Correcting Common Problems in Your Air Conditioning Ductwork
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Correcting Common Problems in Your Air Conditioning Ductwork

Facts about ductwork and some simple rules you can use to determine if your ductwork is sized correctly and able to efficiently provide cooling in your home.

Most residential air conditioning systems are fairly straightforward and should follow a handful of guidelines without being designed according to industry standards. The best known of these procedures is the Air Conditioning Contractors of America (ACCA) Manual D.

Best Practices for Ductwork Runs

• Changes in airflow direction should be accomplished as infrequently and as smoothly as possible.

• The inner liner of flex duct should be pulled to its full extent to avoid the spiral “accordion” effect which causes a lot of flow resistance, or use smooth metal duct.

• Start with the size of the fan outlet/inlet connection. Use that size if the duct length is fairly short (less than 10 ft.) Increase the duct size 1 inch if the duct run is not long (less than 25 ft.) and there are few fittings (less than 3). Go up 2 inches in duct size if the duct run is long or there are many fittings. Size the wall cap or roof jack to match the final duct size.

• Seal all joints with long-lasting sealant material such as duct mastic. Some UL 181 listed tapes will work well on clean surfaces, but do not use cloth-backed tape.

The outlet connection for standard bathroom exhaust fans is usually 3 inch diameter. The better fans are 4 inch. Remote fans usually have 4, 6, or 8 inch diameter inlet or outlet connections. Smooth transition fittings should be used to change duct sizes. Odd-sized diameter duct is less common, but they are available and can make achieving the right air flow much easier.

If you are designing the duct system, when laying out duct runs and sizes, plan for air velocity of:

Supply ducts should be sized for 500 ft/min or less. To determine this velocity you need to know the amount of air supplied by your unit. It may be stamped on the label near the model and serial number or you can look it up in the owner’s manual. If all else fails you can research it online through the manufacturers website. Divide the amount of air (cubic feet per minute, CFM) by the area of the main supply trunk in square feet. Measure the metal duct, not the insulation around it; for fiberboard ductwork, which is usually an inch thick, subtract 2” from the height and width of the ductwork. Example: Air handler the supplies 1,600 CFM through a metal duct that measures 30” x 16”

1,600 CFM / ((30” x 16”)/144) = 480 ft/min < 500 ft/min; velocity is within recommended range.

(To convert the duct dimensions from square inches to square feet divide the area of the duct 144 square inches per square foot)

Return ducts should be sized for 350 ft/min.

By keeping the velocity below 500 ft/min will help keep static pressure and noise down, while keeping throw and efficiency up. Throw and efficiency have to do with how well the air is injected into the room so that it mixes well with room air. It will also keep energy cost down by keeping the motor from working too hard and maintaining the proper temperature.

General conventions for branching round ducts are:

• One 5” duct branches to two 4” ducts

• One 6” duct branches to two 5” ducts

• One 8” duct branches to two 6” ducts

Exhaust fans in the same dwelling can share a common discharge duct, but each fan must have a back-draft damper to prevent movement of air from one fan back through another.

Stamped metal grilles are inexpensive, but they are not adjustable and are inefficient. Aluminum grilles should be used where moisture will be present instead of standard painted steel. Grilles with adjustable curved blades that can direct the airflow to suit the room layout are the best choice. Grilles with a means to adjust the volume of air flow can help with balancing, but cannot limit the flow as well as volume dampers mounted in the duct branch. They can also be noisy if the are less than 50% open.

Transfer Grilles or Jump Ducts

Jump ducts are a popular method for improving the energy efficiency of homes with forced-air heating and cooling systems. They address the critical issues of equalizing air pressure in various parts of the home and of handling return air. When doors are closed, as with the case of bedrooms, it can be difficult to return enough air back to the air handling unit and create negative pressure inside the house. This would cause unconditioned air to enter the home through cracks and leaking windows or doors.

Inside the room, if more air is supplied than is returned, the air pressure in the room increases and exceeds the pressure outside it. Without balanced air pressure, energy efficiency decreases as conditioned air escapes through the building envelope, through uncontrolled routes such as windows or around electric outlets.

When return ductwork or plenums are installed, the cavities between studs and floor joist are used by attaching a piece of sheet metal to one side and using the floor joists and subfloor to complete the return. Inside walls, two studs and the layers of drywall are used and a hole is cut in the bottom plate of the wall. It a 2x4 framed wall, the area is about 0.35 square feet. If more than 125 CFM of air is supplied to the room, the return would be undersized. Return grilles have about 80% free area, so they should be larger than 3 ½ x 14 ½ or about 6 x 15. Larger rooms have holes cut between two stud cavities to increase the increase the area.

The problem with using wall and floor cavities is that they leak and can introduce humidity into the structure during the cooling season. The holes may not be cleanly cut and can compromise the wall or floor structure.

It is a fairly easy process to install a jump duct. A grille on the ceiling of the room is connected through ductwork to a grill in the hallway's ceiling. Air exits the room through the jump duct into the hallway, and then moves through the hallway to the main return duct. A transfer grille can also be installed near the ceiling to accomplish the same thing. A baffle should be installed to prevent light from leaking through. If noise and privacy are concerns, a length of six to eight feet of flexible duct will help prevent light pollution, as well as dampen noise moving between a room and the hallway. Low-cost stamped-metal grilles are typically used to provide a finished look on the ceiling. Doors should still be undercut by at least 1” as well.

Typical Jump Duct

Transfer Grille

Transfer Grille (Top View)

If you are experiencing problems related to return air, such as extreme inefficiency, dampness or other comfort problems, jump ducts are worth considering. It is relatively straightforward to gain access to the space above the bedrooms and hall, and a central return can be accommodated in the main space of the house. In a few homes I inspected, the wall cavity used for a return was open to the attic and was pulling in unconditioned air year-round. Insulation and dust was also pulled into the system and reduced efficiency and reduce indoor air quality.

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Comments (7)

However, There can be various reasons for the poor functioning of your air conditioner, which can occur either from inside or outside your home. Some of the problems occurring inside the home may include; filter clogging, clogging of fins due to collection of debris from dirty air filter, a low Freon pressure.

Very true which is why I discuss these other problems in my other articles about cleaning AC coils, spring start-up procedures, and fixing frozen coils. This article focuses on ductwork problem which in most cases are less common and specific to a room and not necessarily the whole system.


There is a gaping hole in our return duct under the house. Is this a problem? Why wouldn't our home inspector mention it before we bought the house?

I can't speak to why an inspector did not mention a defect, odds are that he did not notice it or it did affect the supply air temperature of the air. The hole should be repaired to improve the efficiency of the system, it will also control the temperature better. You state the hole in the ductwork is under your house, if it is in a crawlspace it will be pulling in outside air and also bring in moisture and dirt into the airstream. If the ductwork is sheet metal it should be fairly easy to repair, probably the piece came apart at a seam. If it is fiberboard ductwork you may have to replace the whole section of duct as it does not last much longer than 25 to 30 years, especially if exposed to moisture.

Air Conditioning Static Pressure

Its amazing very technical information as well common know how for every one about air conditioning duct work, one thing is the FE-1000 is an insertion type airflow sensing element designed for quick, easy installation through a small cutout in the existing ductwork. Where multiple elements are required for proper duct traversing, the output ports are manifolded together, external to the ductwork. Flow transducers are sold separately.

We are awaiting for more information for more efficiency.........


Thank you Daniel! It is in the crawlspace.. the duct work is a flexible silver tube type. Would that be easy to repair/replace? should it be done professionally? Do you think we should get a duct cleaning throughout the house?

Flex duct is probably the easiest type of duct to replace. Most home centers have 6, 7, and 8" flex duct in stock. If it is larger you can purchase it at an HVAC supply house. Measure the collar where it connects to the register or at the main trunk duct take off. They sell special zip-ties which are larger that connect the duct to the metal collar. You are better off replacing the entire length, the flex duct comes in 25' lengths, but it should be no longer than 10 feet. you can look inside the main trunk with a flashlight when you have the old flex duct off. If it looks like there is a lot of dust stuck to the inside you may want to have them professionally cleaned. I wrote an article about duct cleaning that you can read here: