TELL-TALE SIGNS OF PROBLEM DUCTS

Now that you know here each branch duct leads, you are in a better position to ask whether your system is likely to be a big energy loser. Here are the things to look for.

Uninsulated Ducts in Unconditioned Spaces

Heat transfer through duct walls can contribute significantly to energy losses. Conductive heat losses are typically at least as great as the energy losses due to air leakage. If the duct system runs through an attic or vented crawl space and is not insulated, you can be sure that much energy is being wasted. If the ducts are in a basement, you will have to weigh the fact that insulating the ducts will cause the basement to get colder. If both the ducts and the basement walls are uninsulated, you should consider insulating the basement walls instead of the ducts.

Disconnected, Torn, or Damaged Ducts

A thorough inspection of the duct system should be made to look for holes large enough to see. Some sections of duct that are supposed to be joined together may have fallen away from each other, leaving a gap through which large quantities or air can leak. Flexible duct sections may have been torn during installation or afterward. Fiberglass ductboard sections are subject to damage if weight is placed on them. Whatever the cause, visible holes in ductwork are a clear indication that the system needs fixing.

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 this register will then be too cold. If it is an important room, the thermostat setting may be raised in an attempt to get enough heat to this room. If a room always seem too cold or a register doesn't seem to have any air flowing out of it, it may be worth investigating.

Inadequate Return-Side Ductwork

As we've noted, it is common to find building spaces pressed into service as part of the duct system. These tend to be leaky, especially on the return side. Even worse, some homes are designed without any return ductwork at all. In that case, unless the furnace is in the conditioned space, it will be surrounded by cold basement or crawl-space air and will have to use more energy to warm this cold air for delivery to the home than it would have if warmer air from the living space were available from return ducts. A system without return ductwork can also depressurize the furnace room, giving rise to the health hazards we've already discussed.

Other Evidence of Supply-and Return-Side Leakage

In any kind of duct system, the joints between duct sections should be sealed against 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. Duct sealing should be done using a silicone caulking or a special cement (mastic) with an embedded fabric. 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. This dark area is a coating of dust occurring, leakage that could, with reasonable effort, be eliminated.

Another fairly common type of energy-wasting air leakage is found in systems where ducts, water pipes, or vent pipes lead between the basement and the attic. If there are openings around these pipes that allow heated air to flow out or cold air to flow in, then the pressure difference between the basement and the attic is likely to increase air infiltration into the basement. It is usually a good idea to seal this flow path.

These are all signs that serious duct leakage may be which over time has accumulated on the surface as the air is being pulled through the insulation.

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