It is important that you are able to check for static pressure on the range for your duct system. This is something that can be very beneficial to you for several different reasons.
Step 1 - Select Critical Path
You will need to begin by selecting your critical path as the longest branch that is between the terminal outlets and the fan. Get the total pressure losses assigned to each of your critical paths as the recommended amount of pressure loss per the unit of the length. You will multiply this by the section’s actual length.
Step 2 - Calculate
The cross sections for your critical path need to be calculated by using the total pressure losses. The pressure loss that you have can’t be calculated until you have the branched cross sections assigned properly. Ignore the pressure loss and assume constant pressure for the junctions. Get the pressure losses in the critical path’s sum and choose a fan. Total pressure should be assigned at the critical path node.
Step 3 - Use Software
It is usually a good idea that you have software that can help you with all of this as it is difficult to do on your own.