Minimizing Energy
Losses in Ducts

In new home construction or in retrofits, proper duct system design is critical. In recent years, energy-saving designs have sought to include ducts and heating systems in the conditioned space.

Many existing duct systems lose a lot of energy from leakage and poor insulation. Sealing and insulating your ducts can save significant amounts of energy and money. Existing ducts may also be blocked or may require simple upgrades.

Designing and Installing New Duct Systems

Efficient and well-designed duct systems distribute air properly throughout your home without leaking to keep all rooms at a comfortable temperature. The system should provide balanced supply and return flow to maintain a neutral pressure within the house.

Since even well-sealed and insulated ducts will leak and lose some heat, many new energy-efficient homes place the duct system within the conditioned space of the home. The simplest way to accomplish this is to hide the ducts in dropped ceilings and in corners of rooms. Ducts can also be located in a sealed and insulated chase extending into the attic or built into raised floors. In both of these latter cases, care must be taken during construction to prevent contractors from using the duct chases for wiring or other utilities.

Illustration of ducts inside conditioned space shows a duct within framing dropped from the ceiling. Above the duct space is solid blocking to serve as a draftstop.

Ducts located within the conditioned space avoid the energy losses associated with most duct systems.

In either case, actual ducts must be used: chases and floor cavities should not be used as ducts. Regardless of where they are installed, ducts should be well sealed. Although ducts can be configured in a number of ways, the "trunk and branch" and "radial" supply duct configurations are most suitable for ducts located in conditioned spaces.

Illustration of  supply ducts shows four configurations. The trunk and branch configuration consists of two large ducts extending in opposite directions from the air source, with many smaller ducts attached at right angles to the large ducts. The radial design features many small ducts extending radially out from the central air supply. The perimeter loop design again features radial ducts, but they connect to a loop that runs along the perimeter of the house, with vents located along the loop. The spider design features a few large ducts extending radially from the central air supply, then connecting to mixing boxes from which several smaller ducts branch out.

"Trunk and branch" and "radial" configurations of supply ducts are most suitable for incorporating within the conditioned space of a home.

Air return duct systems can be configured in two ways: each room can have a return duct that sends air back to the heating and cooling equipment, or return grills can be located in central locations on each floor. For the latter case, either grills must be installed to allow air to pass out of closed rooms, or short "jumper ducts" can be installed to connect the vent in one room with the next, allowing air to flow back to the central return grilles. Door undercuts help but are usually not sufficient for return air flow.

Illustration of return air techniques shows supply air returning through grilles in doors and walls, under gaps beneath undercut doors, through offset 'transfer grilles' that use the wall cavity to carry return air, and through a 'jumper duct' that runs over the ceiling to connect grilles in two rooms.

A variety of approaches are available to allow supply air to flow back to a central return air grille.

Maintaining and Upgrading Existing Duct Systems

Aside from sealing your ducts, the simplest and most effective means of maintaining your air distribution system is to assure that furniture and other objects are not blocking the air flow through your registers, and to vacuum the registers to remove any dust buildup.

Existing duct systems often suffer from design deficiencies in the return air system, and modifications by the homeowner (or just a tendency to keep doors closed) may contribute to these problems. Any rooms with a lack of sufficient return air flow may benefit from relatively simple upgrades, such as the installation of new return-air grilles, undercutting doors for return air, or installing a jumper duct.

Some rooms may also be hard to heat and cool because of inadequate supply ducts or grilles. If this is the case, you should first examine whether the problem is the room itself: fix any problems with insulation, air inleakage, or inefficient windows first. If the problem persists, you may be able to increase the size of the supply duct or add an additional duct to provide the needed airflow to the room.

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