Report in ARBU: Forced Air Systems

Technological University of the Philippines
College of Architecture and Fine Arts
Report in ARBU
Forced Air systems
(Single and Dual Duct constant volume
System)
Evangelista,Jhennel A.
BSA-3B
Forced Air systems
 A forced-air system is one which uses air as its heat transfer medium. These
systems rely on ductwork, vents, and plenums as means of air
distribution, separate from the actual heating and air conditioning systems.
The return plenum carries the air from several large return grills (vents) to a
central air handler for re-heating. Thermostats are used to control forced
air heating systems.
 Forced air heating is probably the type most commonly installed in North
America. It is less common in Europe, particularly in the United Kingdom
(where it is typically referred to as "warm air" heating).
 Fueled by gas, electricity, or oil, a forced-air distribution system is just what

the name implies. Air is forced from the furnace through ducts to registers
in various rooms. Besides warming the air, the blower system that
distributes the
warmed air also
returns the cold
air to the furnace
so it can be
rewarmed and
distributed to the
rooms again.
Types
• Natural gas/propane/oil
Heat is produced via combustion of fuel.
• Electric
A simple electric heating element warms the air.
• Heat pump
Extracts heat from outdoor air via the refrigeration cycle
• Hydronic coil
 Combines hydronic (hot water) heating with a forced air

delivery
Advantages and disadvantages of forced air systems
Advantages
 Can accommodate central air conditioning, humidifiers, HRVs, and whole-

house air cleaners
 Excellent for programmable thermostats - takes a short time to recover
 No risk of structural damage due to water leaks

Disadvantages
 When improperly installed, they are prone to air infiltration; especially if the
ductwork is located in an un-conditioned space. Air infiltration typically lowers
humidity in the heated space and increases energy consumption.
 More noisy than hydronic systems
 Can distribute allergens and cooking odors throughout heated space.
 Requires a filter, which must be replaced periodically.
 Duct Materials
 Ducts can be made out of a variety of materials.
• Polyurethane and Phenolic insulation panels (pre-insulated air

ducts)
 Traditionally, air ductwork is made of sheet metal which is

installed first and then lagged with insulation as a secondary
operation. Ductwork manufactured from rigid insulation
panels does not need any further insulation and is installed in a
single fix. Light weight and installation speed are among the
features of preinsulated aluminium ductwork, also custom or
special shapes of ducts can be easily fabricated in the shop or
on site parts are typically cut at 45 degree.
Fiberglass duct board (preinsulated non metallic ductwork)
 Also the fiberglass panels provide built-in thermal insulation

and the interior surface absorbs sound. helping to provide
quiet operation of the HVAC system.
Flexible Ducting
 Flexible ducts, known as flex, have a variety of configurations,

but for HVAC applications, they are typically flexible plastic
over a metal wire coil to make round, flexible duct
Fabric
 Fabric ducting, also known as air socks, duct socks or textile ducts,
are designed for even air distribution throughout the entire length.
Usually made of special permeable polyester material, fabric ducts
act like a conventional system with much more diffusers.
 Fabric ducts are normally used where even air distribution is

essential.
Polyurethane duct Fiberglass duct
Plastic flexible duct

Fabric air duct
Single duct constant

volume System
 Single Duct – Single Zone
Distribution
-Simplest form of system
 -Most inexpensive
 -Responds only to 1 zone or space condition
-Can be single or dual fan
 -Can incorporate re-heat coils for de-humidification
 -Economizers can be used
 -Constant or variable volume
 -Energy efficiency = good
 Single Duct, Constant Volume, Reheat
A typical single duct, constant volume air handling system with reheat delivers a
fixed volume of cool supply air to multiple zones and reheats this air as demanded
by the thermostat in each zone. The supply air temperature is set low enough to
meet the zone with the highest demand for cooling. Terminal equipment at each
zone consists of a set of steam, hot water, or electric reheat coils controlled by
the zone thermostat.
Dual Duct constant volume system

 In the CAV system instead of using a single-duct system, the dual-duct
constant-volume system can also be used. The dual-duct system has a
single supply and return fan but two sets of duct (one for cold and one for
hot).
 The heating duct and the cooling duct merge in a mixing box controlled by a
reverse-acting damper to combine the two different air types to reach the
desired temperature of thermostat in each zone. Some benefits of using a
dual-duct CAV system are good humidity control and very quick and precise
temperature control.
 However, there are also a lot of disadvantages. One of the important

disadvantages is that the system is not energy-efficient since there is both
heating and cooling of the air.
Dual Duct constant Volume System

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Technological University of the Philippines

College of Architecture and Fine Arts

 Fueled by gas, electricity, or oil, a forced-air distribution system is just what

Heat is produced via combustion of fuel.

A simple electric heating element warms the air.

Extracts heat from outdoor air via the refrigeration cycle

 Combines hydronic (hot water) heating with a forced air

Advantages and disadvantages of forced air systems

 Can accommodate central air conditioning, humidifiers, HRVs, and whole-

 Excellent for programmable thermostats - takes a short time to recover

 No risk of structural damage due to water leaks

 More noisy than hydronic systems

 Can distribute allergens and cooking odors throughout heated space.

 Requires a filter, which must be replaced periodically.

• Polyurethane and Phenolic insulation panels (pre-insulated air

 Traditionally, air ductwork is made of sheet metal which is

Fiberglass duct board (preinsulated non metallic ductwork)

 Also the fiberglass panels provide built-in thermal insulation

 Flexible ducts, known as flex, have a variety of configurations,

 Fabric ducts are normally used where even air distribution is

Plastic flexible duct

Single duct constant

-Can be single or dual fan

 -Can incorporate re-heat coils for de-humidification

 -Economizers can be used

 -Constant or variable volume

 -Energy efficiency = good

 Single Duct, Constant Volume, Reheat

Dual Duct constant volume system

 However, there are also a lot of disadvantages. One of the important

Dual Duct constant Volume System

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