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Best Practices For Duct Systems
A manufactured home can have top quality carpeting,
a Jacuzzi, and Palladian windows. But if the air distribution system
performs poorly, the customer will be uncomfortable -- and dissatisfied.
"Customers don't think about it because it's unseen,"
says Frank Kern, national sales manager for Heatilator. "But
it's the artery system of the home."
Unfortunately, poorly designed or installed duct systems are one
of the more common problems in the industry. The good news is that
most things that go wrong can be avoided by following the "best
practices" in a guide recently published by the Manufactured
Housing Research Alliance. The audience includes everyone involved
in the design, engineering, fabrication and installation of new
manufactured homes.
"This is not a design or a comprehensive technical manual,"
says Kern, who served as project chair for publication of Manufactured
Housing Duct Systems: Guide To Best Practices. "Instead, it
is an easy-to-read guide that brings together design methods, installation
practices and material recommendations gathered from the cumulative
experience of leading industry experts. It helps industry professionals
choose the duct system best suited to the climate, as well as the
design and layout of the home, based on currently available technologies.
The guide has to do with selecting the type of system, whether you're
motivated by optimum performance or optimum economy, and making
sure the system is properly designed and installed," says Kern.
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As the diagram illustrates, the performance
difference between an insulated and permanently sealed duct
system and one that is uninsulated and poorly sealed can be
dramatic. |
The project grew out of an earlier MHRA initiative
that provided guidelines for sizing cooling equipment. One of the
assumptions made in developing recommendations for properly sized
cooling equipment is efficient distribution of the conditioned air.
Committee members were not at all confident that current practices
for designing and installing duct systems were performing near optimum,
a basic underlying assumption in selecting equipment capacity. As
Kern notes, "Leaky ducts can double a homeowner's energy costs
while leaving rooms drafty and uncomfortable. Ducts and supply registers
that are sized or located incorrectly can leave cold spots and cause
drafts. Oversized equipment is installed to compensate, further
raising costs and reducing efficiency. It's something that we in
the industry have grown used to," says Kern. "It made
us come to grips with the need to improve duct system design and
installation in the plant and in the field."
This single-volume source has chapters on design decisions, selecting
the system type, planning the system, making the connections, and
evaluating the system. Each point is clearly explained and supported
with diagrams showing best practices and, in some cases, practices
to avoid. Charts make it easy to compare the advantages and disadvantages
of different systems: underfloor vs. overfloor; in-line air supplies
vs. perimeter air supplies; straight duct vs. graduated duct, and
all the various combinations. "For the most part, a graduated
perimeter system is the optimum system for any home, but we're not
saying if you install an in-line system you're wrong, says Kern.
"The economics must be taken into account when selecting a
duct system type and design. The guide offers strategies for optimizing
performance regardless of the system selected."
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Strategically locating the truck graduations
in a graduated duct design is important to balance airflow and
assure that all spaces receive an adequate supply of conditioned
air. |
Connections Are Critical
From Kern's perspective, the most important section
in the book is entitled "Making the Connections." Most experts agree
that quality and performance are easily compromised by poor workmanship
and inferior joinery materials in making the system connections,
such as joining the boot to the truck duct intersection. "Even the
best system designs will underperform if they're not properly connected,"
says Kern. As the guide suggests, following three simple rules aimed
at tightening duct connections can dramatically improve system performance:
(1) cut an accurate hole using a template; (2) fasten components
mechanically; and, (3) seal leaks using a high grade tape or, preferably
mastic.
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Strategically locating the truck graduations
in a graduated duct design is important to balance airflow and
assure that all spaces receive an adequate supply of conditioned
air. |
Perhaps number two on the list of ways to optimize
duct system performance is the proper placement of the supply registers,
which is covered in the "Planning the System" section of the guide.
"You need to make sure the heat is going to the space where it's
needed, not where it's convenient to place a register," notes Kern.
SIDEBAR: Register placement is the key to comfort
Homeowner comfort is closely linked to register location. When properly
located, registers provide even temperatures throughout the home
without blowing air directly on occupants. Register location is
most important in extreme climates and in homes with a large heating
and/or cooling demand. Large window areas, for example, can create
hot or cold spots within a room that can be balanced, to some extent,
by carefully locating registers near windows and exterior walls.
Register location is less critical in well insulated, highly energy
efficient homes.
Among the other points covered in the guide are furnace position,
trunk gradations, sizing ducts and supply registers, duct insulation,
and sealing joints. "You can look through this guide, see how you've
done things in the past, and get ideas on how to do things differently
to achieve better performance in the future," adds Kern.
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