Air Conditioning
Clinic
VAV Systems
One of the Systems Series
TRG-TRC014-EN
VAV Systems
One of the Systems Series
A publication of
The Trane Company
Preface
© 2001 American Standard Inc. All rights reserved
TRG-TRC014-EN
ii
The Trane Company believes that it is incumbent on manufacturers to serve the
industry by regularly disseminating information gathered through laboratory
research, testing programs, and field experience.
The Trane Air Conditioning Clinic series is one means of knowledge sharing.
It is intended to acquaint a nontechnical audience with various fundamental
aspects of heating, ventilating, and air conditioning. We have taken special
care to make the clinic as uncommercial and straightforward as possible.
Illustrations of Trane products only appear in cases where they help convey the
message contained in the accompanying text.
This particular clinic introduces the concept of variable air volume or “VAV”
systems.
A Trane Air Conditioning Clinic
VAV Systems
Figure 1
TRG-TRC014-EN iii
Contents
period one What Is Variable Air Volume? .......................... 1
Why VAV? ............................................................... 7
period two Components of a VAV System ...................... 12
period three System Configurations ..................................... 40
Perimeter Spaces .................................................. 41
Interior Spaces ...................................................... 46
Changeover/Bypass VAV System ........................... 47
System-Level Control Modes ................................ 48
period four Fan Modulation ................................................... 52
Fan Performance Curve ....................................... 52
System Resistance Curve .................................... 53
Fan Modulation Curve ......................................... 56
Fan Modulation Methods .................................... 58
System Static-Pressure Control .......................... 64
period five Application Considerations ............................. 68
period six Review ................................................................... 76
Quiz ......................................................................... 81
Answers ................................................................ 84
Glossary ................................................................ 85
iv TRG-TRC014-EN
TRG-TRC014-EN 1
notes
period one
What Is Variable Air Volume?
A variable-air-volume (VAV) air-conditioning system varies the volume of
constant-temperature air that is supplied to meet the changing load conditions
of the space.
For the purpose of comparison, we will look at a traditional constant-volume
(CV), variable-air-temperature system.
This system delivers a constant volume of air to the space and, to maintain the
required space temperature at all load conditions, varies the temperature of this
air. In this example, the temperature of the air is varied by controlling the
capacity of the central cooling coil.
VAV Systems
period one
What Is Variable Air Volume?
Figure 2
Constant-Volume,
Variable-Temperature System
space
OA
supply
supply
fan
fan
SA
cooling
cooling
coil
coil
thermostat
thermostat
RA
Figure 3
2 TRG-TRC014-EN
notes
period one
What Is Variable Air Volume?
This equation describes the performance of a constant-volume, variable-
temperature system at various load conditions.
With these design conditions:
space sensible heat gain = 40,000 Btu/hr [11,724 W]
space dry-bulb (DB) temperature = 75°F [23.9°C]
supply air dry-bulb (DB) temperature = 55°F [12.8°C]
constant = 1.085 [1,210]
the system balances the load with 1,840 cfm [0.87 m
3
/s] of 55°F [12.8°C]
supply air.
Sensible Heat Gain
Constant × (Space DB – Supply DB)
Supply
Airflow
=
40,000 Btu/hr
1.085 × (75°F – 55°F)
Supply
Airflow
=
1,840 cfm
=
Constant Volume–Full Load
11,724 W
1,210 × (23.9°C – 12.8°C)
Supply
Airflow
=
0.87 m
3
/s
=
Figure 4
Supply Airflow (cfm)
Sensible Heat Gain
Constant (Space DB Supply DB)–×
------------------------------------------------------------------------------------------------------=
Supply Airflow
40,000 Btu/hr
1.085 75°F55°F–()×
----------------------------------------------------------
1,840 cfm==
Supply Airflow
11,724 W
1,210 23.9°C12.8°C–×
----------------------------------------------------------------
0.87m
3
/S==
TRG-TRC014-EN 3
period one
What Is Variable Air Volume?
notes
Rearranging the equation and using the constant supply airflow of 1,840 cfm
[0.87 m
3
/s], the supply air temperature required to balance a lesser 20,000
Btu/hr [5,862 W] space sensible load is:
Therefore, as the space sensible load drops from 40,000 Btu/hr to 20,000 Btu/hr
[11,724 W to 5,862 W], this system modulates the temperature of the constant
1,840 cfm [0.87 m
3
/s] supply air from 55°F to 65°F [12.8°C to 18.3°C].
Removing less energy from the air takes less energy from the central system.
Therefore, with this particular system, refrigeration energy savings are realized
at part load conditions, although control of space humidity levels suffers due to
the warmer supply air condition.
75°F –
20,000 Btu/hr
1.085 × 1,840 cfm
Supply
DB
=
65°F=
Constant Volume–Part Load
Space
DB
Sensible Heat Gain
Constant × Supply Airflow
–
=
Supply
DB
23.9°C –
5,862 W
1,210 × 0.87 m
3
/s
Supply
DB
=
18.3°C
=
Figure 5
Supply DB Space DB
Sensible Heat Gain
Constant Supply Airflow×
---------------------------------------------------------------------------–=
Supply DB 75°F
20,000 Btu/hr
1.085 1,840 cfm×
------------------------------------------------
65°F=–=
Supply DB 23.9°C
5,862W
1
,
210 0.87m
3
/
S
×
-------------------------------------------------–=
4 TRG-TRC014-EN
notes
period one
What Is Variable Air Volume?
However, because this type of system can respond to the demands of only one
thermostat, it can serve only those building spaces with similar cooling
requirements. If a building has many spaces with diverse cooling needs, each
must be served by its own system.
Other constant-volume system designs can serve the cooling requirements of
more than one space with a central fan and cooling coil. However, to do so, the
cool primary air must be either reheated or mixed with warm air to produce the
supply temperatures needed to balance the various space cooling loads.
I’m
freezing!
I’m
freezing!
I’m
fine.
I’m
fine.
I’m
hot!
I’m
hot!
It’s stuffy
in here!
It’s stuffy
in here!
Constant Volume–Multiple Spaces
single thermostat
Figure 6
Constant Volume–Multiple Spaces
reheat coilreheat coil
reheat coil
80°F
80°F
[26.7°C]
[26.7°C]
55°F
55°F
[12.8°C]
[12.8°C]
55°F
55°F
[12.8°C]
[12.8°C]
65°F
65°F
[18.3°C]
[18.3°C]
65°F
65°F
[18.3°C]
[18.3°C]
air mixingair mixing
air mixing
Figure 7
TRG-TRC014-EN 5
period one
What Is Variable Air Volume?
notes
The terminal reheat system uses a central air handler and cooling coil to deliver
cool primary air to all the spaces. Each space has its own heating coil to temper
the air to satisfy the space load. Of course, any heat added to meet the part-load
requirements of a space becomes a cooling load that the refrigeration system
must overcome. This can result in a nearly constant refrigeration load, even
when the building is at part-load conditions.
Therefore, reheating cooled air to achieve part-load supply air temperature
control is not very energy efficient and is used only in special constant-volume
applications, or when there is a “free” source of heat (i.e., heat recovery).
Primary air (PA) is the air delivered by a central supply fan to a terminal unit.
Supply air (SA) is the air delivered to a space.
Terminal Reheat System
EA
PA
OA
RA
supply
supply
fan
fan
cooling
cooling
coil
coil
reheat
reheat
coil
coil
thermostat
thermostat
SA
Figure 8
6 TRG-TRC014-EN
notes
period one
What Is Variable Air Volume?
In contrast, a variable-air-volume (VAV) system delivers the primary air at a
constant temperature and varies the airflow to maintain the required space
temperature at all load conditions.
Similar to the constant-volume system, at design conditions the VAV system
will balance the same 40,000 Btu/hr [11,724 W] space sensible load with the
same 1,840 cfm [0.87 m
3
/s] of 55°F [12.8°C] supply air.
Variable-Air-Volume (VAV) System
EA
OA
RA
variable
variable
-
-
speed drive
speed drive
supply
supply
fan
fan
cooling
cooling
coil
coil
thermostat
thermostat
VAV
VAV
box
box
PA
SA
Figure 9
VAV–Full Load
40,000 Btu/hr
1.085 × (75°F – 55°F)
Supply
Airflow
=
1,840 cfm
=
11,724 W
1,210 × (23.9°C – 12.8°C)
Supply
Airflow
=
0.87 m
3
/s
=
Figure 10
TRG-TRC014-EN 7
period one
What Is Variable Air Volume?
notes
However, at part load, the VAV system produces a balance by modulating the
volume of constant 55°F [12.8°C] supply air.
In this example, the air volume required to balance the lesser 20,000 Btu/hr
[5,862 W] space sensible load is:
That is, 920 cfm [0.43 m
3
/s] of 55°F [12.8°C] supply air.
Why VAV?
One reason to use a VAV system is the potential for part-load energy savings.
The part-load energy savings inherent with the VAV system are twofold. First,
Air Conditio ning Clinic TRG-TRC0 14-EN
Air Conditio ning Clinic TRG-TRC0 14-EN© Amer ican Standard Inc. 2001
VAV–Part Load
20,000 Btu/hr
1.085 × (75°F – 55°F)
Supply
Airflow
=
920 cfm
=
5,862 W
1,210 × (23.9°C – 12.8°C)
Supply
Airflow
=
0.43 m
3
/s
=
Figure 11
Supply Airflow
20,000 Btu/hr
1.085 75°F55°F–()×
----------------------------------------------------------
920 cfm==
Supply Airflow
5,862 W
1,210 23.9°C12.8°C–×
----------------------------------------------------------------
0.43m
3
S⁄==
▲ Reduced fan energy
▲ Reduced refrigeration energy
Why VAV? Energy Savings
Figure 12
8 TRG-TRC014-EN
notes
period one
What Is Variable Air Volume?
the air volume reduction creates an opportunity to reduce the fan energy
required to move this air. The amount of energy saved depends on the method
used to modulate the capacity of the fan (to be discussed in Period Four).
Second, the reduced airflow across the cooling coil causes the refrigeration
system to throttle back in order to stabilize the primary air temperature. In turn,
this results in a reduction in refrigeration energy compared to full load.
Another reason for VAV is to cost-effectively provide improved comfort. A VAV
system is capable of controlling space temperature in many spaces with
dissimilar cooling and heating requirements, while using only one central air
handling unit (AHU). This is accomplished by providing one VAV terminal unit
and thermostat for each independently controlled space. When the sun is
beating against the west side of the building in the late afternoon, a VAV system
can provide an increased amount of cool supply air to keep the spaces on the
west exposure comfortable, while throttling back the airflow to the spaces on
the east exposure so as not to overcool them.
Why VAV? Comfort
Ahhh!
Ahhh!
I’m
fine.
I’m
fine.
I’m
quite
cozy.
I’m
quite
cozy.
It’s nice
in here!
It’s nice
in here!
▲ Dedicated terminal units
▲ Dedicated thermostats
Figure 13
TRG-TRC014-EN 9
period one
What Is Variable Air Volume?
notes
Contrasting the three systems discussed in this example, the simple, constant-
volume, variable-temperature system consumes constant fan energy while
providing refrigeration energy savings at part load. However, it can only deliver
comfort to spaces with similar loads.
The terminal reheat system can serve many spaces with dissimilar load
requirements, but consumes constant fan energy and nearly constant
refrigeration energy at part load. It also uses an increasing amount of reheat
energy at part load.
On the other hand, the variable-air-volume system is capable of providing both
fan and refrigeration energy savings at part load, making it the most energy
efficient of these three systems. In addition, the VAV system can efficiently
serve many spaces with dissimilar cooling and heating load requirements.
System Comparison
constant
constant
-
-
volume,
volume,
single zone
single zone
constant
constant
-
-
volume,
volume,
terminal reheat
terminal reheat
◆
◆
Fan energy
Fan energy
savings
savings
◆
◆
Refrigeration
Refrigeration
energy
energy
savings
savings
◆
◆
Delivers comfort
Delivers comfort
to many spaces
to many spaces
efficiently
efficiently
◆
◆
Constant fan
Constant fan
energy
energy
◆
◆
Refrigeration
Refrigeration
energy
energy
savings
savings
◆
◆
Delivers comfort
Delivers comfort
to only one
to only one
thermal zone
thermal zone
◆
◆
Constant fan
Constant fan
energy
energy
◆
◆
Nearly constant
Nearly constant
refrigeration
refrigeration
energy
energy
◆
◆
Delivers comfort
Delivers comfort
to many spaces
to many spaces
inefficiently
inefficiently
◆
◆
Reheat energy
Reheat energy
increases at part
increases at part
load
load
VAV
VAV
Figure 14
10 TRG-TRC014-EN
notes
period one
What Is Variable Air Volume?
There are some basic building characteristics that favor the application of VAV
systems. We will discuss three:
n Variable thermal load profiles in the spaces.
n The need for multiple, independently-controlled spaces.
n The existence of a common return air path.
A key feature of a VAV system is its ability to realize both fan and refrigeration
energy savings at part-load conditions. A building with construction and
utilization characteristics that cause the thermal load profiles of the spaces to
vary throughout the day and year is an excellent application for a VAV system.
The fact that the west-facing space in this example has a very different load
profile than the east-facing space allows the VAV system to take advantage of
this energy savings at all hours of the day.
▲ Variable thermal load profiles
▲ Multiple, independently-controlled spaces
▲ Common return air path
VAV Building Characteristics
Figure 15
Variable Thermal Load Profiles
100
75
50
25
0
100
75
50
25
0
percent space loadpercent space load
6126
6126
noon
noon
a.m.
a.m.
p.m.
p.m.
west
west
-
-
facing
facing
space
space
east
east
-
-
facing
facing
space
space
Figure 16
TRG-TRC014-EN 11
period one
What Is Variable Air Volume?
notes
Another feature of the VAV system is the ability to efficiently satisfy the comfort
requirements of many different spaces within the building. As we saw from the
comparison of the VAV and terminal reheat systems, the VAV system is the
most efficient at performing this task.
Because the VAV system uses one central cooling coil and fan to serve many
spaces, the need for a common path to allow the air to return from these spaces
is significant. A return air plenum is commonly used. If return ductwork is
required to connect each conditioned space to the central air handling unit, the
system becomes more difficult to balance and control.
Independent Space Control
▲ Dedicated terminal units
▲ Dedicated thermostats
Figure 17
Common Return Air Path
Space 1
Space 1
Space 2
Space 2
Space 3
Space 3
Figure 18
12 TRG-TRC014-EN
notes
Next, we will examine the individual components that compose a VAV system.
A simple VAV system includes the following:
n Central air handler with a variable-volume supply fan, a cooling coil,
possibly a heating coil, controls, filters, a mixing box, and a return or relief
fan
n Supply duct
n VAV terminal unit, or “box,” with a thermostat and supply diffusers for each
independently controlled space
n Thermostat and unit controller for each terminal unit
n Return plenum or duct
The VAV terminal units modulate the airflow supplied to each space.
VAV Systems
period two
Components of a VAV System
Figure 19
supply
supply
ductwork
ductwork
VAV
VAV
box
box
thermostat
thermostat
return air path
return air path
Components of a VAV System
central
central
air handler
air handler
supply
supply
fan
fan
relief
relief
fan
fan
Figure 20
period two
Components of a VAV System
TRG-TRC014-EN 13
period two
Components of a VAV System
notes
VAV Terminal Units
A VAV terminal unit is a sheet-metal assembly installed upstream of its
respective space diffusers. The unit consists of an air-modulation device,
control hardware and, depending on the system application, possibly a heating
coil, a filter, and a small terminal mixing fan. Modulating the airflow to each
individual space is accomplished using a temperature-controlled mechanical
device that varies the airflow resistance in the supply duct to that space. The
rotating blade damper changes airflow resistance by rotating the damper
into the air stream, restricting the size of the air passage to the space. It is very
cost-effective and flexible. Typically, either a pneumatic or electric controller
can be used to adjust the damper. An understanding of the common VAV
terminal unit types is important to understanding VAV systems.
VAV Terminal Units
primary
primary
air
air
supply
supply
air
air
airflow
airflow
modulation
modulation
device
device
▲ Heating coil
▲ Filter
▲ Mixing fan
controller
controller
Figure 21
14 TRG-TRC014-EN
notes
period two
Components of a VAV System
The simplest of all VAV terminal units is the single-duct, cooling-only
terminal unit. It consists of an airflow modulation device with controls
packaged in a sheet-metal enclosure. The unit can only modulate the primary
airflow to the space. The primary air is supplied by a single, central air handler.
This VAV terminal unit is typically used for those zones that require year-round
cooling, like the interior zones of a building. It is the most common and basic
type of single-duct VAV terminal unit.
This graph indicates how the air supplied to the space by the terminal unit
varies as the space loads change. The vertical axis indicates the total airflow
supplied to the space. The horizontal axis indicates the space load.
The supply airflow to the space is reduced as the cooling load in the space
decreases. Responding to the space thermostat, the primary airflow is
modulated between maximum and minimum settings. The maximum setting is
Single Duct, Cooling Only
primary
primary
air
air
supply
supply
air
air
Figure 22
space load
design
cooling load
design
heating load
% airflow to space
100%
minimum
primary
airflow
0%
primary air
primary air
Single Duct, Cooling Only
maximum
primary
airflow
remote source of heat
remote source of heat
heating
heating tempering
tempering
Figure 23
TRG-TRC014-EN 15
period two
Components of a VAV System
notes
determined by the design cooling load of the space and the minimum setting is
normally determined by the space ventilation requirement or minimum airflow
for proper diffuser selection.
Most cooling-only units are applied to spaces that have no need for heat. These
units would operate in the region on the right-hand portion of this chart,
modulating between design and minimum primary airflow. When cooling-only
units are applied to spaces that do have heating requirements, the heat is
provided by a remote source such as finned radiation along the wall. In these
spaces, when the cooling load drops below the minimum airflow setting for the
unit, overcooling the space, the remote heat source activates. When space
heating is required, the remote heat source satisfies the space heating load.
Most terminal unit controllers provide an output signal to control this remote
source of heat.
In addition to controlling this remote source of heat (perimeter baseboard
radiation, in this example), single-duct VAV terminal units can directly provide
heat to a space. This can be accomplished by adding a heating coil to each
cooling-only unit or by mixing the primary air with warm plenum air before it is
delivered to the space.
Space Heating with a VAV System
remote
remote
heat source
heat source
heating
heating
coil
coil
plenum air
plenum air
terminal
terminal
mixing fan
mixing fan
Figure 24
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
Page is loading ...
/
