Ductwork

DEFINITION
CONSIDERATIONS
COMMERCIAL STATUS
IMPLEMENTATION ISSUES
GUIDELINES

  1. Duct System Design Elements
  2. Airflow Factors
  3. Sealing the Duct System

CSI Numbers:

157 250
157 480


DEFINITION:

A high quality duct system greatly minimize energy loss from ductwork. Leaks in ductwork can contribute 20 to 60 percent of the air leakage in a house. The system should be airtight, sized and designed to deliver the correct air flow to each room. MADAIR (Mechanical Air Distribution And Interacting Relationships) refers to the principles underlying duct installations.


CONSIDERATIONS:

Duct leakage is a major source of energy loss in homes and a contributor to poor indoor air quality. Good system design, improved duct and sealing materials combined with proper installation reduce energy losses. Some design strategies are simple, such as allowing adequate airspace under interior doors to permit air to return to plenums outside of the room.

Depressurization and overpressurization can occur in homes due to faulty duct system characteristics. As a result, air is exchanged (through infiltration and exfiltration) with outside air and hotter attic air.

Depressurization can be caused by closing interior doors, blocking air that is supplied to rooms 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 can include the flue from a gas water heater, which is why some water heaters show signs of flame “rollout” (burn marks on the outside of a gas water heater). In the same scenario, the rooms receiving air from the system are overpressurized. The air can exfiltrate (migrate outdoors) and cause energy losses or enter wall cavities possibly introducing moisture problems.

Poorly sealed duct connections permit attic air to enter the system in cases of depressurization or become energy losses 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 a serious health, safety, and energy issue.

Commercial
Status
Implementation
Issues
T
E
C
H
N
O
L
O
G
Y
S
U
P
P
L
I
E
R
S
C
O
S
T
F
I
N
A
N
C
I
N
G
A
C
C
E
P
T
A
N
C
E
R
E
G
U
L
A
T
O
R
Y
High Quality Duct System Satisfactory Satisfactory Satisfactory in most conditions Satisfactory Satisfactory in most conditions Satisfactory
Legend
Satisfactory Satisfactory
Satisfactory in most conditions Satisfactory in most conditions
Satisfactory in limited conditions Satisfactory in Limited Conditions
Unsatisfactory or Difficult Unsatisfactory or Difficult


COMMERCIAL STATUS

TECHNOLOGY:

Well documented but not well distributed.

SUPPLIERS:

Limited.

COST:

5% to 10% higher than conventional installations.


IMPLEMENTATION ISSUES

FINANCING:

Available.

PUBLIC ACCEPTANCE:

The public is generally not aware of the role ductwork plays in energy use or air quality.

REGULATORY:

Sections 503.9 and 503.10 of the Model Energy Code address ductwork.


GUIDELINES

1.0 Duct System Design Elements

Overall system design should include the following elements:

1.1 Load Calculation

Load calculations are based on the use of Manual J (developed by Air Conditioning Contractors Association or ACCA). A short form Manual J is now available. This was developed by LCRA and is available from LCRA and the City of Austin. The load calculation will tell what the cooling and heating requirement is for a house. An easy-to-use computer program for Manual J is also available (see Resources).

1.2 Air Side Design

Sizing of the ductwork is calculated using Manual D. A short form Manual D is available from the City of Austin which permits rapid duct sizing.

Air duct calculator or “ductalator” sizing is acceptable when all other guidelines herein are adhered to.

The correct friction loss must be used based on the total equivalent length of each duct run.

1.3 Equipment Selection

Proper equipment selection is based on the information provided by using Manuals J and D. Optimal equipment sizing depends upon accurate information from proper duct design methods.

2.0 Airflow Factors

2.1 Doors

Opening and closing doors in a home has a significant effect on the operation of a ducted heating and cooling system. Supply registers are typically located in each room. Returns air grills (usually only one) are typically located in a central part of the house.

The air that is supplied to a room with closed doors will overpressurize the room if the doors are not sufficiently undercut to allow air flow underneath (accounting for carpets). The room with the return air is then “starved” for air or depressurized.

An unbalanced situation of depressurization and overpressurization leads to infiltration of unconditioned air and exfiltration of conditioned air, resulting in higher energy costs.

2.2 Flex ducts

Flex duct should have a foil exterior reinforced with fiber scrim or a listing approving installation exposed to UV light.

Air flow through a flex duct is greatly restricted due to the convoluted interior surface. This must be accounted for in the system design.

Use duct that is insulated to at least R-6, with a reflective outer surface.

2.3 Metal Ducts

Are most durable and can be cleaned.

Can be unacceptably noisy.

All joints and connections must be sealed before duct is insulated.

2.4 Ductboard

Fiberglass insulation on interior is uncovered. Indoor air quality specialists question the potential for fibers to become airborne.

All seams must be sealed.

Precise cutting tools should be used for all penetrations in the ductboard.

3.0 Sealing the Duct System

The proper sealing of plenums, air handlers, and ducts is key to eliminating leaks in a duct system. Duct mastic is a preferred flexible sealant that can move with the expansion, contraction, and vibration of the duct system components. It is often strengthened with fiberglass strands for increased strength. Mastic is not a substitute for mechanical fastening of duct system components. Choose water based products that are the least toxic and easier 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 except to seal the joints on access doors.

The application process for mastic requires all duct connections be mechanically fastened with screws, rivets, or, when using flex duct, with metal 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, the tape should be U.L. 181 tape. The tape rating must be stated on the tape and must meet U.L. 181 A-P. However, mastic is preferred over tape.

All connections (splices, Y’s, T’s, 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 and outer linings need to be sealed (do not extend the duct liner through the wall of the plenum to the interior of the plenum).

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) and sealed. The support platform should be sealed on all sides. Penetrations into the plenum, such as refrigerant lines, must be sealed. The sealing of the support platform can be added to the tasks of insulating/sealing contractor for the house.

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.