Many forced-air furnaces use galvanised ductwork to distribute conditioned air to designated spaces. Installers must consider a variety of factors to ensure each room has the proper airflow. Ductwork connects to a plenum, a rectangular-shaped galvanised sheet metal box attached to the furnace, which serves as a chamber for mixing warm or cool air. Ductwork moves the conditioned air to various rooms. The installer must decide on the type of ductwork distribution that works best for the particular project -- radial or extended plenum.
Radial ductwork consists of round ducts that connect directly to the main or central plenum. The ducts run to a wall, ceiling or floor register to each room. Position the heating unit in a central location. Radial pipes work best in attics or crawl spaces. Install pipes between floor joists. Do not use radial pipes in lengthy or oddly shaped structures.
An extended plenum system employs a combination of round and rectangular ducts to circulate conditioned air to spaces. The proper size for an extended plenum line will depend on the number of round pipes the installer plans to attach to the component. Compute the plenum's width by counting the number of round pipes connection. When installing 6-inch round pipe, multiply the number of pipes by two and add two. This will give you the necessary duct width. For 8-inch pipe, multiply the number of pipes by three and add two to obtain the width of the trunk line.
Round pipe offers greater efficiency. Use round pipe to draw return air or for main and branch lines air distribution. Typically made of 30-gauge sheet metal, round pipes have 3- to14-inch diameters. Install 6-inch diameter pipe for heating only. For heating and cooling, put in 8-inch pipe. Connect the pipes by inserting the crimped end into another pipe. Seal the joint with mastic.
Rectangular pipes also come in 30-gauge sheet metal. Use this type of pipe to form extended plenums or trunk lines. Run the ductwork in attics or between floor joists. Install a collar or take-off on the extended plenum and connect round duct for branch lines leading to the supply registers.
Branch Line Size
Calculate the square footage for each room -- length x width. Compute the "cubic feet per minute" (CFM) for the heat or air flow needed for each room. For example, one cfm heats or cools 1 to 1.25 square foot of floor space. A 150-square-foot room would require 150 to 188 CFM. A 6-inch diameter pipe carries 110 cfm; therefore, the space would need an 8-inch pipe for sufficient air supply.
One of the most important aspects of duct installation concerns outlining the straight lines for the ductwork routes. Install a duct run along an existing beam to establish a straight line. Another technique for creating a straight line of travel for ductwork consists of pulling a string pull taut. Install temporary support wires to ease the installation process. Many installers fabricate permanent hangers out of rolls of 16- or 20-gauge galvanised steel. Follow local building codes to determine the final placement of the hangers. For example, some rules require one hanger for every 10 feet of main galvanised duct, and a hanger for every 10 feet or less or branch lines.
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