Hvac Air Duct Principles 89113
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Today's principles of duct design correct many energywasting errors of
the past.
Owners of many older homes are struggling with duct efficiency as low as 60
percent in their heating, ventilation and air conditioning systems. That
means nearly half the conditioned air conveyed by the ducts is lost to
leakage or thermal loss or gain. With current skyhigh heating and cooling
expenses, efficiencies of more than 80 percent are the norm, and even
higher ratings are typically required by states that mandate duct inspection
and testing whenever HVAC equipment is upgraded.
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Ducts In Unconditioned Zones
As much of the ductwork as possible should be located inside the home’s
conditioned zone. Ductwork routed through unconditioned zones, such as
attics, crawl spaces, or inside exterior walls, is exposed to acute summer and
winter temperatures. Thermal gain or loss and diminished efficiency result.
Where ductwork must be installed in unconditioned zones, it should be
insulated to meet the current standards of the International Energy
Conservation Code, which stipulates that ducts in attics should be insulated
to an Rvalue of 8, and ducts in other unconditioned areas should be
insulated to R6.
No Substitute For Proper Ductwork
All ductwork should be fabricated of approved permanent materials such as
metal or fiberglass. Moneysaving substitute methods of the past included
simply enclosing wooden wall voids or the channels between ceiling joists to
convey conditioned air, instead of installing proper “hard” ductwork in
those spaces. Such shortcuts are not appropriate to today’s standards for
energy efficiency.
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Keeping In Balance
Install manual dampers where each room branch duct joins the central
trunk duct. This facilitates customizing air flow to individual rooms. The
volume of air entering a room under positive pressure through supply ducts,
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HVAC Air Duct Principles
by Gus Stephens, Demand Media
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and the amount of air drawn out of the room through return ducts under
negative pressure, must balance to maintain neutral air pressure in each
room, which is the optimum condition for heating and cooling efficiency.
Dampers should incorporate a mechanism to permanently secure the
position of the damper and prevent gradual loss of adjustment over time.
A Clear Air Path
Ideally, every room with its own supply duct should have a dedicated return
duct. Where this isn’t feasible, a central return in a common area, such as a
hallway, is standard in many residences. To maintain neutral air balance in
all rooms, measures must be taken to ensure the air path from supply vents
in individual rooms to the central return remains unobstructed at all times.
This may involve installing air passthrough grilles in doors of rooms
without returns, or jumper ducts in ceilings that pass air from room to room
to ensure it reaches the central return.
Sealing And Testing
During installation, all duct segments should be mechanically fastened
together with sheet metal screws, and joints should be sealed with mastic,
metal foil tape or both. Before permanently connecting HVAC equipment to
the air ducts, the entire span of ductwork should be leaktested using a
pressure device such as a duct blaster to verify that leakage in ratio to total
air volume meets local or state standards.
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References
U.S. Department Of Energy: Air Distribution System Design PDF
California Energy Commission: Ducts And Vents
About the Author
Gus Stephens has written about aviation, automotive and home
technology for 15 years. His articles have appeared in major
print outlets such as "Popular Mechanics" and "Invention &
Technology." Along the way, Gus earned a Bachelor of Arts in
communications. If it flies, drives or just sits on your desk and blinks, he's
probably fixed it.
Photo Credits
Comstock/Stockbyte/Getty Images
C L E A N A I R E U R O P E
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