Duct leakage is a major source of energy loss in buildings and a contributor to poor indoor air quality. Leaks in HVAC ductwork can account for 10 to 30 percent of total heating and cooling costs in a building. Good system design, improved duct and sealing materials combined with proper installation reduce energy losses.

Depressurization and over pressurization can occur in homes because of faulty duct systems. As a result, air is exchanged (through infiltration and exfiltration) with outside air and attic air.

Depressurization can be caused by closing interior doors, blocking air from reaching the return air plenum (usually located in a central area of the house). The return air plenum will consequently be starved for air and pull air from wherever it can. This is how the carbon monoxide-filled flue gases from water heaters and furnaces can be drawn into the living space, poisoning the occupants. In the same scenario, the rooms receiving air from the system are over pressurized. The air can exfiltrate (migrate outdoors) and cause energy losses or enter wall cavities, possibly introducing moisture problems.

Special consideration should be given to locate ductwork within the conditioned space to help improve overall energy efficiency. Many of the problems caused by leaky ducts can be reduced or eliminated by bringing the ducts and air-conditioning system inside the conditioned space, so that the air temperature in the ducts is not affected by outside air temperature or quality. If that is not possible, the crawl space or basement is usually preferred over the attic due to the summertime heat buildup in the attic. Exterior walls are bad locations for ducts because they displace wall insulation and can cause condensation problems.

Ductwork, Fig. 1

	Ductwork and HVAC system are located within an insulated attic space.
	Ductwork and HVAC system are located within a conditioned space below a vented attic.

If ducts are located in the attic, poorly sealed duct connections permit attic air to enter the system on the return air side and become an energy drain as system air is lost into the attic. Proper techniques, materials, and training to address these problems are available. A growing number of regulatory and energy conservation agencies throughout the nation are actively seeking to eliminate poor ductwork systems as serious health, safety, and energy issues.

The Austin Energy Green Building is promoting the concept of home commissioning. This procedure involves testing the duct system prior to occupying the home. This test involves the use of a "duct blaster" machine to test leakage rates of the system. Air conditioning contractors are beginning to purchase such machines and will be available to conduct testing for building owners and builders. The driver for this new trend will most likely be consumer demand for ducts with little to no leakage.

Duct system design should include the following elements:

Load Calculation

Load calculations are based on the use of Manual J (developed by the Air Conditioning Contractors Association or ACCA). The load calculation will tell what the cooling and heating requirement is for a building. An easy-to-use computer program for Manual J is available (see Resources section). With this calculation, the system designer now knows the air flow requirements of each room in the building.

Air Side Design

Sizing of the ductwork is calculated using ACCA's Manual D .

Air duct calculator or "ductalator" sizing is acceptable when all other guidelines are met.

Sizing the duct must take friction loss into consideration. Friction losses depend on the type of duct material, the length of the duct run, any special fittings, inlets, outlets, and duct dimensions.

Basically, try to centralize the location of your system and duct runs. Trunk lines should be as short and straight as possible. All flex connections should be short, straight and well supported. Flex duct should not be used for runs over 10 feet.

Ductwork, Fig. 2

Duct system design should take into account issues such as varying building loads, pressurization, and circulation.

Ductwork, Fig. 3

Keep duct layout simple.

Airflow Factors

Doors

Opening and closing doors in a home has a significant effect on the operation of a ducted heating and cooling system especially when the return air grill is located in a central part of the house. In most residential systems there are air supply registers in each room, but no return air path or grille from the room.

The air that is supplied to a room with closed doors will over pressurize the room if there is no return air path from the room. In Central Texas the common practice is to undercut doors to allow air flow underneath. Unfortunately, this space is too small for adequate flow. The room with the central return air grille is then 'starved" for air or depressurized. An unbalanced situation of depressurization and over pressurization leads to infiltration of unconditioned air and exfiltration of conditioned air, resulting in higher energy costs, reduced comfort, and potential safety and health hazards. This may be alleviated by using a vent, transom, or return air path from the room.

Flex Ducts

Flex duct should have a reflective foil exterior reinforced with fiber scrim or a listing approving installation exposed to ultra-violet light. Airflow through a flex duct is greatly restricted because of the convoluted interior surface. This must be accounted for in the system design. Use duct that is insulated to at least R-6.

Ductwork, Fig. 4

Components of a duct system using flex duct.	Sealing flex duct connections.

Metal Ducts

Metal ducts are the most durable, provide the most unrestricted airflow, and can be cleaned. All joints and connections must be sealed before duct is insulated on the exterior. If ductwork is to be exposed in the conditioned or living space, metal is usually the material of choice.

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Ductwork, Fig. 5

Components of a metal duct system	Sealing metal ducts.

Ductboard

Traditionally, ductboard contained uncovered fiberglass insulation. Indoor air quality specialists question the potential for fibers to become airborne. Therefore, lined or coated ductboard is now available for those who have concerns about exposed fiberglass in the duct. All seams must be sealed and precise cutting tools should be used for all penetrations in the ductboard.

Efficient Duct Design

Efficient duct design involves minimizing the length of duct, number of turns, and transitions in the system (the use of trunk ducts will help address many of these concerns). The use of air scoops, turning veins, and dampers has a significant effect on improving the flow of conditioned air. The duct design should be completed and included in the final plans of the house.

Sealing the Duct System

The proper sealing of plenums, air handlers, and ducts is key to eliminating leaks in a duct system. Latex duct mastic is a preferred sealant because it is flexible and can move with the expansion, contraction, and vibration of the duct system components. It is often strengthened with fiberglass strands or a mesh tape. Mastic is not a substitute for mechanical fastening of duct system components. Choose water-based products that are the least toxic and easy to clean up.

If a gap exceeds 1/4 inch, reinforce mastic with fiberglass mesh tape. This tape is similar to drywall fiberglass tape but is wider and is treated to reduce smoke development. Conventional duct tape should not be used in a duct system.

The application process for mastic requires that all duct connections be mechanically fastened with screws or rivets or, when using flex duct, with metal or plastic bands. The area to be joined should be wiped clean with a dry rag. The mastic is then applied with a trowel or brush (according to its viscosity) and spread one inch beyond the opening. For 1/4 to 1/2 inch openings use fiberglass mesh tape under the mastic. A larger gap needs a rigid material covering.

If tape is used for sealing ducts, it should be U.L. 181 foil tape. The tape rating must be stated on the tape and must meet U.L. 181 A-P (ductboard) or U.L. 181B-FX (flex duct). However, mastic is preferred over tape.

All connections (splices, Ys, Ts, and boots) must be sealed. Additionally, boots should be sealed to the sheetrock (a wire can be used to keep it from pulling loose). Penetrations into the plenum must be sealed. Flex duct inner liner requires air sealing, while the outer jacket needs only to be secured with a band or draw-tie (do not extend the duct liner through the wall of the plenum to the interior of the plenum).

Ductwork, Fig. 6

Sealing rigid duct splices	Sealing splices between flex duct and rigid duct

The air handler closet and air handler itself must be sealed, including sealing the air handler to the platform. Next, the return plenum should be lined on the interior with duct board (foil face in), sheet rock, or OSB and sealed. The support platform should be sealed on all sides. Penetrations into the plenum, such as refrigerant line chases, must be sealed. The sealing of the equipment support platform can be added to the tasks of the insulating/sealing contractor for the building.

Ductwork, Fig. 7

Sealing the air handler.

Return air grills should be sealed at the point of penetration through the walls. Any structural cavities also must be sealed. Place duct board (cut to fit) on all four sides with the foil sides facing in and seal them in place. Seal boots to sheetrock with polyseal.

Ductwork, Fig. 8

Sealing at grilles/registers.

A new technology that is available in Austin is an internal aerosol duct sealing system. This system is able to seal all air gaps up to 1/2 inch in diameter. The sealing process can be done by a trained, licensed professional. This method is not meant to replace traditional duct sealing techniques, but its application is often suitable for retrofit situations.

See "Energy Conservation Products & Services" in Yellow Pages

Aeroseal
115 E. St. Elmo; Suite B
Austin, TX 78745
(512) 445-2504
www.aeroseal.com
Internal aerosol duct sealing system

Austin Energy
(512) 494-9400
www.austinenergy.com
Rebates and loans for efficiency upgrades

Building Performance + Comfort
251 CR 264
Leander, TX 78641
(512) 259-2324
ACCA Manuals J & D, consultation, MADAIR test

Air Conditioning Contractors Association (ACCA)
1712 New Hampshire Ave., NW
Washington, D.C. 20009
(202) 483-9370
www.acca.org
Air conditioning and ductwork sizing methods

Air Diffusion Council (ADC)
1901 N. Roselle Rd., Suite 800
Schaumburg, IL 60195
(847) 706-6750
www.flexibleduct.org
Flex duct

The Energy Conservatory
5158 Bloomington Ave. S.
Minneapolis, MN 55417
(612) 827-1117
www.energyconservatory.com
Duct diagnostic tools

Lawrence Berkeley National Laboratory
Introduction to Residential Duct Systems
http://epb1.lbl.gov/aerosol/
Very thorough, visual explanation of residential ductwork

NAIMA (National Association of Insulation Manufacturers of America)
44 Canal Center Plaza, Ste. 310
Alexandria, VA 22314
(703) 684-0084
www.naima.org
Insulation standards

SMACNA (Sheet Metal & Air Conditioning Contractors National Association)
420 Lafayette Dr.
Chantilly, VA 20151
(703) 803-2980
www.smacna.org
Ductwork standards

US Consumer Product Safety Commission
Room 529
Washington, DC 20207
(800) 638-2772
www.cpsc.gov
Info on carbon monoxide poisoning