Duct Scenarios

scenairo 2Have you ever wondered how to use the Duct Scenario in the Duct Load Factor screen? If you haven’t played around with the Duct Scenarios in RHVAC this will take a little explanation.

In many houses ductwork is run through different areas in the structure. Think of it this way: the equipment is in the basement of a two story house. Some duct might run through the basement to feed the rooms on the first floor. But some of the duct has to go up through the walls to the second floor.  You might run that duct all the way to the attic and install runouts to feed each room on the second floor.

Each one of these is a “scenario.”

Scenario 1 – the duct running in a closed crawl space.

Scenario 2 – the duct that runs up the wall to the attic.

Scenario 3 – the duct that runs through the attic.scenario 3

Your loss and gains will be different because of the different locations.

In the System Data screen select the Duct Load Factors.

First define the properties for the duct run in the closed crawl space.

scenario 4Then select the drop down box in the Duct Surface Area field.

The total square foot of the house is automatically entered into the Supply and Return Floor Area sections. Our Sample house has a total of 4370 sq ft.scenario 5

But the duct that is running through the crawlspace is only serving the first floor – half the house – or 2185 sq ft.


scenario 6b





So we change the Floor are from 4370 to 2135 for both the supply and the return.

Click OK and then the Refresh Button in the middle.scenario 8




Next create a new Duct Scenario

You add a new scenario by clicking on the double arrows >> at the bottom of the screen.

scenario 10We define the duct properties for the duct running for the duct running in the attic. Now when you open the Duct Surface Area calculator we enter the amount of floor space the duct running in the attic will serve. In our example the half of the total square foot of the house is on the second floor and will be served by the duct running in the attic. Enter the Floor area in both the Supply and return section, define how many returns you plan to use and click on OK.

Now click the Refresh button in the middle.scenario 11




The duct in the inside walls is in Conditioned (Cond.) Space and won’t have a load. So we don’t need to do a scenario for it.

The system will calculate the duct surface area for the duct running in the closed crawl space and the gain and loss for this duct. The software then adds the two together to determine the gain and loss for the duct.

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