Anytime you have ductwork that is in an unconditioned space, it is crucial to model heat gains and losses for an accurate load calculation. The good news is that it is super easy to do.
Overview
Scan all the rooms in the building
Add one or several duct systems to the home
Scan all the rooms in the building
In order to add duct systems, you must first scan all conditioned rooms in the building and take care to set proper assumptions for the ceilings and floors.
This is important since the attic and crawl/basement duct locations only available for duct locations if you have identified them as ceilings or floors in scanned rooms. I.e. If an attic ceiling doesn't exist in at least one room, an attic duct location will not be available for duct losses.
Add one or many duct systems
Note: Only add duct systems that are in partially or fully unconditioned spaces. E.g. an unconditioned attic or unconditioned basement /crawl space. Modeling loads for ductwork in a fully conditioned space, like a finished basement, will result in incorrect and grossly oversized loads.
Select "Ducts" in the Building Profile
Set the HVAC System Type to ducted and add a duct system
Indicate that the HVAC System Type is a Ducted system if you haven't already. Ducted is the correct value for a fully ducted system or a mixed application with both ducted and ductless systems present in the home.
Select the "+" to add a duct system
Select a location
Note: attic and crawl/basement duct locations only available for duct locations if you have identified them as ceilings or floors in scanned rooms. I.e. If an attic ceiling doesn't exist in at least one room, an attic duct location will not be available for duct losses.
Answer questions about the type of return, supply and layout
Set final assumptions for the duct system
After setting a location and layout, you can specify more granular details:
Duct insulation level: R-value of the insulation on the ducts
Leakiness: How sealed the ducts are.
Percent Unconditioned: Adjust this if a fraction of the ducts are in conditioned space. For example, for a partially finished basement where some of the ducts are in the conditioned area and a fraction are in unconditioned space.
Stories Covered: Indicate what stories of the building the duct system serves. For example, an attic system in a larger home will often only serve the second story of the building.
Use Explicit Areas: If you know the exact amount of duct surface area (both supply and return) you can enter it here. This is typically only used if you are proposing brand-new ductwork and have the surface area information available.
Add additional duct systems where applicable
If you have several independent duct systems, such as a basement system and an attic system in a larger home, simply add another duct system so that they can be modeled independently.
Duct loads and rooms
Duct loads are allocated to rooms that the ducts are connected to automatically based on the Stories Covered by the duct system. For example, if an attic duct system is labeled as serving the 2nd story of the home, then losses/gains for that duct work will be allocated to all the rooms in the 2nd story.
Duct load fractional allocation
Duct loads are allocated to each room they are related to, based on the non-duct loads in each room.
For example, let's focus on heating loads only, ignoring cooling sensible and latent for simplicity for now.
Let's pretend that we have an:
Attic duct load of 6,000 Btu/h
And on the 2nd story of the home, we have three rooms with these heating loads:
Bed 1 = 5,000 Btu/h (non-duct load)
Bed 2 = 4,000 Btu/h
Bath = 1,000 Btu/h
Total 2nd story heating loads = Bed 1 + Bed 2 + Bath =
5,000 + 4,000 + 1,000 = 10,000 Btu/h
For reference, Attic duct load + 2nd story heating loads = 6,000 + 10,000 = 16,000
That means that:
Bed 1 has 50% of 2nd story loads
Bed 1 / Total 2nd story heating loads
Bed 2 has 40%
Bath has 10%
So, we then allocate the duct loads based on those fractions.
Total room loads become:
Bed 1 + 50% x duct load = 5,000 + 0.5 x 6,000 = 8,000 Btu/h
Bed 2 + 40% x duct load = 4,000 + 0.4 x 6,000 = 6,400 Btu/h
Bath + 10% x duct load = 1,000 + 0.1 x 6,000 = 1,600 Btu/h
So, the final numbers are:
Bed 1 = 8,000 Btu/h
Bed 2 = 6,400 Btu/h
Bath = 1,600 Btu/h
Notice that 8,000 + 6,400 + 1,600 = 16,000, which is the same total we had before for Attic duct load + 2nd story heating loads above.
This example was only for heating loads, but the same methodology is used for allocating cooling sensible and latent loads from ducts to rooms.