You probably wonder how you can know if a duct system is losing large amounts of energy. Although it is often difficult to be sure without testing, some telltale signs should help you convince your customer that testing would be a good idea.

It will help to make a simple diagram of the system. This can be a rough sketch. Start with the central heating unit, which may be in a basement, garage, crawl space, attic, or in the living space itself. Then work forward into the supply duct system and backward through the return system. If it isn’t obvious which are supply and which are return ducts, follow them to the registers and notice whether air is being sucked in or blown out when the fan comes on.

In one common type of installation, the return duct leads down from the basement ceiling to enter the furnace near the floor. The supply duct runs out from the top of the furnace.

Another common installation has all the supply ducts branching directly from the furnace like the arms of an octopus. You should be able to identify every register in the house, every supply and return branch of the duct system, how they connect to major supply or return trunks (if present in the home) and the connections to the plenums at the furnace or heating unit. Nearly every house is different.

You will feel more confident, and gain your customer’s confidence as well, if you know your way around the particular system on which you are proposing to work.


While you are at it, you might want to locate the filter, which is usually within the central fan unit or at the return register. The filter removes dust and other small particles that otherwise could interfere with the operation of the blower and the furnace heat exchanger.

A detailed discussion of filter selection is beyond the scope of this article. What can be said is that the filters supplied with hvac equipment often do little more than protect the fan against impingement by large particles such as sand and grit. Reducing the volume of smaller particles, which are the ones that typically cause health problems, usually requires an add-on filter.

Regardless of filter type, however, the homeowner should be cautioned to guard against clogging of the filter, which can drastically reduce airflow. (Even electrostatic filters have mechanical prefilters that can and do clog if not cleaned.) Cleaning or changing the filter two or three times a year is a very worthwhile practice.

If one does install an additional filter, it is important to make sure that the filter cabinet is adequately sealed against leakage from the outside. Add-on filters installed near the return plenum — at exactly the point where the pressure that drives air leakage is usually greatest — often contribute a significant fraction of the air leakage into the return duct.


To save money, builders sometimes use the building structure itself as part of the duct system. One common tactic is to use the spaces between basement or ceiling joists as ducts. (Joists are the horizontal-running boards — generally 2 by 10 in. or 2 by 12 in. — that support the floor above.)

Although this type of construction can be made to operate efficiently, it often leads to significant energy losses. One reason is that joist-space ducts are likely to be uninsulated. Another problem is that they may have unintended leakage paths to the outside, typically through the end of the joist cavity.

With returns, it is even more common to see portions of the building structure used as part of the duct system. Some homes have no return at all; the furnace simply has an intake grille through which air is drawn in to be warmed and distributed to the home.


Now that you know where each branch duct leads, you are in a better position to ask whether a duct 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, remember that insulating the ducts will cause the basement to get colder. If both the ducts and the basement walls are uninsulated, consider insulating the basement walls instead of the ducts.

Disconnected, torn, or damaged ducts: Thoroughly inspect the duct system; 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 torn during installation or afterward. Fiberglass ductboard sections are subject to damage if weight is placed on them.

Blind-alley ducts: Occasionally found in duct systems that use joist spaces or other parts of the building structure to channel airflow, blind-alley ducts occur as a result of mistakes made during installation. A blind-alley duct leads nowhere (except possibly to the outside); 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: Building spaces pressed into service as part of the duct system, while common, 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 health hazards.

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.

If the 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 which over time has accumulated on the surface as air is pulled through the insulation.

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 openings around these pipes 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.


In addition to looking for direct evidence of problems in an existing duct system, several general conditions point toward a probable customer benefit from a duct upgrade:

Possible comfort, health, or safety issues have been noted.

If the customer complains of rooms that are too cold or too warm, or if conditions exist that are likely to give rise to health and safety problems, the duct system should be tested for leakage.

The home is heated and cooled with a heat pump.

Most heat pumps have electric-resistance backup coils, which come on during cold weather when heating loads are high and the heat pump compressor’s ability to pull heat in from the outside is at its lowest ebb. Repairing a leaky or poorly insulated duct system not only reduces the part of the heating load arising from duct losses, it reduces the amount of inefficient backup heat that the heat pump must supply. The “double-dip” benefit is legitimate and real.

The heating or cooling equipment needs to be replaced.

When equipment needs replacing, it is a good opportunity for fixing the duct system as well. By improving the efficiency of the duct system, the peak load is reduced and it may be possible to select a smaller unit. In the case of an air conditioner, this will save on first cost and will probably improve thermal comfort as well.

The home is in a hot, humid climate and the ducts are in the attic.

Duct systems in this category are especially poor performers. Seasonal average efficiencies for these systems are often less than 60%, and peak-load duct efficiency tends to be even lower. In some cases, the duct losses are so great that the system cannot keep the home cool during peak summer heat.

Andrews is with the Department of Energy’s Brookhaven National Laboratory. This article is excerpted from the book Better Duct Systems for Home Heating and Cooling. For more information on obtaining the publication, contact the Office of Building Research and Standards, DOE, EE41, 1000 Independence Ave. S.W., Washington, DC 20585-0121; (website).

Sidebar: Safety

It is assumed that contractors are familiar with basic safety rules around home heating systems. The following is just a useful checklist:

  • Guard against falls, cuts, and other personal injuries.

  • Unless you are a trained electrician, do not open up or probe into any electrical devices, wires, or connections.

  • Wear an approved mask if you go into an area with fiberglass or loose fill insulation.

  • Before you touch any uninsulated duct, hold your hand about an inch from it to check if it is hot. This is especially important in furnaces fueled with gas or oil, because what looks like a small duct might actually be the vent pipe, and this might be at a temperature of several hundred degrees Fahrenheit.

    Sidebar: For Further Information

    “Duct Design for Residential Winter and Summer Air Conditioning and Equipment Selection (Manual O).” Air Conditioning Contractors of America, 2800 Shirlington Rd., Suite 300, Arlington, VA, 22206; (website).

    “Flexible Duct Performance and Installation Standards.” Air Diffusion Council, 1000 E. Woodfield Rd., Suite 102, Schaumburg, IL, 60173-5921; (website).

    “A Guide to Insulated Air Duct Systems.” North American Insulation Manufacturers Association, 44 Canal Center Plaza, Suite 310, Alexandria, VA 22314; (website).

    “Installation Standards for Residential Heating and Air Conditioning Systems.” Sheet Metal and Air Conditioning Contractors’ National Association, 4201 Lafayette Center Drive, Chantilly, VA 0151-1209; (website).

    “Air Distribution Design for Small Heating and Cooling Systems” (In Systems and Equipment Handbook). American Society of Heating, Refrigerating and Air-Conditioning Engineers, 1791 Tullie Circle N.E., Atlanta, GA 30329; (website).

    “Energy-Efficient Design of New Low-Rise Residential Buildings.” Standard 90.2-1993. American Society of Heating, Refrigerating, and Air-Conditioning Engineers, 1791 Tullie Circle N.E., Atlanta, GA 30329; (website).

    Publication date: 04/08/2002