Powered damper having automatic static duct pressure relief

According to the present invention a damper for controlling the flow of conditioned air supplied through a supply duct to a conditioned space is provided. The damper includes a support housing which defines a flow passage which communicates the supply duct with the conditioned space. A plurality of damper blades are mounted within the support housing for pivotal movement about respective spaced apart parallel axes. A blade link interconnects the damper blades in a ganged relation so that a common pivoted orientation of the damper blades is determined by the position of the blade link. Means are provided for selectively exerting a force on the blade link which will either move the link toward the second position to increase the flow of air through the damper or for exerting a force on the blade link to move the link toward the first position to decrease the flow of air through the damper. Means are provided which allows the blade link to move toward the second position (maximum flow) in response to a force imparted on the damper blades by a build-up of pressure in the supply duct. Under such circumstances the force imparted upon the damper blades is sufficient to overcome the force exerted on the blade link to move the link toward said first position (flow substantially blocked).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to dampers for use in heating, ventilating and air 
conditioning systems where conditioned air is provided from such a system 
to a plurality of zones, and, more particularly to a damper which provides 
automatic relief when an excessive duct pressure occurs. 
2. Description of the Background Art 
In conventional heating, ventilating and air conditioning ("HVAC") systems 
conditioned air is supplied to a plurality of zones. Zoning systems have 
been developed for these HVAC systems which typically include dampers 
disposed in the ductwork for controlling the air flow of the conditioned 
air to the zones. These zoning systems control the flow of conditioned air 
to the plurality of zones independently so as to allow for independent 
control of the zone environments. 
However, these zoning systems are difficult and expensive to install, both 
as original equipment and as retrofit. Implementation of these systems 
typically requires the installation of dampers in the ductwork, 
installation of power and control wiring for the components of the system 
throughout the building, and installation of thermostats in the building 
walls. Retrofits typically include modifications to the ductwork, power 
and control wiring throughout the building, and thermostat installations 
in walls. Additionally, these zoning systems typically include an 
expensive and difficult installation of a bypass damper system which is 
used to relieve excess static duct pressure. 
Excess static duct pressure may result when a large number of the dampers 
restrict the air flow to the zones. In one implementation of a bypass 
damper system, a bypass damper is connected between the supply and return 
air duct. An airflow sensor is disposed in the supply air duct and is 
connected to the bypass damper. A bypass controller is also connected to 
the bypass damper and is used to modulate the bypass damper in response to 
the airflow measured by the airflow sensor. Thus, if the bypass controller 
determines that the air flow to the supply air duct causes excess static 
duct pressure then the bypass damper will be used to recycle the 
conditioned air to the return air duct. This implementation has the 
disadvantage of being expensive and difficult to install. Additionally, 
recycling the conditioned air can cause the HVAC system to overload. For 
example, if the HVAC system is set in heat mode and the bypass damper is 
activated to relieve excess pressure in the duct, the recycled heated air 
may continue to increase in temperature, as it recycles, which may cause a 
limit switch to shut down the HVAC system. Elimination of the 
aforementioned bypass damper system would reduce the amount of HVAC system 
equipment which in turn would reduce installation and maintenance costs. 
Another implementation of a bypass damper system is similar to the bypass 
system mentioned above with the exception that the conditioned air is 
redirected to a dump, such as an equipment room, instead of being recycled 
to the intake duct. This implementation has the additional disadvantage of 
lost efficiency because the energy used to condition the redirected 
conditioned air is wasted. 
DISCLOSURE OF THE INVENTION 
It is an object of the present invention to provide an inexpensive and easy 
to install zoning system damper for use in providing conditioned air to a 
plurality of zones. 
It is a another object of the present invention to provide an inexpensive 
and automatic means to relieve excessive static duct pressures which 
occur, for example, in zoning systems if too many zone dampers are closed. 
According to the present invention a damper for controlling the flow of 
conditioned air supplied through a supply duct to a conditioned space is 
provided. The damper includes a support housing which defines a flow 
passage which communicates the supply duct with the conditioned space. A 
plurality of damper blades are mounted within the support housing for 
pivotal movement about respective spaced apart parallel axes. A blade link 
interconnects the damper blades in a ganged relation so that a common 
pivoted orientation of the damper blades is determined by the position of 
the blade link. The blade link is moveable to any position which lies 
between a first position wherein the damper blades cooperate with one 
another so that air flow through the passage is substantially blocked and 
a second position wherein air flow through the passage is at a maximum. 
Means are provided for selectively exerting a force on the blade link 
which will either move the link toward the second position to increase the 
flow of air through the damper or for exerting a force on the blade link 
to move the link toward the first position to decrease the flow of air 
through the damper. Means are provided which allows the blade link to move 
toward the second position (maximum flow) in response to a force imparted 
on the damper blades by a build-up of pressure in the supply duct. Under 
such circumstances the force imparted upon the damper blades is sufficient 
to overcome the force exerted on the blade link to move the link toward 
said first position (flow substantially blocked). 
The foregoing and other objects, features and advantages of the present 
invention will become more apparent in light of the following detailed 
description and accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now in detail to the drawings, FIG. 1 is a block diagram 
illustrative of a zoning system 10 of the type for use with the present 
invention. The major components which make up the system include a user 
interface 15, a temperature sensor 20, a zone damper means 25, and a main 
control 30. A means for conditioning air 35 which is a component of the 
HVAC system and controlled by the present invention is also shown. 
The user interface 15 can be any device which allows a user to select 
temperature setpoints and system operating modes. For example, a hand held 
infra-red remote control may be used, such as the Sanwa CES0110032-00. The 
user may select the setpoint and operating mode to achieve the desired 
zone temperature. One user interface may be employed and carried from zone 
to zone, or multiple user interfaces may be employed, one in each zone as 
desired. 
The temperature sensor 20 may be any device which produces an output 
responsive to its surrounding temperature, such as the MCI 10K THERMISTOR. 
The temperature sensor 20 may be attached to a wall in a zone with a screw 
or, self adhesive pad, or any conventional securing means. 
The zone damper means 25 controls the flow of conditioned air to the zones 
and, according to the invention, provides an automatic and inexpensive 
means to relieve excess static duct pressure. The zone damper means 25 is 
disposed at each zone outlet in the zones which receive conditioned air. 
The main control 30 is used to control the system mode and equipment 
capacity (for variable capacity equipment) of the HVAC system. The HVAC 
system may be any ducted system which supplies conditioned air to a 
plurality of zones. 
FIG. 2 shows a zoning system arranged in a two zone structure which 
includes a zone damper means 25 including master zone dampers 40 and slave 
zone dampers 45. In the illustrated embodiment each zone damper means 25 
includes a master zone damper 40 and a slave zone damper 45. It should be 
understood, that any one zone damper means 25 may include only a master 
zone damper 40, or alternatively may include a master zone damper 40 and 
one or more slave zone dampers 45. In both zone 50 and zone 55, a master 
zone damper 40, a slave zone damper 45, and a temperature sensor 20 are 
shown. The main control 30 and a means for conditioning air 35 are both 
shown in an equipment room 60. The means for conditioning air 35 is a 
conventional component of the HVAC system which conditions the air and 
supplies the conditioned air to an air distribution system which supplies 
the conditioned air to the plurality of zones. Typically, the means for 
conditioning air 35 has several system modes such as auto, heat, cool, fan 
and off modes. The means for conditioning air 35 may have variable 
capacity capability or fixed capacity capability. 
Referring to FIGS. 1 and 3, the user interface 15 transmits a command 
signal to the master zone damper 40 which is responsive to the command 
signal in a manner which will be described in more detail hereinbelow. The 
user interface 15, for example, may transmit the command signal by way of 
an infrared light beam 65, or alternatively, a radio frequency signal. The 
command signal includes a temperature setpoint and an operating mode 
signal; both of which the user controls from the user interface 15. The 
operating mode signal represents a request, from the respective zone, to 
the HVAC system for auto, heat, cool, fan or off mode. The setpoint signal 
represents the desired temperature for that particular zone. 
The temperature sensor 20 is connected to the master zone damper 40 by way 
of a thin cord 70 which couples with a sensor plug 75 located on the 
damper 40. As will be more fully understood as the description of the 
system continues, the master zone damper 40 receives, and is responsive 
to, a temperature signal from the temperature sensor 20. The temperature 
signal represents the actual temperature in the zone in which a sensor 20 
is located. 
Referring now to FIGS. 1, 2 and 3, the master zone damper 40 transmits a 
damper position signal to the slave zone dampers 45. The damper position 
signal controls the position of the flow control mechanism of the slave 
zone dampers 45, such that the master zone dampers 40 control the air flow 
of conditioned air to the zones though the slave zone dampers 45. The 
master zone dampers 40 also transmit an operating mode signal and a 
temperature related control signal to the main control 30. The operating 
mode signal is set by the user as described above. The temperature related 
control signal is calculated by the master zone damper 40 and may be a 
temperature error signal which is the difference between the zone setpoint 
temperature selected by the user and the actual zone temperature. 
In the illustrated embodiment the damper position signal, the operating 
mode signal, and the temperature error signal are transmitted by a power 
line carrier ("PLC") means (shown in FIG. 6). Such a circuit is well known 
in the art and no further description of the circuit, which allows 
information to be transmitted across a power line, is considered necessary 
for a full understanding of the present invention. It should be readily 
apparent to someone skilled in the art that these signals could be 
transmitted by means of radio frequency ("RF") or by hardwiring the 
relevant system components. 
Referring to FIGS. 1 and 2, the main control 30 is connected to a means for 
conditioning air 35 and is responsive to the operating mode signal and the 
temperature error signal from the master zone damper 40 such that the main 
control 30 provides the HVAC system with a system mode signal and a system 
capacity signal as will be appreciated as the system operation is 
described below. 
Referring now to FIGS. 4 and 4A, the zone damper means 25 includes a zone 
damper assembly 80, zone damper circuitry 85, and a motor 130 for opening 
and closing the zone damper assembly 80, in response to inputs from the 
zone damper circuitry, as will be described hereinbelow. The zone damper 
assembly 80 of the type for use with the present invention is common to 
both the master zone damper 40 and the slave zone damper 45 and is shown 
in simplified form. While the zone damper circuitry is generally 
designated by reference numeral 85, it will be seen that the master zone 
damper circuitry 150 (shown in FIG. 5) is different from the slave zone 
damper circuitry 270 (shown in FIG. 8). The damper assembly 80 is sized 
such that it may be operatively installed in place of a conventional 
conditioned air outlet diffuser in a typical air distribution system. 
The damper assembly 80 includes an outside grill 90 which is attached to a 
support housing 95. The housing 95, shown only in outline, may be formed 
from sheet metal or other suitable material. Operatively mounted to the 
support housing 95 are a plurality of damper blades 110. Each of the 
damper blades 110 is pivotally mounted about a pivot point 115 in the 
support housing 95 for movement from a substantially vertical position, 
wherein the air flow through the damper is blocked, to a horizontal 
position wherein the air flow through the damper is at a maximum. Each 
damper blade 110 is shown in an intermediate position in FIG. 4. 
The damper blades 110 are interconnected with one another at an 
intermediate pivot point 116 thereof by a vertically extending blade link 
120. As a result the blade link 120 moves vertically and horizontally, as 
the blades 110 move, together in a ganged relationship, about their 
respective pivot points 115. An arcuately shaped slot 137 is provided in 
the left hand side of the blade link 120, as viewed in the drawing 
figures, for operationally cooperating with the damper assembly 25 to open 
the damper blades 110 as will be described hereinbelow. An actuating arm 
125 couples the blade link 120 to the motor 130 such that as the actuating 
arm 125 is turned by the motor 130 the blade link 120 causes the damper 
blades 110 to open or close. One end 132 of the actuating arm 125 is 
connected to the motor 130. As best shown in FIG. 4A a pin 135 is mounted 
to the other end 133 of the actuating arm 125. The pin 135 is sized such 
that it is received in and operationally engages, the slot 137 on the 
blade link 120, as the actuating arm 125 moves counter clockwise. As a 
result the blade link 120 is caused to move upwardly and to the right, 
which, in turn, opens the damper blades 110. 
A coil spring 140 is connected at one end to the arm pin 135 such that the 
spring 140 is operationally disposed at the second end 133 of the 
actuating arm 125. The other end of the spring 140 is connected to a pin 
145 mounted on the blade link 120 at a location above the slot 137. The 
spring 140, as so mounted, is in tension and, as a result, as the 
actuating arm 125 turns clockwise the spring 140 pulls on the blade link 
120 to close the damper blades 110. Thus, the spring 140 is used to 
regulate the closing motion of the damper blades 110. It should be 
understood by one skilled in the art that a solenoid may be used in place 
of the motor 130. 
This arrangement provides automatic pressure relief which prevents 
excessive static duct pressures. For example, if the force against the 
damper blades 110, caused by the static pressure in the duct, is higher 
than the spring force, the blades 110 will swing open against the spring 
force to relieve the static duct pressure. The spring force may be 
adjusted by moving the end of the spring 140 to different blade link pins 
145 corresponding to different calibrated pressure relief settings. Thus, 
the spring 140 is used to regulate the opening motion of the damper blades 
110 caused by excess static pressure. As will now be described in detail 
the zone damper circuitry 85 is used to control motion of the motor 130. 
FIG. 5 shows a block diagram of the master zone damper circuitry 150. The 
master zone damper circuitry 150 comprises a sensor plug 75, an a/d 
converter 155, a microprocessor 160, a user interface receiver 165, a 
motor control 170, motor control terminals 175, a d/a converter 180, a PLC 
circuit 185, power line terminals 190, a power cord 195, and a power 
supply 200, all electrically connected as shown. 
The temperature sensor 20 is connected to the sensor plug 75 such that the 
sensor plug 75 receives the temperature signal. The sensor plug 75 is 
connected to the a/d converter 155 and the a/d converter 155 is connected 
to the microprocessor 160 such that the a/d converter 155 converts the 
temperature signal to a digital temperature signal which is transmitted to 
the microprocessor 160. A Harris CDP68HC68A2 may be used for the a/d 
converter 155 and an Intel 80C52 may be used for the microprocessor 160. 
The user interface receiver 165, which is connected to the microprocessor 
160, is used to receive the command signal from the user interface 15 and 
transmit the command signal to the microprocessor 160. The microprocessor 
160 also is connected to the serially connected combination of the motor 
control 170, motor control terminals 175, and the damper motor 130 such 
that the microprocessor 160 causes the motor control 170 to operationally 
regulate the damper motor 130 for opening or closing the damper blades 
110. An Allegro UNC58D4B may be used for the motor control 170. The 
microprocessor 160 is also connected to the serially connected combination 
of the d/a converter 180, the PLC circuit 185, the power line terminals 
190, and the power cord 195 for allowing the master zone damper 40 to 
transmit signals across the power line to the slave zone dampers 45 and 
the main control 30. A Harris AD7520 may be used for the d/a converter 
180. One known embodiment of the PLC circuit 185 is shown in FIG. 6. The 
power supply 200 is used to provide electrical energy to the master zone 
damper circuitry 150. 
Referring to FIG. 7, the logic programmed into the microprocessor 160 in 
the master zone damper circuitry 150 is illustrated. Beginning at the 
block 205 labeled "start" the first step performed 210 is to determine the 
mode and setpoint from the command signal from the user interface 15. The 
next step 215 is to determine the zone temperature from the temperature 
signal. Then in step 220, the zone temperature is subtracted from the 
setpoint to determine the temperature error. If the mode is set to auto 
mode the microprocessor moves to step 230 where, if the temperature error 
is greater than one (1) the heat mode is selected in step 235. If the 
temperature error is less than negative one (-1) the cool mode is selected 
in step 245. If the temperature error is between one (1) and negative one 
(-1) the microprocessor 160 moves to step 250 and the fan mode is 
selected. 
After the proper mode is selected the microprocessor 160 moves to step 255 
and determines the damper position as the absolute value of the 
temperature error multiplied by fifty percent (50%). The damper position 
is limited to a value of 100% which corresponds to a fully opened damper. 
The damper position is transmitted to the motor control 170 (shown in FIG. 
5) causing the damper motor 130 to adjust the damper blades 110 (shown in 
FIG. 4) on the master zone dampers 40 to the position indicated by the 
damper position step 260. The microprocessor 160 also transmits the damper 
position to the d/a converter 180 which transmits the damper position to 
the PLC circuit 185 which in turn transmits the damper position to the 
slave zone dampers 45 through the power line. The slave zone dampers 45 
use the damper position to adjust the damper blades 110 (shown in FIG. 4) 
on the slave zone dampers 45 to the position indicated by the damper 
position step 260. The microprocessor 160 in step 265 transmits the 
operating mode signal and the temperature error signal to the d/a 
converter 180 which transmits these signals to the PLC circuit 185 which 
in turn transmits these signals to the main control 30 through the power 
line. 
Referring to step 225, if the auto mode is not selected then the process is 
the same as above with the exception that at step 225 the microprocessor 
160 moves directly to step 255, instead of to step 230. When the auto mode 
is not selected, the mode selected by the user, such as heat, cool, fan, 
or off is transmitted in step 265 to the main control 30. 
FIG. 8 shows a block diagram of the slave zone damper circuitry 270. The 
slave zone damper circuitry 270 comprises a microprocessor 160, an a/d 
converter 155, a PLC circuit 185, power line terminals 190, a power cord 
195, a motor control 170, motor control terminals 175, and a power supply 
200, all electrically connected as shown. 
The microprocessor 160 is connected to the serially connected combination 
of the a/d converter 155, the PLC circuit 185, the power line terminals 
190, and the power cord 195 for receiving the damper position signal 
transmitted across the power line from the master zone damper 40. A Harris 
CDP68HC68A2 may be used for the a/d converter 155 and an Intel 80C52 may 
be used for the microprocessor 160. One known embodiment of the PLC 
circuit 185 is shown in FIG. 6. The microprocessor 160 is also connected 
to the serially connected combination of the motor control 170, motor 
control terminals 175, and the damper motor 130 such that the 
microprocessor 160 causes the motor control 170 to regulate the damper 
motor 130 for opening or closing the damper blades 110. An Allegro 
UNC58D4B may be used for the motor control 170. The power supply 200 is 
used to provide electrical energy to the slave zone damper circuitry 270. 
Referring to FIG. 9, the logic programmed into the microprocessor 160 in 
the slave zone damper circuitry 270 is illustrated. Beginning at the block 
275 labeled start, the first step performed 280 is to receive the damper 
position signal from the master zone damper 40 using the above mentioned 
PLC circuit 185. In step 285 the damper position signal is transmitted to 
the motor control 170 (shown in FIG. 8) for causing the damper motor 130 
to adjust the damper blades 110 (shown in FIG. 4) on the slave zone 
dampers 40 to the position indicated by the damper position signal. 
The main control 30 may be used for a variable capacity or a fixed capacity 
HVAC system. Shown in FIG. 10 is a block diagram of the main control 
circuitry 290 for a fixed capacity HVAC system. The main control circuitry 
290 for a fixed capacity HVAC system comprises a microprocessor 160, an 
a/d converter 155, a PLC circuit 185, power line terminals 190, a power 
cord 195, relays 295, signal terminals 300, control wiring 305, and a 
power supply 200. 
The microprocessor 160 is connected to the serially connected combination 
of the a/d converter 155, the PLC circuit 185, the power line terminals 
190, and the power cord 195 for receiving the system mode and the system 
capacity signals transmitted across the power line from the master zone 
damper 40. A Harris CDP68HC68A2 may be used for the a/d converter 155 and 
an Intel 80C52 may be used for the microprocessor 160. One known 
embodiment of the PLC circuit 185 is shown in FIG. 6. 
The microprocessor 160 also is connected to the serially connected 
combination of the relays 295, the signal terminals 300, the control 
wiring 305, and the means for conditioning air 35 for controlling the 
system mode of the means for conditioning air 35. The power supply 200 is 
used to provide electrical energy to the main control circuitry 290. 
Shown in FIG. 11 is a block diagram of the main control circuitry 310 for a 
variable capacity HVAC system. The main control circuitry 310 for a 
variable capacity HVAC system comprises a microprocessor 160, an a/d 
converter 155, a PLC circuit 185, power line terminals 190, a power cord 
195, a serial communication transceiver 315, signal terminals 300, control 
wiring 305, and a power supply 200. 
The microprocessor 160 is connected to the serially connected combination 
of the a/d converter 155, the PLC circuit 185, the power line terminals 
190, and the power cord 195 for receiving the system mode and the system 
capacity signals transmitted across the power line from the master zone 
damper 40. A Harris CDP68HC68A2 may be used for the a/d converter 155 and 
an Intel 80C52 may be used for the microprocessor 160. One known 
embodiment of the PLC circuit 185 is shown in FIG. 6. The microprocessor 
160 also is connected to the serially connected combination of the serial 
communication transceiver 315, the signal terminals 300, the control 
wiring 305, and the means for conditioning air 35 for controlling the 
system mode and the system capacity of the means for conditioning air 35. 
A linear LTC485 may be used for the serial communication transceiver. The 
power supply 200 is used to provide electrical energy to the main control 
circuitry 310. 
Referring to FIG. 12, the logic programmed into the microprocessor 160 in 
the main control circuitry 290, 310 for both a fixed capacity and a 
variable capacity HVAC system is illustrated. Beginning at the block 320 
labeled "start", the first step performed 325 is to receive the operating 
mode and temperature error signals from the master zone dampers 40. In 
step 330 it is determined if there are any zones calling for heating or 
cooling from the information in the received operating mode signals. If 
there are no heat or cool zones then the microprocessor 160 moves to step 
335 to determine, from the operating mode signal, if there are any fan 
zones. If no fan zones exist then the system mode is set to "off" in step 
340. If at least one fan zone exists, then the system mode is set to fan 
mode in step 345. 
If in step 330 it was determined that there is at least one heat and/or 
cool zones then the microprocessor 160 moves to step 350 to determine if 
the number of heat zones is equal to the number of cool zones. If the 
number of heat zones is equal to the number of cool zones the 
microprocessor 160 moves to step 355 and sets the system mode to the mode 
of the zone with the largest absolute temperature error. If in step 350 
the number of heat zones is not equal to the number of cool zones the 
microprocessor 160 moves to step 360 and sets the system mode to the mode 
with the larger number of zones. Once the system mode is determined it is 
transmitted to the means for conditioning air 35 in step 365. In step 370 
the microprocessor 160 determines whether the HVAC system is a fixed 
capacity system or a variable capacity system. If the HVAC system is a 
fixed capacity system the microprocessor 160 moves back to step 320. If 
the HVAC system is a variable capacity system then the microprocessor 160 
moves to step 375 and sets the system capacity according to the following 
formula. System capacity=((100%/2 Deg. F.)/(Total No. of Zones)) * (Sum of 
ABS(Temperature Error) of zones with System Mode). In step 380, the system 
capacity is transmitted to the means for conditioning air as described 
above and the microprocessor 160 moves back to step 320. 
The following is an example of the operation of the present invention in a 
two zone environment. Assume zone 1 has a temperature of 71.5 degrees, 
zone 2 has a temperature of 72 degrees and that the user has selected 
auto mode and a setpoint of 70 degrees for both zone 1 and zone 2. Also 
assume that the HVAC system has a variable capacity and that both zone 1 
and zone 2 each have one master zone damper 40 and one slave zone damper 
45. 
Referring to FIG. 7, the master zone dampers 40 in zone 1 and zone 2 
determine that the auto mode and a setpoint of 70 degrees are selected in 
step 210. In step 215, the master zone dampers 40 determine a zone 1 
temperature of 71.5 and a zone 2 temperature of 72 degrees. The 
microprocessor 160 calculates the temperature errors in step 220; in zone 
1 the temperature error is -1.5 and in zone 2 the temperature error is -2. 
The cool mode for both zone 1 and zone 2 is selected in step 245 because 
both temperature errors are less than -1 and the auto mode was selected in 
both zones. Next, the microprocessor 160 calculates the damper position to 
be the absolute value of the temperature error multiplied by 50%; the 
damper position in zone 1 is 75% and the damper position in zone 2 is 
100%. The master zone damper 40 uses the damper positions to adjust the 
damper blades 110 on the master zone dampers 40 to a corresponding 
position. For example, the zone 1 damper position will adjust to a 75% 
open position. The damper position signals are transmitted to the 
respective slave zone dampers 45 in step 260 and the operating mode and 
temperature error signals are transmitted to the main control 30 in step 
265. 
Referring to FIG. 9, each slave zone damper 45 receives the damper position 
signal from its respective master zone damper 40 in step 280 which is 
used, in step 285, to adjust the respective slave zone damper openings to 
the above mentioned positions. 
Referring to FIG. 12, the main control 30 receives the operating mode and 
temperature error signals from the master zone dampers 45 in step 325. 
Since there are two cool zones and no heat zones, the microprocessor 160 
moves to step 360 to calculate the system mode. In step 360, the 
microprocessor 160 sets the system mode to the cool mode, which is the 
mode with the maximum number of zones. The system mode is sent to the 
means for conditioning air 35 so as to set the means for conditioning air 
35 to the cool mode. In this example, the microprocessor 160 moves to step 
375 because the HVAC system has a variable capacity. The system capacity 
is calculated as ((100%/2 degrees F.)/(2)) * 3.5 in step 375. Thus, the 
system capacity, in this example, is calculated as 87.5% and in step 380 
is transmitted to the means for conditioning air 35 so that the means for 
conditioning air 35 is adjusted to 87.5% of its maximum capacity. 
Although the invention has been shown and described with respect to a best 
mode embodiment thereof, it should be understood by those skilled in the 
art that various other changes, omissions and additions in the form and 
detail thereof may be made therein without departing from the spirit and 
scope of the invention as set forth in the attached claims.