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Minimizing Energy Losses in Ducts

June 24, 2012 - 5:45pm

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Placing ductwork in conditioned space can help reduce energy losses. | Photo courtesy of ©iStockphoto/SimplyCreativePhotography

Placing ductwork in conditioned space can help reduce energy losses. | Photo courtesy of ©iStockphoto/SimplyCreativePhotography

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, but you can reduce that loss by sealing and insulating your ducts. 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.

Even well sealed and insulated ducts will leak and lose some heat, so 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.

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.

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 airflow.

You can perform a simple check for adequate return air capacity by doing the following:

  1. Close all exterior doors and windows
  2. Close all interior room doors
  3. Turn on the central air handler
  4. "Crack" interior doors one by one and observe if the door closes or further opens "on its own." (Whether it closes or opens will depend on the direction of the air handler-driven air flow.) Rooms served by air-moved doors have restricted return air flow and need pressure relief as described above.
     

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.

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 airflow 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 airflow 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 leakage, 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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