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Appendix B: Duct Leakage and House Pressure Concepts

In their most basic configuration, forced air central heating and cooling systems circulate air through a distribution system that includes a return duct, an air handler, and a supply duct network. Conditioned air from the house is drawn into the return duct by a fan in the air handler. The fan pushes the conditioned return air over a heating or cooling coil in the air handler. After passing over the coil, air flows through supply ducts to the various supply registers throughout the house where it reenters the conditioned space.

Ideally, the same amount of conditioned air is removed from the house (via the return duct) that is put back in the house (via the supply ducts): volume of return air should be equal to the volume of the supply air. When there is an imbalance, a positive or negative pressure with respect to the outdoors will result. This is true for individual rooms also. If more air is being supplied to a room than is being removed, a positive air pressure (with respect to adjacent spaces, such as the main body of the house) will result. Vice versa, if more air is being removed from a room than is being supplied, a negative pressure (with respect to adjacent spaces) will result.

Extra energy is used to condition outside air in both situations. When rooms or whole houses are pressurized or depressurized, air will move through small cracks and holes to equalize the air pressure. A pressurized room is like a balloon with a small hole. Air is pressing out all sides and seeping through even the smallest break in the air barrier. In a house, this might mean that warm moist air is being pushed into the exterior wall cavity, toward the cold backside of the exterior wall finish. A depressurized room will pull air in from outside bringing outside air in contact with the backside of interior finishes. The resulting conditions may support mold growth, condensation, rot, or material wetting. Thus, the implications of duct leakage range from energy efficiency, to health, durability, and safety.

Air Distribution System Components and Joints
In a 2002 study conducted by the Florida Solar Energy Center, 69 new, installed air handler cabinets were tested and found to have an average leakage of 20.4 cfm when under a 25 pascal pressure difference (Q₂₅, similar to CFM25). This includes return and supply side leaks. Average leakage at the joint between the air handler and supply plenum was 1.6 cfm Q₂₅ and at the joint between the air handler and the return plenum was 3.9 cfm Q₂₅. These numbers pale in comparison to the leakage measured in the 20 full duct systems tested: average Q₂₅,Supply of 53 cfm, Q₂₅,Return of 134 cfm, for a Q_25Total 187 (Cummings, et al, 2002).

In these 20 homes, the full duct system leakage exceeds the air handler leakage (including plenum connections) an average of 13 to 1. Also note that the return duct leakage exceeded supply duct leakage 3 to 1.

Though ducts and plenums are made of continuous, virtually leak free materials such as foil backed duct board, sheet metal, and sleeved flex duct, they consistently leak. Though flex duct is vulnerable to puncturing, this can not explain the tremendous leakage occurring in even new duct systems. The explanation lies in the fabrication joints and the connections between components.

Joints between ducts are part of the air barrier and have historically been sealed with duct tapes. UL 181 approved tapes are still in use today. An alternative method of sealing duct joints combines the tensile strength of fiberglass mesh and the elasticity of mastic. The seal created by mesh and mastic enjoys a more durable, longer life than the seal formed by tape.

Primary Leakage Points
Most new homes have a single central return with a return grill or a simple return system with far fewer inlets than supply registers. Supply registers are commonly scattered throughout the house to spread conditioned air evenly through the space. Between the return grill and the supply register, there are a multitude of joints and connections presenting possible leakage sites. The following list of common duct system joints combines elements of different types of systems and materials common in the manufactured housing industry:

Ceiling Systems
Return grill to dry wall (sheet metal/duct board)
Return grill to return plenum (sheet metal/duct board)
Return plenum (sheet metal/duct board) to air handler or drop out to package unit
Air handler cabinet joints
Supply plenum (sheet metal/duct board) to air handler
Supply plenum (sheet metal/duct board) to main trunk line(s) (flex)
Main trunk lines to flex collars
Flex collars to supply ducts, both ends (flex)
Flex collar to supply boot (duct board)
Supply boot to drywall
Supply boot to supply register
Supply register to drywall

Floor Systems
Return grill to dry wall
Return grill to return plenum (sheet metal/duct board)
Return plenum (sheet metal/duct board) to air handler
Air handler cabinet joints
Supply plenum (sheet metal/duct board) to air handler
Supply plenum (sheet metal/duct board) to main supply trunk line(s) (sheet metal/duct board)
Main trunk line(s) (sheet metal/duct board) to floor riser or collar
Floor riser (sheet metal) to sub-floor (In-line floor system)
Collar to branch duct (sheet metal/flex) (Perimeter floor system)
Branch duct (flex/sheet metal) to collar
Collar to floor riser/supply boot (sheet metal)
Riser/supply boot (sheet metal) to subfloor

In addition to these joints, all the surfaces of all the components in the air distribution path should be free of holes, tears, cracks, punctures, gaps, etc. This includes the fabrication seams in the components. The entire inside of the duct system from the return air inlet to each supply air outlet* should be an air tight, planned air flow path.

A note about return air paths
An additional factor contributing to unbalanced house pressures include inadequate return air paths from bedrooms. This creates positive pressure in the bedrooms when the bedroom door is closed and air is trapped. At the same time, a negative pressure results in the main body of the house as the air handler continues to draw the same amount of return air from the open part of the house. Thus infiltration is induced in the main body, while exfiltration is induced in the closed room(s). Though this is not a direct result of duct leakage, it is a result of having a central return rather than a ducted return from each private room. Methods of overcoming the limitation of return air from bedrooms include providing ducted passive return paths (jump ducts or saddle ducts) or through wall passages (high-low vent or over door) for return air to move from bedrooms to the air handler. In diagnostic testing, BAIHP has documented the impact of this phenomenon on house pressure, resulting in recommendation to balance supply and return air into private rooms using passive devices such as through wall/door grills, high-low wall grills, jump ducts (also called saddle ducts), or individual return grills ducted to a central return plenum.

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Cummings, J.B., C. Withers, L. Gu, J. McIlvaine, J. Sonne, P. Fairey, M. Lombardi, 2002. Field Testing and Computer Modeling to Characterize the Energy Impacts of Air Handler Leakage. FSEC-CR-1357-02, Florida Solar Energy Center, Cocoa, Florida.