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Duct loads

How to model losses and gains for ductwork in unconditioned space

Eric Fitz avatar
Written by Eric Fitz
Updated this week

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

  1. Scan all the rooms in the building

  2. 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 are limited based on the ceilings and floors that are specified 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.

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.

  1. Select the "+" to add a duct system

  2. Select a location

    1. Note: Attic and crawl/basement duct locations are limited based on the ceilings and floors that are specified 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

  3. 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.

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