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Working With It - Troubleshooting

Possible Problems

Broken Duct If comfort or air circulation problems occur, the ductwork should be investigated as contributing to or causing the problem.

Uninsulated ducts in unconditioned spaces
Disconnected, torn or damaged ducts
Blind-alley ducts
No return-side ductwork
Supply- and return-side leakage
Poor duct layout
Health hazards
Unbalanced airflow

Uninsulated ducts in unconditioned spaces

Conductive heat loss through uninsulated ducts is typically as great or greater than losses from air leakage.

Ducts running through unconditioned spaces like an attic or crawlspace should be insulated. If the ducts are in a basement, it should be considered that insulating them will make the basement colder.

If both the ducts and the basement walls are uninsulated, consider insulating the basement walls instead of the ducts.

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Disconnected, torn or damaged ducts

Most ducts are fairly delicate and easily damaged. Duct systems should be inspected for visible holes and, while operating, feel for air leakage.

Some duct sections that are supposed to be joined together may have pulled away from each other leaving gaps through which large quantities of air can leak.

Flexible duct sections may have been torn during installation or afterward. Almost any duct can be damaged by someone stepping on it or handling it roughly. Fiberglass ductboard sections are subject to damage if weight is placed on them.

Whatever the cause, fixing holes in ductwork should be a high priority on an energy improvement list.

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Blind-alley ducts

Occasionally found in duct systems that use joist spaces or other parts of the building structure to channel air flow, blind-alley ducts occur as a result of mistakes made during installation.

A blind-alley duct leads nowhere, except possibly to the outside, while the register it was supposed to serve has no source of heat. The room containing the register is therefore cold.

If a room always seems too cold or a register doesn't seem to have any air flowing out of it, it may be worth investigating.

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No return-side ductwork

Some structures have no return-side ductwork at all, a design that assumes the air delivered to rooms will probably make its way back to the air handler.

In such systems, the air handler is usually in a cold basement or crawlspace and has to do more work warming this cold air for delivery to the rooms than it would have if warmer air from the occupied space were available from return ducts.

This type of system is also likely to be unbalanced.

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Supply- and return-side leakage

In any kind of duct systems, the joints between duct sections should be sealed against supply- and return-side leakage.

If duct tape was used for this purpose, it often loses adhesiveness after a few years. In such cases, you can see it falling off the ducts or you can easily pull it away.

If your return ducts are insulated, you may see accumulations of soot or other dark material on the insulation where it covers loose duct joints.

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Poor duct layout

Poor duct layout can cause pressure differences. Many buildings have a supply duct for each room, but only a single return duct located in central area.

When interior doors are closed, it may be difficult for the air in these rooms to circulate back to the return duct.

The pressure in the closed-off rooms increases and the pressure in rooms open to the return decreases. This uneven pressure can increase air leakage into one area of the house while increasing leakage out of another.

Installing return ducts in rooms frequently isolated from the central return duct alleviates this problem. Transfer ducts, which allow air to circulate from one room to another, are another option.

At a minimum, all interior doors should be undercut at least one inch to allow for air to flow to the return duct. Be certain that carpeting does not block the undercut area.

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Health hazards

Leakage in the duct system can present health hazards. For example, low pressure in a home's basement can draw in flue gases from the furnace and radon gas from the soil.

Flue gases containing carbon monoxide are dangerous, and exposure to radon gas is the second leading cause of lung cancer, after smoking.

Return air leaks can also contribute to health problems. Air drawn from attics, crawl spaces and basements can be laden with toxic chemicals, mold, excess humidity, dust and other contaminants.

Experts disagree about how common these hazards are, but upgrading a duct system reduces the risks.

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Unbalanced airflow

Studies show this unbalanced airflow can increase infiltration up to 200 percent when the forced-air system operates.

Supply and return ducts are designed to be in balance, meaning that the amount of air supplied is the same as the amount returned the air handler, and the pressure inside the building is neutral. If either the supply or return ducts have air leaks, this balance is disrupted and the entire building pressure can be skewed.

For example, most homes have leaks in the return ducts. And since most return ducts are located outside the conditioned space in the crawlspace, basement or attic, these leaks draw in outside air.

This excess air increases the pressure in the home which forces conditioned air outside through cracks. The outside air drawn into the return ducts is hotter in summer and colder in winter than room air, so comfort drops and energy costs rise.

Even when return ducts are located inside the house, they can draw in outside air. Often these ducts are hidden from view inside wall, floor or ceiling cavities, but are not sealed.

If the supply ducts leak, then conditioned air is lost to the outside. Supply leaks create a negative pressure in the building that draws in outside air. This air, too, can be unhealthy and increases unhealthy energy and moisture problems.

Buildings usually have a combination of supply and return leaks as well as other duct problems. As a result, one area may have a positive pressure while another has a negative pressure.

While pressure imbalances inside are bad, partially correcting duct leakage can also pose a risk. For example, equal supply and return leaks can balance each other such that the pressure remains neutral.

Sealing only some of the leaks can create a pressure imbalance. Of particular concern to human health is the quality of air drawn into the building by a leaky return duct.

If a return duct is near a flue or chimney, it can draw in combustion by-products. Fixing return leaks without sealing supply leaks can also create an unsafe pressure imbalance. The best course is to ensure that both supply and return leaks are sealed and that the pressure inside the building is neutral.

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