Note: Descriptions are shown in the official language in which they were submitted.
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FORCED AIR FURNACE CONTROL SYSTEM
AND METHOD OF OPERATION
BACKGROUND OF THE INVENTION
1. Field of the Invention
5 This invention relates in general to forced air furnaces of the type used for
residential and office building heating, and more particularly relates to systems
for controlling operation of forced air furnaces.
2. Descliylion of the Prior Art
The prior art co~ ..elcial and residential forced air heating systems employ
10 a heat exchanger which seyala~s the furnace combustion chamber from the
path of air leading to rooms or other spaces in the building which are to be
heated. During normal operation, the heat exchanger provides a barrier
preventing escape of the combustion products into the air flow. Cracks or
other defects in the heat exchanger can release the combustion products into
15 the air flow and building airspace. Components of the combustion products
which are poisonous to hllm~n~, yuillciyally CO gas, is detrimental to health,
and in many cases causes death to the building oc~ y~ . Because CO gas
is colorless and odorless, the occupants are oftentimes unaware of the problem
and can become asphyxiated.
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Among the prior art ~uel~ts to provide occllp~nt~ with warning of abnormal
furnace operation is U.S. patent 4,171,944to Hilsch"lA~ The patent
provides a smoke detector electrically connected to shut a furnace down when
excessive smoke escapes into the room co,~l~il,i"~ the furnace. U.S.patent
5 RE.30,936toKmetz provides atemperature-dependent control systemwhich
senses air telllpeldlulc in exhaust ducting to cut out the burners of a furnace
when the e~h~llst gases are too hot. U.S.patent 3,582,247to Faure provides
a system for ~hlltting a boiler down when leaks of combustible fluid are
~letecte~l. U.S.patent 4,715,214to Tveter detects leakage from a pressurized
10 system by sensing and calclll~ting dirrelcllces in gas pressure values. U.S.
patent 4,221,206to Haas relates to automobile systems and provides a system
for ~hlltting off an internal combustion engine responsive to carbon monoxide
detectors.
OBJECTS AND SUMMARY OF THE INVENTION
15 It is a general objection of the present invention to provide a forced air
h~ting system with improved safety features preventing buildup of dangerous
concentration of noxious gases within a building ~ir~pare.
Another object is to provide a safety control system which continuously
monitors the presence of CO gas in the air exit plenum of a forced air furnace
20 and ~ulolll~tic~lly shuts the furnace down when the CO concentration is
unsafe.
Another object is to provide a safety control system of the type described
which reacts to the presence of CO in the air flow from a forced air furnace
before the CO concentration in the building airspaces reaches unsafe levels.
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Another object is to provide a safety control system
of the type described in which the C0 sensor element is in the
furnace exit air plenum where the heated air is dry to obviate
the problem of relative humidity interference with the sensor.
Another object is to provide a safety control system
of the type described in which the C0 sensor element is
positioned in the air flow from the furnace where the gas
concentration is relatively much higher than in the building
airspace so that C0 detector sensitivity is less difficult to
achieve.
The invention in summary provides a safety control
system and method of operation in which a C0 gas sensor is
placed in the air exit plenum which is downstream in the flow
of heated air from a forced air furnace. The control system
includes means for producing an output signal when the sensor
detects that the concentration of C0 gas in the plenum reaches
a predetermined unsafe level. Control circuit means then
triggers a disable control signal responsive to the output
signal, and disable circuit means connected with the furnace
control circuit switches the furnace to a nonoperating mode.
Latch circuit means is provided to thereafter maintain the
furnace in its nonoperating mode independent of the disable
control signal. A reset circuit is also provided to
selectively override operation of the latch circuit means and
re-enable control of the furnace in its operating mode.
The invention may be summarized, according to one
aspect, as forced air furnace apparatus for heating a building
airspace with hot air, including the combination of a furnace
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having a combustion chamber for combusting fuel to produce hot
combustion products which contain C0 gas, heat exchanger means
in the combustion chamber for normally separating the
combustion products from a first air flow path and for
transferring heat from the combustion products to heat air
within said first path, said furnace having an outlet opening
with a given cross sectional area through which air in said
first path is directed at a first rate of flow out of the
furnace, means connected with said outlet opening for
providing a plenum chamber for directing air from the first
path along a second path within said plenum chamber, said
plenum chamber having a cross sectional area which is sized
greater than said given cross sectional area in an amount
sufficient to cause air to flow within the second path at a
second rate which is slower than said first rate, means for
directing air from the plenum chamber along a third path for
distribution to the building airspace, control circuit means
for switching the furnace between an operating mode for
burning fuel to produce said combustion products and a non-
operating mode in which combustion products are not beingproduced, sensor means positioned in the second path of hot
air in the plenum chamber for sensing the concentration of C0
gas therein and for producing an output signal responsive to
the concentration of C0 gas in the plenum chamber reaching a
predetermined level, and means for operating said control
circuit for switching the furnace to said non operating mode
responsive to said output signal.
The invention may be summarized, according to
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another aspect, as a method for controlling the operation of a
hot air furnace to prevent buildup of dangerous concentrations
of C0 gas within a building airspace being heated by hot air
distributed from an air exit plenum which is downstream in the
flow of hot air from the furnace, the furnace having a heat
exchanger which separates air circulating through the furnace
from the furnace's combustion products which contain C0 gas,
the method including the steps of controlling the furnace in
an operating mode by combustion fuel for producing said
combustion products, directing hot air from the heat exchanger
out of the furnace through an outlet path having a
predetermined cross sectional area while causing the air to
flow through the outlet path at a first rate, directing hot
air from the outlet path through a plenum path having a cross
sectional area which is greater than said predetermined cross
sectional area while causing air to flow through the plenum
path at a second rate which is slower than said first rate of
air flow, positioning a C0 gas sensor in the plenum path of
hot air while said furnace is in its operating mode, operating
said sensor to sense C0 gas within said plenum path and
generating a disable control signal when said C0 gas
concentration is sensed above a predetermined level, and
disabling said furnace control circuit by switching the
furnace responsive to said disable control signal into a
nonoperating mode in which said fuel combustion is
discontinued.
The foregoing and additional objects and features of
the invention will appear from the following specification in
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which the several embodiments have been set forth in detail in
conjunction with the accompanying drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view illustrating a forced air furnace system in
accordance with a plcfelled embodiment of the invention;
FIG. 2 is a srhPrn~tic block circuit diagram illustrating components of the
5 control system for the forced air furnace system of FIG. l;
FIG . 3 is a schematic circuit diagram illustrating details of certain components
of the circuit diagrammed in FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS
10 In the drawings FIG. 1 illustrates generally at 10 a forced air furnace
system incorporating a plcÇ~lled embodiment of the invention for use in
heating airspaces of a building, such as a co~lelcial or residential building.
The system includes a furnace 12 which can be of conventional design
comprising open flame burners 14 which burn a fuel, such as natural gas,
15 received from supply line 16 under inflll~nre of furnace control valve 18.
A heat exchanger 20 forms a combustion chamber 21 for confining the
gaseous combustion products. Normally the combustion products flow
upwardly into a flue 22 which exits through a wall of the furnace and into
a stack 24 for exhaust from the building. A box-shaped outer furnace
20 housing 26 encloses an air chamber 28 around the heat exrh~nger, and an
opening 30 conn~cte~ with a conduit 32 directs inlet air into the chamber.
At the bottom of the furnace housing an opening 34 directs heated air into
an exit air plenum 36. Hot air is directed from the plenum along ducting 38
to the airspaces in the building which are to be heated.
FIGS. 1 and 2 illustrates schematically a circuit 40 of the
furnace control system. The control system incorporates a
conventional building thermostat 42 which normally controls
on-off operation of the furnace for regulating airspace
temperatures. The thermostat which is installed as original
equipment with the furnace can be used by connecting circuit
components in series with the thermostat contact switch 43.
The control system of the invention can thereby be retrofit
into an existing furnace system. Alternatively, the control
system can be incorporated in the design of a furnace system
as original equipment. Thermostat contact switch 42 shown in
FIG. 2 connects in series through a pair of wires 44, 46 with
furnace control 18. Details of the thermostat and furnace
control are conventional and are not shown in the drawings for
purpose of clarity. In the operating mode of the furnace,
thermostat contact switch 43 is closed.
The invention provides a control circuit 48 shown
generally in the block diagram of FIGS. 1 and 2 and in detail
in the schematic of FIG. 3. The control circuit includes a
relay 50 which is connected in series with one of the wires 44
which interconnects furnace control 18 with thermostat 42.
The contacts of relay 50 are closed to enable the furnace for
its operating mode. When the relay contacts are open, furnace
control 18 is switched off and the furnace is shut down.
Power supply 52 provides electric power to the control
circuit.
A carbon monoxide (C0) sensor 54 is mounted on one
side wall of exit air plenum 36 with the active sensor element
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56 projecting into the path of air flow through the plenum.
This location for the C0 sensor element provides a number of
important advantages. From a safety standpoint, the plenum
location enables detection of any buildup of C0 to a harmful
level before the C0 concentration in the building airspace
increases. Next, C0 concentration in the plenum is at least
two times higher that in the building airspace, making it
possible to achieve greater C0 detection sensitivity. With
the C0 sensor placed in the plenum where the air has just
passed through the heat exchanger, a relatively high
concentration of C0 would exist if there is a crack or other
failure in the heat exchanger permitting release of combustion
products from chamber. Also, air in the plenum has been
heated to the point that it is quite dry with low relative
humidity, thereby reducing the problem of relative humidity
interference with the sensor. Another advantage from an
installation standpoint is that the plenum wall typically
affords a convenient location for installation of the detector
in the housing which contains the supporting electronics.
Sensor 54 can be of conventional design adapted for
sensing concentrations of C0 gas in an air atmosphere. The
sensor sold under model number TGS203 by the Figaro Company,
or its equivalent, is suitable for use in the invention.
Details of sensor 54 are shown schematically in FIG.
3. A titanium dioxide (TiO2) ceramic tube 56 is contained
within the sensor body. A pair of heater wires 58, 60 are
mounted within the sensor body and connected through leads to
a cable connector 62. Resistance heating of the wires heats
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the titanium dioxide ceramic tube 56 for degassing the sensor
by driving off water and other volatile components on its
surface. The sensor operates on the principle that titanium
dioxide in a reducing atmosphere, such as a gas with CO, will
reduce and give up oxygen. The titanium dioxide ceramic tube
becomes more conductive as oxygen is given up. A suitable IC
chip 63 is coupled through connector 62 to control the sensor
to operate in successive detect cycles of 2~ minutes duration.
Each cycle comprises a heating phase of approximately 90
seconds when the titanium dioxide ceramic 56 is degassed and
then cooled. In the next phase of approximately 60 seconds
the air, with any entrained C0, comes into contact with the
sensor. The C0 causes the titanium dioxide ceramic tube 56 to
lose oxygen and become slightly more conductive. In the final
phase of the cycle this conductivity is compared with a
reference level by means of a resistor 64 which is controlled
by chip 63. The resistance of the titanium dioxide ceramic
tube 56 goes down as the oxygen is driven off. Resistor 66 is
connected in the circuit to provide temperature compensation.
IC chip 63 contains conventional logic circuits
which are programmed in accordance with known techniques to
operate sensor 54 through its sequential detect cycles. Chip
63 also includes suitable internal comparator circuits which
are programmed in accordance with conventional techniques to
compare the magnitude of change in conductivity of the
titanium dioxide ceramic tube in relation to a known change in
conductivity that would result from a CO concentration which
has been determined to be unsafe. A CO concentration of 200
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ppm is established as the unsafe level in the explanation of
the present invention, although this level could vary
depending upon particular operating conditions. When IC chip
63 determines that the concentration of C0 gas within the
plenum being sensed by the gas sensor reaches the
predetermined unsafe level, a disable control signal is
generated by applying a high signal into line 68 leading to
field effect transistor 70 as well as line 72 leading to
buzzer alarm 74. The C0 concentration level at which the
disable control signal is triggered can be adjusted between
two levels by either installing or removing a jumper in line
76 between the IC chip and ground. The circuit can be
calibrated for the desired concentration levels by testing the
sensor in atmospheres of known C0 concentrations.
Control circuit 48 includes a second field effect
transistor 78 which is coupled with transistor 70 by the
circuit shown in FIG. 3 so that the two transistors can
cooperate to latch out relay 50 for maintaining the furnace in
its nonoperating mode independent of the disable control
circuit and until a reset button 80 in the circuit is pushed.
During normal furnace operation when sensor 54
operates to sense that the C0 concentration in the plenum is
below the unsafe level, then IC chip 63 drives the pin 82 of
transistor 70 high, turning it on. Current then flows from
the +5V power supply at 84 through the coil of relay 50 to
close its contacts and establish the circuit between building
thermostat 18 and furnace control 42. When transistor 70 is
on, its pin 86 is low, making pin 88 of transistor 78 low.
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This turns transistor 78 off. Therefore during normal furnace
operation the transistor 70 is on while transistor 78 is off.
If the IC chip logic determines that the sensed C0
concentration reaches an unsafe level, or if a line or cable
is disconnected, the disable control signal from the chip
causes pin 82 of transistor 70 to go low which in turn causes
its pin 86 to go high to +5V. This deactivates relay 50 and
opens the circuit into furnace control 42 to disable the
furnace. At the same time, the high on pin 86 of transistor
70 makes pin 88 of transistor 78 high. This causes pin 90 of
transistor 78 to go low, which in turn holds pin 82 of
transistor 70 low. This causes transistor 70 to latch and be
held in its off state, preventing relay 50 from closing. The
IC chip cannot pull pin 82 of transistor 70 back into its high
state. In this condition of the circuit, transistor 78 can
only be turned off by manually pushing reset button 80 to
close contacts 92 for overriding the latch and enabling
furnace operation. After C0 is detected in plenum 36, then
for safety reasons this latch feature prevents the circuit
from recovering without a manual reset.
A green LED 94 is provided in the circuit between
+5V power source 84 and pin 86 of transistor 70. When the
furnace is enabled by the circuit, LED 94 is closed and green
light is emitted, and when the furnace is disabled the LED is
open and the light is off. Even if the circuit is latched out
by transistors 70 and 78, if the green light is off then the
furnace is still not enabled. The purpose of this is to
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permit the user to know when the reset button can be pushed to
enable the furnace. If the reset is pushed and if everything
is normal, then the green light will go on.
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Circuit 48 also includes a yellow LED 96 which is controlled by chip 63 to
cycle on and off at 2l/2 minute intervals. A normally open test switch 98is
provided in the circuit for use in combination with the yellow LED to show
that the circuit is working propelly. If test switch 98 is closed then in no
5 more than one measurement cycle of the sensor, buzzer alarm 74 should
be activated and the furnace control should be disabled. Test switch 98
sim~ tes the condition of the sensor detecting an unsafe collcenll~tion of
CO, and the buzzer should sound in no more than one measurement cycle.
If the test switch is closed and yellow LED 96 goes through a complete on-off
10 cycle without the buzzer sounding, then this provides an indication of a
problem in the circuit. This permits the yellow LED to be used in the test
process to verify that the test is complete.
It is a feature of the control circuit that buzzer alarm 74 goes off if the CO
concentration is below the unsafe level even if relay 50 is latched out.
15 Therefore the buzzer alarm gives an on-line "CO exceeded" signal even if
the furnace is shut down. This helps the user delell~ e if CO has been
eli.,~ led asthe source of anyproblems inthe furnace system. Forexample,
should the buzzer stay on after the furnace has been shut down for a time
sufficient to dissipate the CO concentration, then this would tell the user
20 that the problem is in the circuit and not that of CO leakage.
While the foregoing embodiments are at present considered to be prcrellcd
it is understood that numerous variations and modifications may be made
therein by those skilled in the art and it is intended to cover in the appended
claims all such variations and modifications as fall within the true spirit and
25 scope of the invention.
