The duct system, used in air heating and air cooling your home, is a collection of tubes that distributes the heated or cooled air to the various rooms. This system can significently affect the cost and the effectiveness of heating and cooling the home. Furthermore the duct system can affect the health of the occupants through the distribution of indoor air pollution.
Instillation, changes and repairs to the duct system should always be performed by a qualified professional.
A duct system is a branching network of round or rectangular tubes generally constructed of sheet metal, fiberglass board, or a flexible plastic-and-wire composite – located within the walls, floors, and ceilings. Usually, you can see only the outlet, which is a register covered with grillwork. the following is a description of a common type of duct system.
This system consists of supply ducts and return ducts. Central heating or cooling equipment (furnace, air conditioner, or heat pump) contains a fan that forces heated or cooled air into supply ducts leading to the rooms. The fan gets its air supply through return ducts, which in the best systems are installed in every room of the house. To save on installation costs, most homes have one or two return registers located in common areas such as hallways. (Some homes have no return duct systems. Such design shortcuts often result in lower efficiency and higher heating and cooling bills.)
Energy Losses and Costs
- Typical duct systems lose 25 to 40 percent of the heating or cooling energy put out by the central fumace, heat pump, or air conditioner. Homes with ducts in a protected area such as a basement may lose somewhat less than this, while some other types of systems (such as attic ducts in hot, humid climates) often lose more.
- Duct repairs could be the most important energy improvement measure you can do if your ducts are in the attic. If only one half the typical loss of uninsulated and unsealed ducts that are in attics or crawl spaces were saved, it would amount to $160 off the total heating and cooling bill in a typical home. This savings is based on the national average use of natural gas and electricity for central heating and cooling at national average energy costs of 70 cents per therm, and 8 cent per kWh.
- Duct systems lose energy in two ways: by conduction of heat from the warm surface, and air leakage through small cracks and seams.
- One way duct systems lose energy is for the warm air inside the ducts to heat the duct walls, which in tum heat the cold air outside the ducts. If the ducts are in an attic or vented crawl space that is nearly as cold as the outdoors, this heat is completely lost.
- Another way that ducts lose energy is through air leakage. Sometimes this leakage is from accidental holes in the ducts or poorly connected duct sections; but even if the ducts are sealed, their operation can cause the house itself to leak more air than would otherwise be the case.
Leakage in the duct system can be hazardous to your health. Especially problematic are leaky returns in an enclosed space such as a basement or garage that also contains the furnace . If the return ducts leak, their low pressure can pull down the pressure in the basement or garage as well, and this can suck flue gases from the furnace and radon gas from the soil surrounding the home. The flue gases can be hazardous to health if they contain carbon monoxide. Exposure to radon gas from the ground is the second leading cause of lung cancer (after smoking).
Although experts disagree about how common these hazards are, by upgrading the energy efficiency of the duct system you have an opportunity to avoid these potential problems in your home.
Inspection of the Duct System
You probably wonder how you can know if your system is losing large amounts of energy. Although it is often difficult to be sure without testing, some tell-tale signs, if present in your duct system, should make you have it checked by a professional.
It will help to make a simple diagram of the system. This can be a rough sketch. There is no need for blueprint quality here.
The first thing you need to do is find the central heating unit. That should be no problem if it is located in a basement. It is probably something you pass by almost every day. However, it may be located in an attic or crawl space.
Tell-Tale Signs of Problem Ducts
Uninsulated Ducts in Unconditioned Spaces
- Heat transfer through duct walls can significantly contribute to energylosses. 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 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 of air can leak. Flexible duct sections may have been tom 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.
- 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 seems 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 wan-n 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.
These are all signs that serious duct leakage may be occurring, leakage that could, with reasonable effort, be eliminated.
Get A Professional!
Because of the possible effects that changing the duct leakage pattern can have on indoor air pollution, you should not attempt to repair duct leaks. Suppose, for example, that you find several disconnected duct joints in your supply system. Wouldn’t it make sense to hook them back up? Probably it would, but if the return system has leaks you can’t fix, you might end up with an unbalanced system.