Many problems with ductwork occur when a section is run through unconditioned space. This wastes energy and can also cost hundreds of extra dollars each year. Additionally, if duct systems leak and/or do not distribute air properly throughout the house, it may make some rooms too hot and others too cold.
Leaky and unbalanced duct systems may also force conditioned air outside and unconditioned air into the house. This can draw humidity, dust, mold spores, and other contaminants into a home from the attic, crawlspace, or garage, and radon gas from the soil.
Following the procedures in the Air Conditioning Contractors of America’s (ACCA’s) Manual D: Residential Duct Systems will ensure proper sizing, location, and sealing of the system (be sure to request a copy of these calculations from the HVAC professional).
Follow These Steps to Ensure Efficiency and Comfort:
- Select the general type of air distribution system (supply and return duct systems). Consider the type of equipment selected and its location, the local climate, the structural features of the house, zoning requirements, and installation and operating costs.
- Calculate the design heating loads of each room of the house and the loads that are associated with the entire house using ACCA’s Manual J: Residential Load Calculation (eighth edition) (be sure to request a copy of these calculations from the HVAC professional).
- Size and select the specific models of the heating equipment using ACCA’s Manual S: Residential Equipment Selection (be sure to request a copy of these calculations from the HVAC professional). This precedes the duct sizing calculations because in residential applications, the blower (fan) data of the selected equipment establish the duct design criteria.
- Determine the size of all the ducts based on the room loads, blower data, pressure drops of additional components or devices, and equivalent duct lengths following the procedures in ACCA’s Manual D: Residential Duct Systems (be sure to request a copy of these calculations from the HVAC professional).
- Select and size the air distribution system devices (return grilles and supply air diffusers, grilles, and registers) using ACCA’s Manual T: Air Distribution Basics for Residential and Small Commercial Buildings (be sure to request a copy of these calculations from the HVAC professional).
- Select the insulation levels for the sections of the duct system that are outside conditioned space in accordance with the energy code. Most energy codes require minimum duct insulation levels of R-6. Generally, all ductwork should remain inside the conditioned space eliminating the insulation requirement as well as the other problems associated with ductwork running through unconditioned spaces.
("Air Distribution System Design: Good Duct Design Increases Efficiency". Energy Techinical Bulletin. U.S. Department of Energy. Accessed 24 July 2009)
Two excellent tools for affiliates and volunteers are the ENERGY STAR Ductwork Critical Details Checklist and the ENERGY STAR Ductwork "Good vs. Bad" Example Chart. Both offer information that can be helpful to people with little or no ductwork experience and provide guidance on constructing an air-tight and affordable home.