Night setback-morning ready control system for unit ventilators

The invention provides a system for transmitting operational command signals to one or more remote unit ventilator units from a master control panel. A number of selected command signals corresponding to operational modes such as occupied day, night setback, morning ready and load shedding modes may be transmitted over a two-wire electrical transmission line to which the remote units are parallelly connected. The remote units receive and respond to the command signals by initiating the commanded mode.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to temperature control systems and 
more particularly to a system having means for controlling the operational 
mode of remotely located unit ventilators. 
2. Description of the Prior Art 
In unit ventilator type temperature control systems individual 
self-contained operating units are disposed in each room of a building. It 
has become the common practice to provide a master control panel for 
controlling the operational modes of the unit ventilators. Three modes of 
operation are generally provided as follows: night setback, morning 
warm-up and occupied day. During the night setback mode of operation, the 
temperature set point is reduced by up to 20 degrees below the daytime 
temperature set point, the outside air dampers are closed and the 
mechanical refrigeration is disabled. During the morning warm-up mode of 
operation only the daytime temperature set point is reinstated, the 
outside air dampers remain closed and the mechanical refrigeration remains 
disabled. During the occupied day mode of operation, the daytime 
temperature set point controls the system, both heating and refrigeration 
are allowed to operate as required, and the outside air dampers are opened 
to introduce fresh air to the rooms. 
Most prior art devices used pneumatic temperature control systems for 
controlling the three modes of operation by providing three pressure 
levels that are sensed at the unit ventilators by pressure responsive 
relays which initiate the desired mode of operation. Refinements to the 
three-pressure level type pneumatic control systems have been made so that 
two pressure levels in conjunction with a pulsed drop in pressure may be 
used to control the three modes of operation. 
In general, all pneumatic control type systems required a high initial 
installation cost since pneumatic tubing or piping systems had to be 
directed throughout an entire building. 
It is apparent that the three-pressure level type of pneumatic control 
systems require sophisticated and accurate control devices for providing 
accurate and non-fluctuating pressure levels to supply lines. Sensitive 
pressure relays that operate only within narrow pressure ranges are also 
required. Thus, the three-pressure level type systems were relatively 
expensive and were sensitive to fluctuations in control pressure. 
The refined two-pressure level type systems alleviated some of the problems 
encountered with accurate pressure control and pressure fluctuations; 
however, more sophisticated pressure receiving equipment was required that 
could respond to pressure pulses of certain magnitudes and durations. 
Electrical control systems that did not require the expensive installation 
of pneumatic tubing were provided for controlling the operational modes of 
remote unit ventilators. These electronic systems were either complex 
systems requiring sophisticated and expensive control circuitry for 
detecting coded command signals or large conventional systems using a 
number of clock-operated relays and a transmission cable having a wire 
pair for each of the modes of operation. 
With the advent of the energy crisis and the limited electrical energy 
supply in many areas, it has become desirable to provide an additional 
operating mode for unit ventilators wherein either a heating or cooling 
load may be partially shed during "brownouts" when electric utility 
companies experience critical overload conditions. The prior art devices 
could not provide this additional control mode without a substantial 
increase in cost and sophistication of the control systems which would 
require additional pairs of wires in electronic control systems or the 
provision of additional pressure levels in pneumatic control systems. 
SUMMARY OF THE INVENTION 
The present invention contemplates an electrical control system for 
controlling the operational mode of one or more heating, ventilating or 
air-conditioning units from a control panel. This novel system has the 
advantage of a low initial installation cost while providing for the 
control of up to four operational modes, such as occupied day, night 
setback, morning ready and load shedding modes. 
Through the use of unique but simple transmitting and receiving circuitry, 
control signals for four operational modes may be transmitted over a 
two-wire transmission line which may be inexpensively run throughout an 
entire building with the unit ventilators being connected in parallel to 
the two-wire transmission line. The signals used for the four operational 
modes are as follows: no signal; full-wave a.c.; positive d.c.; and 
negative d.c. Thus, the system has been designed so as not to require the 
sensing of a plurality of specific signal levels as in the prior art 
devices which were very sensitive to fluctuations in electrical power. The 
invention uniquely uses four distinctly different signals that may easily 
be detected without the need for complex circuitry. 
A receiving section in each unit ventilator receives the signals from the 
two-wire system and through logic circuitry, provides an output signal at 
a particular one of four terminals so that the desired operational mode is 
initiated. 
The primary objective of the present invention is to provide a simple 
low-cost system for controlling the mode of operation of one or more unit 
ventilators. 
Another objective of the present invention is to provide a mode controlling 
system for unit ventilators that is not sensitive to voltage fluctuations. 
Another objective of the present invention is to provide an operational 
mode control system for unit ventilators that has increased mode control 
capability while using only a two-wire transmission line. 
Other objectives and advantages will appear in the following description 
which, when considered in conjunction with the accompanying drawings, set 
forth three embodiments of the present invention.

DESCRIPTION OF THE INVENTION 
The invention will be described as controlling unit ventilators as an 
example, but it is to be understood that the invention may be used to 
provide operational command signals to any type of remotely controlled 
system where up to four signals are required and a two-wire transmission 
line is available. It is to also be understood that while the invention 
will be described as providing for the control of four distinct 
operational modes, the invention may easily be practiced for the control 
of any lesser number of operating modes and in particular, the 
conventional heating, ventilation and air-conditioning modes of occupied 
day, night setback, and morning ready. 
Referring to FIG. 1, there is shown a master control transmitter 10 which 
may be located in a separate control panel or in one of the unit 
ventilators used in a temperature control system. The master control panel 
10 is connected to remote unit receivers 12 and 14 through a pair of 
transmission wires 16 and 18 to which the remote unit receivers are 
parallelly connected. Wire 16 provides a signal to the remote unit 
receiver while wire 18 is merely a return line. Throughout the 
application, reference will be made to a two-wire system; however, it is 
to be understood that this could be one wire if a good interconnect ground 
were used to replace wire 18. 
Referring to the master control transmitter 10, which may be located in a 
separate control panel or in one of the unit ventilators of the system, 
there is provided a low-voltage transformer 20 having a primary winding 22 
connected across a source of 115 volts a.c. and a secondary winding 24 
which provides a low voltage output such as 24 volts a.c. Timing clocks 
T.sub.1 and T.sub.2 are connected across the 115 volt a.c. source and have 
normally open contact pairs 26 and 28 respectively. It is to be understood 
that timers T.sub.1 and T.sub.2 could be combined into a single timer 
having two sets of contact pairs; however, two separate timers have been 
shown in the drawing since, as a practical matter, two timers each with a 
single contact pair are cheaper than one timer with two contact pairs. One 
end of the secondary winding 24 of transformer 20 is connected directly to 
wire 18 of the two-wire transmitting system, while the other end of the 
secondary winding of transformer 20 is connected to one contact of each of 
the contact pairs 26 and 28. The other contact of contact pair 26 is 
connected to an anode of a diode 30, while the other contact of contact 
pair 28 is connected to a cathode of a diode 32. A cathode of diode 30 and 
an anode of diode 32 are connected to wire 16 of the two-wire transmission 
line. 
A double pole switch 34 may be included in the master control transmitter 
for the purpose of manually providing a load shedding command signal. The 
double pole switch has a pair of normally closed contacts connected 
between diode 30 and contact pair 26 and a normally open pair of contacts 
connected across the contact pair 28. 
Referring to FIG. 3, there is shown an alternate embodiment for the master 
control transmitter 10 wherein the contact pairs 26 and 28 are connected 
in series and the diodes are also connected in series with their cathodes 
connected. The series connected diodes are in parallel with the contact 
pairs and a jumper connects the cathodes of the diodes with the connection 
between the contacts. Switch 34 is connected to short out contact pair 28 
and open contact pair 26 in a manner similar to that shown in FIG. 1. 
As an example of the operation of the master control transmitter, clocks 
T.sub.1 and T.sub.2 may be set so that contact pairs 26 and 28 both close 
at 5:00 P.M., which is normally the end of a working day to initiate night 
setback. Clock T.sub.2 may be set to open contact pair 28 at 6:30 A.M. to 
initiate the morning ready mode, while T.sub.1 may be set to open contact 
pair 26 at 8:30 A.M. to initiate the occupied day operation. Switch 34 is 
provided to manually initiate load shedding when necessary. During the 
period between 8:30 A.M. and 5:00 P.M. when contact pairs 26 and 28 are 
open, no signal is provided to wires 16 and 18 and this zero signal 
condition corresponds to the occupied day operation. At 5:00 P.M. when 
contact pairs 26 and 28 both close, a 24 volt a.c. signal is transmitted 
over lines 16 and 18. This signal corresponds to the night setback mode of 
operation. At 6:30 A.M., contact pair 28 opens so that only a positive 
half-wave d.c. signal is provided to wires 16 and 18 and this signal 
corresponds to the morning ready mode of operation. At 8:30 A.M. when 
contact pair 26 again opens, the system reverts back to the occupied day 
operation with no signal being provided to the wires 16 and 18. In the 
event that load shedding is required, switch 34 is moved from its normal 
position so that contact pair 28 is shorted and the contact pair in series 
with contact pair 26 opens so that a negative half-wave d.c. signal is 
provided to wires 16 and 18, which corresponds to a load shedding mode of 
operation. 
The circuitry for the master control transmitter 10 has been described in 
an embodiment that provides for four distinctly different signals 
corresponding to operational modes for a unit ventilator. In the event 
that only three command signals are required, the circuit may be 
simplified by elimination of the switch 34 and diode 32 so that three 
distinctly different signals would be provided as follows: no signal, 
full-wave a.c. or positive half-wave d.c. signal. It will be apparent to 
one skilled in the art that any combination of less than four signals may 
be used by charging the contact closing sequence or by the elimination of 
certain diodes. 
The remote unit receiver 12 shown in FIG. 1 illustrates one embodiment of a 
remote unit receiver design that may be used with the present invention. 
Remote unit receiver 12 has a resistor 36 connected between wire 16 and an 
emitter element of a PNP transistor 38. Transistor 38 has a base element 
connected to a positive d.c. potention +VDC and a collector element 
connected to a negative d.c. potential -VDC through a parallel connection 
of a capacitor 40 and a resistor 42. A transistor 44 also of the PNP type 
has an emitter element connected to wire 18 and to the base element of 
transistor 38 and is therefore biased at the +VDC potential. Transistor 44 
also has a base element connected to the emitter element of transistor 38 
and a collector element connected to the -VDC negative potential through a 
parallel connection of a capacitor 46 and a resistor 48. The positive and 
negative potentials +VDC and -VDC may have a level in the order of 6 
volts, however, the exact level is not critical in the practice of the 
invention provided it is less than the peak level of the a.c. signal from 
secondary 24 of transformer 20. 
When no signal is provided on lines 16 and 18 during the occupied day 
operation, both transistors 38 and 44 remain in a cut-off condition 
thereby isolating the +VDC potential from the capacitors 40 and 46. Any 
residual charge remaining on capacitors 40 and 46 will be bled off through 
resistors 42 and 48 respectively so that ultimately the -VDC potential 
will be present at points A and B. When the contact pairs 26 and 28 both 
close during the night setback mode of operation, a full-wave a.c. signal 
appears on wires 16 and 18, which signal will cause transistors 38 and 44 
to alternately switch on and off with transistor 38 conducting on the 
positive half cycle of the a.c. signal and transistor 44 conducting on the 
negative half cycle of a.c. signal. When the transistors conduct the +VDC 
potential is alternately connected to capacitors 40 and 46 which charge to 
the +VDC voltage level. The time constant of capacitor 40 and resistor 42 
and capacitor 46 and resistor 48 is such that the capacitors will not 
substantially discharge during the non-conducting period of their 
respective charging transistors so that the +VDC voltage may be developed 
on the capacitors. Thus, during the night setback mode of operation, both 
capacitors are charged so that a positive potential appears at both points 
A and B. 
During the morning ready mode of operation when contact pair 26 is closed 
and contact pair 28 is opened only a positive half-wave d.c. signal is 
transmitted over wires 16 and 18. The signal causes transistor 38 to 
conduct during the positive half-waves, while transistor 44 remains cut 
off. Since transistor 44 is cut off, capacitor 46 discharges through 
resistor 48 so that the -VDC potential appears at point B. The periodic 
conducting of transistor 38 keeps capacitor 40 charged so that +VDC 
potential remains at point A. During a load shedding mode of operation, a 
negative half-wave d.c. signal is provided to the wires 16 and 18, which 
causes transistor 38 to remain cut off, but causes transistor 44 to 
alternately conduct during the negative half-waves. The conduction of 
transistor 44 causes capacitor 46 to charge to the +VDC potential, while 
capacitor 40 discharges through resistor 42 to the -VDC potential. Thus, 
point A has a -VDC potential and point B a +VDC potential. 
Referring to the transistors 38 and 44, it is to be noted that the bases 
are connected to the emitters of the other transistor. Using this type of 
connection the voltage across a reverse biased base-emitter junction never 
exceeds the forward voltage drop across a forward biased emitter-base 
junction and the transistors are protected. 
If a +VDC voltage is equated with a logic level 1 and a -VDC voltage is 
equated with a logic level zero, the four modes of operation may be 
represented by the four binary numbers comprising the combinations of 
one's and zero's that appear at points A and B. 
Using logic elements, the four binary numbers at points A and B may be 
converted to four combinations of signals at four separate output 
terminals. Point A is connected to the input of an inverter 50, while 
point B is connected to the input of an inverter 52. A NAND-gate 54 has 
first and second inputs connected respectively to points A and B and an 
output connected to a terminal 56. A NAND-gate 58 has first and second 
inputs connected to outputs of inverters 50 and 52 and an output connected 
to a terminal 60. A NAND-gate 62 has a first input connected to the output 
of inverter 52 and a second input connected to point A and an output 
connected to a terminal 64. A NAND-gate 66 has a first input connected to 
an output of inverter 50 and a second input connected to point B and an 
output connected to a terminal 68. 
With the previously mentioned logic elements connected as shown and 
described, during the night setback mode of operation when an a.c. signal 
is provided to the transmission wires 16 and 18, a one-level logic signal 
is provided at points A and B. The one-level signals are applied to the 
inputs of NAND-gate 54 so that the NAND-gate provides a zero level output 
to terminal 56. The remaining NAND-gates each have at least one input 
connected to an output of one of the inverters 50 and 52 so that a zero 
level signal is applied to the inputs, and the outputs to terminals 60, 64 
and 68 are at the one-level. During the occupied day mode of operation 
when no signal is provided to the transmission wires 16 and 18, a zero 
level logic signal is provided at points A and B and these zero level 
signals are inverted by inverters 50 and 52 so that one-level signals are 
provided at both inputs of NAND-gate 58 so that a zero level output is 
provided to terminal 60. Each of the other NAND-gates has at least one 
input with a zero level input signal applied thereto and therefore 
one-level signals are provided to terminals 56, 64 and 68. In like manner, 
during the morning ready mode of operation when positive half-wave signals 
are provided to transmission wires 16 and 18, a one-level logic signal is 
provided at point A and a zero level logic signal is provided at point B. 
Thus, one-level signals are provided at both inputs to NAND-gate 62 so 
that a zero level signal is provided at terminal 64 during the morning 
ready mode of operation. During a load shedding mode of operation, a zero 
level signal is provided at terminal 68 while one-level signals are 
provided at terminals 56, 60 and 64. 
Thus, for each of the possible four operating modes, one of the four 
terminals is provided with a zero level signal, while the other terminals 
have provided thereon a one-level signal. 
It is to be noted that in the remote unit receiver 12 the +VDC potential is 
connected to wire 18 and therefore the d.c. bias runs throughout the 
building and the units are all electrically connected. It may be desirable 
to have the units electrically isolated from each other and an embodiment 
of a receiver that accomplishes this is shown in FIG. 1 as remote unit 
receiver 14. The transistors 38 and 44 have been replaced with optical 
couplers 70 and 72, with the remainder of the circuit being substantially 
identical to that of remote unit receiver 12. The optical coupler 70 has a 
light emitting diode 70a optically coupled to a light sensitive transistor 
70b. In like manner coupler 72 has a diode 72a and transistor 72b. The 
optical couplers provide an advantage in that the positive potential +VDC 
remains isolated from the transmission wires 16 and 18 and each unit is 
electrically isolated from another. The optical couplers may be of the 
type sold under the trade name of Litronix, Model ILD 74. The operation of 
optical couplers 70 and 72 is similar to that of transistors 38 and 44 in 
that the transistors 70b and 72b do not conduct when a zero signal is 
provided to the transmission wires 16 and 18 and they both conduct when an 
a.c. signal is provided to the transmission wires. When a positive 
half-wave d.c. signal is provided, transistor 70b is caused to conduct and 
when a negative half-wave d.c. signal is provided, transistor 72b conducts 
so that 0 and 1 level signals are provided at points A and B as in remote 
unit receiver 12. 
Referring to FIG. 2, there is shown how the remote unit receiver 12 is 
connected to the control circuitry of a unit ventilator system. The 
terminals 56, 60, 64 and 68 are respectively connected to one lead of 
relay coils K1, K2, K3 and K4, which have their other leads commonly 
connected to a positive potential +V. Relay coils K1 through K4 are 
low-power sensitive relays that may be operated within the power output 
capability of the NAND-gates used in the remote unit receivers. A 1 level 
signal at an output terminal has a sufficient positive potential so that 
the associated relay remains de-energized. 
Referring now to the fresh air damper control circuit 73 of the unit 
ventilator, a potentiometer 74 is connected between positive and negative 
potentials +V and -V respectively and includes a wiper arm 75 which may be 
positioned to select a minimum percentage of fresh air to be delivered to 
a room during operation of the unit ventilator. Wiper arm 75 is connected 
to an inverting input of an operational amplifier 76 through a resistor 
78. Operational amplifier 76 is a standard 741 type operational amplifier 
and has a non-inverting input connected to a reference signal which may be 
zero volts. Operational amplifier 76 has an output which is connected to 
the inverting input by way of a feedback resistor 80. The output of 
operational amplifier 76 provides a control signal to a fresh air damper 
system 82 of the unit ventilator. Contact pairs K1a of relay K1 and K3a of 
relay K3 are connected in parallel and have one contact thereof connected 
to a positive potential +V and the other contact thereof connected to the 
inverting input of amplifier 76 through a resistor 84. When relays K1 and 
K3 are not energized, the normally open pairs of contacts K1a and K3a have 
no effect on circuit 70 and the fresh air dampers 82 are driven to a 
position as selected by wiper arm 75 on potentiometer 74. However, when 
either of relays K1 or K3 are energized, such as during the night setback 
or morning ready modes of operation, contact pairs K1a or K3a are closed, 
thereby connecting the positive potential +V to the inverting input of 
amplifier 76 through resistor 84, which potential is sufficient to provide 
an output at amplifier 76 to drive the fresh air dampers 82 to a closed 
position. 
A potentiometer 86 is connected between positive and negative potentials +V 
and -V and has a wiper arm 88 which may be adjusted to select a daytime 
temperature set point. A potentiometer 90 is also connected between the 
positive and negative potentials and has a wiper arm 92 which may be 
adjusted to select a nighttime temperature set point. Wiper arm 88 of 
potentiometer 86 is connected to the inverting input of an amplifier 94 
through a normally closed contact pair K1b of relay K1 and through a 
resistor 96. A pair of normally opened contacts K1c of relay K1 are 
disposed between the wiper arm 92 of potentiometer 90 and the point of 
connection between resistor 96 and contact pair K1b. Amplifier 94 is a 
standard operational amplifier of the 741 type and has a non-inverting 
input connected to a reference potential which may be zero volts. 
Amplifier 94 also has an output which is connected back to the inverting 
input through a feedback resistor 98. A resistor 100 is connected between 
the positive potential +V and the inverting input of amplifier 94, while a 
series connected resistor 102 and a temperature sensor 104 are connected 
between a negative potential -V and the inverting input of amplifier 94. 
The temperature sensor 104 may be a negative temperature coefficient 
thermistor which increases resistance when subjected to a decrease in 
sensed temperature. Thermistor 104 is disposed in a location where it is 
subjected to the room temperature. 
The operation of the circuits thus far described is such that when heat is 
required either because of an increase in a temperature set point by the 
movement of wiper arm 88 or 92 or by a sensed reduction in room 
temperature, the output from amplifier 94 moves in a negative direction to 
provide a negative output signal. In like manner, if a reduction in heat 
or mechanical cooling is required, either because of a reduction in the 
temperature set point or because of a sensed increase in room temperature, 
the output of amplifier 94 will increase to provide a positive output 
signal. 
The signals from the output of amplifier 94 are connected to the inputs of 
threshold trigger circuits 106, 108 and 110. Each of said trigger circuits 
includes an input resistor 112 for connecting the signal from amplifier 94 
to a non-inverting input of an operational amplifier 114 of the 741 type. 
The operational amplifier has an output which is connected back to the 
non-inverting input through a resistor 116 for providing positive feedback 
thereto. Each of the operational amplifiers 114 has an inverting input 
connected to a reference signal. The reference signal for the trigger 
circuit 106 may be zero volts d.c. so that any negative signal at the 
output of amplifier 94 will result in a negative output signal from the 
operational amplifier 114. The reference signal connected to the trigger 
circuit 108 may preferably be a negative reference signal so that the 
trigger will not provide a negative output until a larger negative signal 
is provided by amplifier 94. Thus, trigger circuits 106 and 108 will 
trigger successively at different level output signals from amplifier 94, 
rather than simultaneously. The reference signal for trigger circuit 110 
may preferably be a positive d.c. level so that a positive output from 
amplifier 94 will have to increase to a certain threshold level before the 
output amplifier 114 switches from negative to positive level. 
A coil of a relay K5 is connected between a positive potential +V and the 
output of the trigger circuit 106 so that the relay K5 is activated when 
the circuit initially calls for heat and the output of trigger circuit 106 
goes negative. Relay K5 has a pair of contacts K5a which are normally open 
and are connected to a Heat Stage No. 1 of a unit ventilator system. The 
Heat Stage No. 1 is activated when relay K5 is energized. The coil of a 
relay K6 is connected between a positive potential +V and the output of 
trigger 108 through a normally closed pair of contacts K4a of relay K4 so 
that relay K6 is activated when additional heat is called for by a more 
negative signal being provided at the output of amplifier 94. Relay K6 has 
a pair of normally open contacts K6a connected to a Heat Stage No. 2 for 
activating the heat stage when additional heat is called for. During the 
load shedding mode when terminal 68 has a zero logic level output, relay 
K4 is energized so that the normally closed contacts K4a are opened, 
thereby de-energizing relay K6 and shutting off the Heat Stage No. 2 
thereby conserving energy that would be consumed by the additional heat 
stage. 
The output of the trigger circuit 110, which is normally at a low level, is 
connected to a first input of a NAND-gate 118. NAND-gate 118 has a second 
input connected to an output of an inverter 120 which has an input 
connected to ground through a normally closed pair of contacts K1d of 
relay K1. The input of inverter 120 is also connected to a positive 
potential +V through a resistor 122, which supplies a positive level or 
logic level one signal to the input of inverter 120 when contacts K1d of 
relay K1 open during the night setback mode of operation. The coil of a 
relay K7 is connected between a positive potential +V and the output of 
NAND-gate 118 and has a pair of contacts K7a which are normally opened and 
are connected to a mechanical cooling portion of the unit ventilator. 
During normal daytime operation when relay K1 is de-energized, the input 
of inverter 120 is at ground or logic level zero and therefore, the 
inverter provides a one level signal to the NAND-gate 118. When the output 
signal from amplifier 94 goes positive by a sufficient amount to overcome 
the threshold level set in trigger 110, the output of trigger 110 goes 
positive so that the NAND-gate 118 provides a zero level output signal 
which energizes relay K7 and initiates mechanical cooling. During the 
night setback mode of operation when relay K1 is energized, it is 
desirable that the mechanical cooling be disabled and this is provided by 
opening contacts K1d so that a positive or one-level signal is seen at the 
input of inverter 120 and the inverter provides a zero level signal to the 
input of the NAND-gate 118 so that the output of NAND-gate 118 is at a one 
level and prevents energization of the relay K7, thereby disabling 
mechanical cooling. 
If it is desirable not to have the mechanical cooling activated during the 
morning ready period, then rather than disabling the cooling during the 
night setback mode, it may be advisable to enable the cooling only during 
the occupied day mode when relay K2 is energized. In such a case, contacts 
K1d would be replaced with a pair of normally open contacts controlled by 
relay K2 so that the input of inverter 120 would be grounded to a zero 
logic level during the occupied day mode of operation. 
It is to be understood that the invention may be used in place of the 
relays. The relays have been used in the description only because their 
operation is easier to describe and illustrate. 
Thus, the present invention provides an inexpensive and simplified means 
for controlling the operational mode of one or more unit ventilators which 
may be remotely located. The system is simple and inexpensive to install 
since it only requires the installation of a two-wire transmission line 
throughout the building. Using the unique transmitter circuit distinctly 
different and easily detectable command signals are transmitted over the 
two-wire system. When used to control a plurality of unit ventilators, the 
transmission system may be used to transmit command signals from a master 
control transmitter for controlling operational modes, of the unit 
ventilator, such as occupied day, night setback, morning ready and if 
desired, a load shedding. The system, while being less complex and less 
expensive than those of the prior art devices, is not sensitive to voltage 
fluctuations or pneumatic pressure fluctuations as were the devices of the 
prior art. 
Thus, the present invention decreases the actual cost of the control 
system, decreases the installation cost and improves reliability and 
performance of the system.