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Observations

Researchers find variation among manufacturers related to duct system materials, layout, fabrication, assembly, sealing, quality control and air handler position. The primary distinction focused on in this data set is the duct sealing method. Researchers tested 132 floors with mastic sealed duct systems and 58 floors with taped duct systems (Table 1). Depending on the stage of production and the objective of the testing, researchers measured total leakage only, leakage to the outside, or both.

Duct Layout

Ducts systems observed in these tests were installed either in the attic (ceiling systems) or in the belly (floor systems). Perimeter floor systems have a main supply duct with smaller run-out ducts extending to the edges of the house. In-line floor systems have a main supply duct that is directly connected to supply registers with floor boots.

Figure 3. Duct assembly stations become a focus of improvement and quality control when factories strive to meet to qualify for Energy Star. Photo shows perimeter floor duct system with flex duct runs from duct board trunk to registers. Note this floor assembly will be flipped over after insulation is completed.

Occasional a “baby duct” will be run to serve a room that doesn’t intersect the main supply trunk, such as a master bath or a laundry room. The data set represents a mixture of ceiling and floor systems, both perimeter and in-line layout, as summarized in Table 2.

Air Handler Position and Connection to Duct System

Heating and cooling systems are either package units or split systems.

In homes designed for package systems, a duct drops from a central return plenum through the belly into the crawl space for connection to the package unit during set-up. The supply ducts, one from each section of the house, have a similar drop into the belly for connecting to the package unit. Thus, each section is directly connected to the main supply trunk. In overhead duct systems, this connection is sometimes made in the attic during set-up.

In homes designed for split systems, an air handler with a central return is installed during production. If the house is a multi-section unit with floor ducts, a drop-out collar is installed near the air handler in each trunk duct for tying the non-air handler section(s)’s supply ducts into the air handler. These are connected during set using a cross over duct (flex duct) in the crawl space for floor systems. Multi-section overhead systems are often connected through the attic during set-up.

Multiple returns are rare. Through-the-wall grills are sometimes used to provide a passive return air path from bedrooms. Researchers have observed that the HUD Code requirement of 1in² of return air area for every 5ft² of floor area served is not always adequate to prevent the main body of the house from depressurizing when bedroom doors are closed. BAIHP highly recommends adequate return air provisions to all bedrooms using passive, through wall grills or ducting from bedroom ceiling to main body ceiling.

Duct Materials

Manufactures typically install ducts made of sheet metal, duct board, and flex duct.

Duct board components which are generally assembled in the factory include supply ducts, return plenums, ceiling boots, distribution boxes, etc.

Flex ducts are used in conjunction with prefab sheet metal collars to connect duct board components. Flex ducts are secured to collars with plastic zip ties. Collars are secured to duct board with built in tabs that fold out around the edge of the duct board.

Sheet metal components, such as floor boots are generally pre-fabricated by suppliers in standard sizes. Sheet metal trunk ducts are fabricated in the factory. Table 3 summarizes the duct materials represented in the data set.

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