stems supply conditioned air to the interior space of the
building. Fresh outside air (ventilation air) is provided to these units from separate outside
air dampers, which are split into fixed minimum and maximum air sections. The fixed
minimum air section damper opens simultaneously when the fan is energized and the air is
kept at around 55º F, which would require cooling the air stream in the summer and
heating the air in the winter. When the outside air temperature is between 55º F and 60º
F, no energy is added or removed from the fixed minimum air stream. The maximum air
section of the outside dampers is interconnected to the return air dampers, and their
motion is an opposing form of operation. This means that as one damper is completely
closed, the other one is completely open, and any variation in between.
The position of the maximum-air-section damper and the return-air damper is determined
by a mixed-air temperature controller. The mixed-air controller senses the temperature of
the outside air and the temperature of the return air, and chooses the air stream with the
greatest potential for heating or cooling. The fixed minimum-air streams and the
maximum-air streams are combined before entering the supply-air fan, and from this point
it flows throughout the air-distribution system. The air is then reheated at the various
zones in the distribution system to get the desired discharge temperature of the air to
satisfy the room load. The supply air is reheated with hot-water coils in the various zones,
with the temperature of the air leaving the reheat coils determined by the sensor in the
return-air stream, which tries to maintain a return-air temperature of approximately 75º F.
This type of mechanical configuration is called a constant-volume terminal reheat system.
The constant-volume terminal reheat system is one of the simplest systems to design and
does a very good job of controlling room comfort. However, this system wastes a relatively
large amount of energy. The greatest waste of energy in this system is the continuous
running of the supply and return fan motors at their full load. There is no need for a
constant supply of conditioned air to the space when it is not required, based on the local
demand. Also, the constant-volume terminal reheat system cannot be easily reset when
there is a reduction in occupancy at night and during weekends. This system can be
converted, however, to a Variable Air Volume system (VAV), which will save energy and
satisfy the space needs.
As the name implies, the temperature of the space is controlled by varying the amount of
air from the supply-air distribution system with a Variable Air-Volume system. This is
accomplished by a space thermostat positioning the dampers in a VAV box to match the - 2 -
supply air to the space needs. This reduces reheat energy and fan energy. The reheat
energy is saved because there will be moments when less supply air is needed than the
minimum air supplies; therefore, a lower amount of energy will be needed to reheat the
coils. A static pressure sensor will be installed in the duct system to transmit a signal to the
frequency drive-type motor controller to reduce the fan speed in proportion to the decrease
in air supply.
An alternating current fan motor works at its design speed and torque at 60 cycles per
second, or 60 Hertz. A variable frequency motor controller controls the frequency of the
alternating current supplied to the motor, thereby changing the speed of the motor. This
saves energy and motor horsepower, which is equal to the speed-reduction ratio cubed
times the original horsepower. This means that any reduction in supply air need translates
into fan motor speed reduction, which translates into substantial fan horsepower reduction
because of the cubed function. There are several methods that could be used with the VAV
system to obtain savings in horsepower. However, the frequency-drive-type motor-
controller method is the best technique for closely matching the cubed function in the
horsepower savings equation. The VAV systems with the combination of lesser need for
the reheat coils and the reduction in horsepower requirements will save a tangible amount
of energy over the constant-volume terminal reheat system.
Variable-frequency drives can accomplish substantial energy savings, mainly for centrifugal
loads with varying demands. A minor reduction of 10% in cubic feet per minute of air can
yield a 27% reduction in energy savings. Furthermore, a 20% reduction in fan speed can
reduce electric energy consumption by 50% (see illustration). Any equipment with variable
loads such as fans, pumps, and others is a good candidate for retrofit with variable-
frequency drive technology. - 3 -
The Energy Effects on Fan Horsepower by Reducing the
Quantity of Air the Fan Supplies
Fan Law:
H
1
= H
2
x (Q
1
/Q
2
)
3
H
1
= New Horsepower
H
2
= Original Horsepower
Q
1
= New CFM
Q
2
= Original CFM
Example #1:
10% Reduction in CFM or 90% of Original CFM
H
1
= 72.9% of the Original Horsepower
Savings in Horsepower = 1 - 0.729 = 27.1%
CFM Versus Original Horsepower
and Horsepower Savings
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
100%
80%
60%
40%
20%
0%
Percentage of Original CFM
Percentage of Origin
a
Horsepower
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Percentage of Horsepower Savin
g
Graph by Henry Manczyk - 4 -
Constant Volume Terminal Reheat System
Outside
Air
Intake
Filters
Pre-
heat
coil
Cooling
Coil
Constant
Speed
Supply Fan
Exhaust
Air
Constant
Speed
Return/
Exhaust Fan
Terminal
Box with
Reheat coil
(typical)
Thermostat
controls reheat
valve
Conditioned
Space
Modulating
Return Air
Damper
Supply
Air
T
T
Minimum
outdoor
air damper
Modulating
outdoor
air damper
Minimum
exhaust
air damper
Modulating
exhaust
air damper
77°F
55°F
T
T
T
T
Diagram prepared by Henry Manczyk - 5 -
Modifications to Terminal Reheat System
for Variable Air Volume Operation
Outside
Air
Intake
Filters
Pre-
heat
coil
Cooling
Coil
Variable
Speed
Supply Fan
Variable
Speed
Return/
Exhaust Fan
Supply
Air
T
T
VSD
Differential
Pressure
Controller
P
Maintains slightly positive
building static pressure
VSD
Pressure sensor to
adjust fan speed to
maintain proper duct
pressurization.
Minimum
exhaust
air damper
Modulating
exhaust
air damper
Modulating
Return Air
Damper
Minimum
outdoor
air damper
Modulating
outdoor
air damper
77°F
55°F
Conditioned
Space
P
VAV Box
with
Reheat coil
(typical)
Thermostat
controls VAV box
damper and
reheat valve
T
T
T
T
P
Diagram prepared by Henry Manczyk