Chapter 8: Duct Design and Sealing

To be used with the Guide to Building Energy Efficient Homes in Kentucky

Ductwork

Poorly sealed ductwork is• Common• An easily solved problem

Image Courtesy of Energy Services Group

Ductwork

Duct leakage• Increases heating and cooling loads• Diminishes comfort

Duct leaks = 10-30% of heating/cooling loads

Location of Ducts

• Locating ducts in conditioned space eliminates many problems with leakage.

• Ducts are often installed in chases. • It is critical to seal chases and other hidden

areas completely from unconditioned spaces.

Duct Leaks and Air Leakage

Effective materials for sealing:

• Duct sealing mastic with mesh tape

• Rated tapes that are UL-labeled

UL Listed

UL Labeled

Duct Tightness Test

Balanced Systems

Forced-air heating and cooling systems should be balanced.

Supply SupplyReturn

Return

Pressure Imbalances

Pressure imbalances can create dangerous air quality in homes including:

• Potential backdrafting of combustion appliances• Increasing air leakage from the crawl space to the

home• Pulling pollutants into the air handling system via

return leaks

Pressure Imbalances

Typical causes and concerns of pressure imbalances:

• HVAC systems with excessive supply leaks can cause homes to become depressurized.

• HVAC systems with excessive return leaks can cause homes to become pressurized and create negative pressures around the air handling unit.

Pressure Imbalances

Typical causes and concerns of pressure imbalances:

• Homes with central returns can have pressure imbalances when the interior doors to individual rooms are closed.

• Tighter homes with effective exhaust fans may experience negative pressures when these ventilation devices operate.

• Large kitchen exhaust fans can create large pressure imbalances.

Testing for Duct Leakage

• The best method to ensure airtight ducts is to pressure test the entire duct system.

• In most test procedures, a technician temporarily seals the ducts by taping over the supply registers and return grilles.

Leakage Inspection for Air Ducts

Blower Door Test

Sealing Air Distribution Systems

Duct leakage should be eliminated.

UL-181 A

Sealing Air Distribution Systems

Sealed and insulated ducts prevent conditioned air from leaking into unconditioned space.

mastic sealant

Upgraded Exhaust Ventilation

Dislodged Boot in Floor Space

Where do you seal the air distribution system?

EVERYWHERE!

High Priority Leaks

Areas that have the highest priority for sealing:• Disconnected components

− Takeoffs that are not fully inserted − Plenums or ducts that have been dislodged− Tears in flex-duct− Strained connections between ductwork

Disconnected Ducts

Dislodged Boot

High Priority Leaks

• Connections between the air handling unit and the supply and return plenums

• All of the seams in the air handling unit, plenums, and rectangular ductwork

− Look particularly under components and in tight areas

− Seal the holes for the refrigerant, thermostat, and condensate lines

− Almost all air handling cabinets come from the factory with leaks

Areas that have the highest priority for sealing:

Sealing Leaks in Air Handler Unit

High Priority Leaks

Areas with the highest priority for sealing:• Condensate lines of many systems contain a

trap with a vertical vent that freely leaks air– If the vent is needed, it can be reduced in size by

drilling a hole in a standard plumbing cap and placing the cap on the open vent

High Priority Leaks

Areas for the highest priority for sealing:• The return takeoffs, elbows, boots, and other

connections • The takeoffs from the main supply plenum and

trunk lines• Any framing in the building used as ductwork

Moderate Priority Leaks

• Connections near the supply registers• Joints between sections of the branch ductwork

Seal All Duct Leaks

Seal Leaky Boots

Low Priority Leaks

Seal these low priority leaks:• Longitudinal seams in round metal ductwork

Duct Design

Duct Materials• Three common types of duct material

used in home construction:– Metal – Fiberglass duct board – Flexible duct

Duct Material

Duct Materials

• Flexible duct – Long, continuous pieces between the register and plenum

box, the plenum box and air handler, or between the register and air handler

– Long flexible duct runs can severely restrict air flow, so they should be sized and installed carefully

• Metal duct – Round and rectangular – Must be sealed with mastic and insulated during installation– Used for plenums and larger trunk duct runs, are often

insulated with duct liner

Duct Materials

Metal ducts • Use fiberglass insulation having an attached

metal foil vapor retarder• The duct insulation should be at least R-8 • The vapor retarder should be installed to the

outside of the insulation—facing away from the duct

• The seams in the insulation are usually stapled together around the duct and then taped

Sizing and Layout

The proper duct size depends on:• The estimated heating and cooling load for

each room in the house• The length, type, and shape of the duct• The operating characteristics of the HVAC

system

Duct Configuration Flow Rates

Sizing and Layout

• The lower temperature of the heated air affects the placement of the registers.

• In standard duct placement and design, supply registers are almost always located on outside walls under or above windows. –Return registers are placed towards the

interior, typically in a central hallway.

Sizing and Layout

• Some energy efficient builders are able to trim both labor and material costs for ductwork by locating supply and return ducts near the core of the house.

• In standard duct design, virtually all supply ducts are 6-inch flex-duct or round metal pipe.– Most standard designs have only one return

for each floor.

Sizing and Layout

Keeping all ducts a standard size may work for some homes, but can create operating problems for others, including:• Too much heating and cooling supplied to small

rooms• Inadequate airflow• Over pressurization of rooms when interior doors are

closed

Standard Duct Installation

Ductwork Summary

Supply

Size Number

5” 3

6” 5

7” 7

10” 1

Summary

Ductwork is• Out of sight, but not out of mind• Hidden, but not forgotten

Summary