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Working With It - Troubleshooting

Possible Solutions

Broken Duct Blower door tests
Finding leaks
Areas to seal
Duct sealing materials
New buildings

Blower door tests

The most rigorous way to locate duct leaks is with a blower door test or similar analysis performed by an energy services professional. This analysis measures the magnitude of duct leakage and identifies its location.

Some companies offer ductwork sealing services with a follow-up blower door check to ensure duct leakage has been reduced to acceptable levels.

A blower door is a special instrument used to measure air leakage in a building shell and its ductwork. The equipment consists of a temporary door covering installed on an external door and a blower that forces air into or out of the building.

The blower door measures how leaky the building and ductwork are, and can be sued to find the location of the major leaks. Without a blower door, finding the leaks in the ductwork can be difficult since the ducts are often in hard-to-reach areas such as the attic or crawlspace, and the leaks are usually hidden from view by insulation.

Finding leaks

Finding leaks is easiest with uninsulated ductwork. However, insulated ducts can also have significant leakage.

The fiberglass insulation covering the ducts is not an effective air seal. The inside surface of this insulation can offer clues to whether the ducts leak. Look for streaks in the insulation, which appear as dirt is filtered from air passing through duct leaks.

As a general rule, it is most important to fix large holes first, such as a disconnected duct or gaps between pieces of ductwork and the holes closest to the air handler because the pressure is greatest there.

Areas to seal

When it comes to finding areas to seal air distribution systems, the solution is "just about everywhere." Here are the key locations to be sealed, in order of priority:

  • Disconnected components, including tears in flexduct, and strained connections between ductwork.

  • All seams in the air-handling unit. Also seal the holes in the air-handling unit for the refrigerant, thermostat and condensate lines. Use tapes rather than mastic to seal between the seams in the panels of the air-handling unit so they can be easily removed during maintenance.

  • The connections between the air handling unit and the supply/return plenums. After the air handling unit, the plenums have the greatest pressure, so even small holes result in large leaks. Remove insulation if necessary and seal all corners and connections.

  • The return and supply trunk lines, takeoffs, elbows and other connections.

    Duct sealing materials

    Duct sealing materials should last as long as the ducts themselves. The most commonly used materials - cloth duct tape - is not recommended because the adhesive dries out and the tape falls away. Many foil duct tapes also do not have quality adhesives.

    The recommended duct-sealing materials include mastic, UL-181 aluminum tape and high-quality caulking or foam sealant.

    Mastic is the most effective material to seal all types of ductwork and can be purchased in tubs or caulk tubes. Mastic looks like drywall joint compound and dries in hours. It sticks well to most surfaces and requires little preparation. For larger gaps, a fiberglass mesh tape serves as a backing, and is then covered with mastic.

    Mastic and better grades of duct tape can be purchased from contractor supply houses. These better grade tapes cost more than standard products; however, cheap tape is no bargain as it can lose its seal within months.

    New buildings

    If a geothermal system will be installed in a new building or home, take the opportunity to make sure the duct system will deliver top-notch comfort and efficiency by specifying a leak-free, insulated duct system.

    An even better option is to locate the ducts within the conditioned space and hiding them so that they don't show. It is possible to box in ductwork installed near the intersection of a wall and the ceiling, or to use other builders' tricks so the raw duct materials will not be visible.

    This isn't as difficult as it seems because an energy-efficient duct system in an energy-efficient building can be less bulky than a standard duct system. This is because the heating and air conditioning loads are smaller, permitting the use of a smaller heat pump that requires less airflow through the duct system.

    When it is possible to reduce the size of the duct system and the central unit, consumers can save on equipment, materials and installation costs, possibly enough to pay for the cost to hide ducts that are located within the conditioned space.

    The ductwork should be designed to provide adequate airflow to each area of the building or home. Contractors should avoid general "rules of thumb," such as a four-inch diameter duct for a bedroom, and should calculate the optimum size duct to meet the specific heating and cooling needs for a room. Ducts should be designed using industry standards, such as Manual D published by the Air Conditioning Contractor's Association.

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