. This will enable the schedule editor.
Select the hyperlinked schedule to open the schedule editor dialogue.
Enter the stairwell temperature values in Table 4 for each hour so that the Temperature over Time looks like Figure 4. Leave the function Shape as Rectangular. You can also copy/paste the schedule from the "Schedules" sheet of the excel workbook, included in the ZIP folder.

vnts scrn view 3temps zone temp set — Figure 3. Set a zone temperature schedule.

vnts scrn dialog 3temps scheduled temp change — Figure 4. Ventus stair zone temperature schedule.

Putting it together

For each weather scenario, Ventus will run a 3 hour simulation and simulate the stair temperatures changing at 1 hour and 2 hours. Therefore, the viable data is presented in Table 4

Table 4. Putting Together Scenario and Schedule Data.

Weather	Scenario Ambient Temperature	Stair Temperature	Simulation Time
Low	-4 F (-20 C)	8 F (-13 C)	0-1h
Isothermal	73 F (22 C)	73 F (22 C)	2h
High	105 F (41 C)	89 F (32 C)	3h

Weather

Scenario Ambient Temperature

Stair Temperature

Simulation Time

Low

-4 F (-20 C)

8 F (-13 C)

0-1h

Isothermal

73 F (22 C)

High

105 F (41 C)

89 F (32 C)

But how will temperature schedules interact with the weather scenarios?

By changing the temperature of these zones each hour and exporting data away from the model temperature changes, this model will be quasistatic. The model is solving airflow at each time step, without accounting for density variations between time steps. A visual representation of this method to create temperature differentials is presented in Figure 5. Static data can be taken at 30 min, 90 min, and 150 min. In the Simulation Parameters menu, change the Airflow Simulation Method to Transient with a Simulation Time of 3 h and a Time Step of 30 min.

Vis of method — Figure 5. Visualization of temperatures over time for the Ventus model.

Run the model

Immediately after running the model, the Ventus time-step bar pops up on the bottom of the view window. Investigating the results, uh-oh! There are negative flow paths for airflow and pressure at the doors to each condo unit at 0:00.

Bad sign fp — Figure 6. Getting a negative sign for flow through the door on Level 2 in Ventus 3D results.

The flow paths between unit 1 and the hallway, on all levels, were defined positive from condo to hallway. According to the requirements, a positive direction should be defined from the hallway to the condo unit. The current results have an opposite sign convention, requiring a change to settings.

Change the direction of some flow path sign conventions

On each level, select the Door-SC type flow path between Zone04 and Zone00. These flow paths should all be tagged as part of the pds-study group. The name will be similar between levels 2 through 7.

Ventus assigns the names of zones by numbering them upon creation, and then appending the name when the flow path is copied. For example, since the flow paths between unit 1 and the hallway were copied from level two to levels 3-6, the flow paths are copies of flowpath42 as in Table 5. This way, when filtering for results, copies of the flowpath, which are often aligned vertially, share letters, making them easily searchable.

Table 5. Names of flow path between Unit 1 and the hallway.

Level Name	flowpath name
Level 1	flowpath25
Level 2	flowpath42
Level 3	flowpath42_1
Level 4	flowpath42_1_1
Level 5	flowpath42_1_2
Level 6	flowpath42_1_3
Level 7	N/A
Roof	N/A

Level Name

flowpath name

Level 1

flowpath25

Level 2

flowpath42

Level 3

flowpath42_1

Level 4

flowpath42_1_1

Level 5

flowpath42_1_2

Level 6

flowpath42_1_3

Level 7

N/A

Roof

N/A

With a flow path selected, the properties panel will show the direction of positive flow Simply select the arrow to switch the direction as in Figure 7.

switching fp sign — Figure 7. Changing the flow path sign convention with the properties direction arrow.

Repeat for all the flow paths in Table 5. The model must be run again to change the sign of results associated with these flow paths.

Re-run the model

Now, result values will adhere to the sign convention of the requirements. Examine the flow paths. Note that the values have changed in the Results panel, but the graphics in Ventus remain in the same direction as in Figure 8. This is because the vectors themselves point in the direction of flow.

Good sign fp — Figure 8. Corrected results sign convention on Level 2, to obtain positive pressure and flow values from Ventus.

Export Results

Open the results Path Data results panel, which contains a results table and filters.

To isolate relevant data:

Use the tag study to quickly select all relevant flow paths. Tip: use ctrl+z if you lose the selection group at any point.
Toggle the selection tool icon ON to filter for results associated with the selected objects.
Toggle the time step filter tool OFF to show all transient data
Ensure that the scenarios filter is set to <All> data. The settings should match those in Figure 9.

Figure 9. Filters for efficient data mining.

To transfer the data:

Use ctrl+c to copy the data.
Open the appropriate excel workbook for your unit selection. In this demonstration, we open condo-three-temperatures-EN.xlsx.
After reading the release notes, switch to the sheet Ventus-results-table and paste the data from ventus into the table.

You have now successfully exported the data.

Filter between scheduled zones and weather data

It is important to isolate data between each scenario and the time-dependant stairwell temperatures. Remember that viable data is a combination of the scenario weather and the scheduled stair temperature in Figure 5. Use the Data slicers in excel to isolate the appropriate data. For example, the data for the low temperature scenario can be filtered and analyzed as in Figure 10.

Examine criteria and simulation data

With the data in Excel, create your own plots, pressure differential requirements, and air flow evaluation. The included spreadsheet is a starting point for whatever workflow you follow.

excel scrn view 3temps dP criteria not met — Figure 10. This pds fails to maintain the differential pressure requirement.

The pressure differential criteria is not met for this pds system. We will need to modify the stairwell pressurization by increasing the AHS Supply flow rates.

If desired, you can export the results data from ventus before changing the model.

In the toolbar, under Results select Show Link-Path Flows to dump data into your web browser.
From your browser, save the exported .html file something like condo-three-temperatures-2500scfm.

Change the model, and re-run the simulation

Iterate to improve the PDS:

On Level 2, select both AHS supply objects.
Enter a Design Flow Rate of 3300 SCFM.
Select run, over-write previous results, and follow steps in Export Results again to update the excel data.

The scenarios and schedules enable you to update geometry once only while Ventus updates all three seasonal conditions for analsis! Export and examine the new results.

excel scrn view 3temps dP criteria met — Figure 11. This pds does maintain the differential pressure requirement.

excel scrn view 3temps flow criteria met — Figure 12. This pds also meets the airflow requirements.

Examine specifics in context

In context of ventus, you can identify and troubleshoot any relevant flow path. For example, if the Authority Having Jusisdiction (AHJ) decides that the maximum pressure differential should be lowered to 0.25 inWC (62.2 Pa), you would realize that the stair exit doors have pressure differentials exceeding the requirement for AHS supply units providing 3300 SCFM in the isothermal and high temperature conditions. To troubleshoot, you can use the playback bar, selection tools, and the results filters as in Figure 13.

vnts scrn view 3temps isolate door1 — Figure 13. This flow path would fail the new requirements.

Conclusion

This tutorial provided an example process to efficiently group scenarios in one Ventus model and maintain data integrity.

To download the most recent version of Ventus, please visit the Ventus Download page. Please contact support@thunderheadeng.com with any questions or feedback regarding our products or documentation.