Instructions / Assembly

Heating and Cooling Essentials
28.4.4 Insulated Flexible Duct
As shown in Figure 28-13, insulated flexible duct
consists of several layers, each with a specific func-
tion. The core is usually two plies of polyester film
used to encapsulate the galvanized steel wire helix.
The purpose of the wire helix is to help flexible duct
keep its shape. The insulation is a fiberglass blanket,
varying in thickness, depending on the R-valuc. Many
states have adopted energy codes for flexible duct that
require certain R-values for code approval. A basic
requirement is an R-value of 4.3, but Florida requires
an R-6, and Washington and Oregon require R-8.
Scrim is a high-tensile-strength, fiberglass mesh that
provides more protection protect the insulation and
inner core. The vapor barrier is a plastic jacket or
sleeve that encloses the product.
Major Duct Types
CORE
shields fiberglass
Vapor barrier
HELIX
steel wire coil
SCRIM
protects insulation
Figure 28-13. This cutaway view shows the construction of
a typical insulated flexible duct.
Round metal
pipe
Insulated
flexible
duct
Rectangular
duct
Pros
1. Excellent airflow
characteristics
2. Longer-lasting
than flexible duct
1. Pre-insulated
2. Ease of
installation
3. Reduced labor
costs
4. Eliminates some
fittings
1. Spacesaver; can
use space
between floor
studs
2. Longer-lasting
than flexible duct
Cons
1. If required,
insulation must
be installed
1. Not as long-lasting
as metal duct
2. Some reduction
in airflow
3. Must be fully
extended (cannot
make sharp
bends)
1. If required,
insulation must
be installed
Figure 28-14. Advantages and disadvantages of three types
of air duct.
28.5.1 Sizing the Extended Plenum
The extended plenum is sized according to the
number and sizes of round pipe it serves. It is easier to
have an extended plenum the same width throughout,
therefore, all the pipes are counted and the entire
extended plenum is sized accordingly. Rectangular duct
Comparison of ductwork types
Various types of ductwork are available for use in a
heating and air conditioning system. Because they
were all designed for a particular application, they
have different strengths and weaknesses. Figure 28-14
is a list of some pros and cons of the major duct types.
28.5 System Efficiency #
An efficient residential heating/cooling system will
move air at the proper velocity and quantity to each
room, and enter that room at the proper location.
Velocity is important: air that is moving too fast creates
uncomfortable drafts and makes more noise as it comes
through the system. Air moving too slowly will not have
enough speed to be thrown across the room or up the
wall. Sec Figure 28-15.
Rules of Thumb
Air should travel from the furnace/air
conditioner throughout the system at an
average velocity of about 700 ft./min.
On average, most system resistance to airflow
is approximately 0.08 (0.075) static pressure
(0.10 Supply air, 0.05 return air).
With an air velocity of about 700 ft/min,, air
conditioners are designed to move 400 CFM
PER TON, and furnaces at 12 CFM/1000Btu.
In general, 1 CFM of air is required to heat or
cool 1 or 1-1/4 sq. ft. of floor area.
Figure 28-15. Regular use of these rules of thumb will help
you solve forced-air system problems.