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 crawlspace 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.

Content from D.O.E Reaseach & Develpment

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