Abstract: The present application relates in general to an apparatus and to methods for testing the performance of an automotive catalytic converter under conditions simulating those which occur in motor vehicles over extended driving conditions. The application provides a novel swirl plate and a novel fuel injector which enable the burner to run stoichiometric for extended periods of time.

Abstract: The burner preferably exclusively burns substantially explosible solid fuels and preferably has instant ON-OFF thermostat control, wastes no energy preheating the enclosure or external air supply, achieves stable combustion the moment the powder-air mix is ignited in our burner, is used in the upward vertical mode except for oil burner retrofits, burns a solid fuel in a single-phase regime as if it were a vaporized liquid or gas, is designed to complete combustion within the burner housing itself rather than in a large, high temperature furnace enclosure which it feeds, has an ultra-short residence time requirement, is a recycle consuming burner with self-contained management of initially unburned particles, is much smaller, simpler and lower cost, has a wider dynamic range/turndown ratio, is more efficient in combustion completeness and thermal efficiency, and operates with air-fuel mix approximately at the flame speed.

Abstract: A method and apparatus for controlling a fuel-fired appliance is provided. The appliance enters a wait state in which burner operation ceases if a sensor indicates the presence of flammable vapors that are above an acceptable and/or safe vapor level. The appliance returns to a run state if the vapor level returns to an acceptable and/or safe vapor level within a period of time, but enters a lockout state if the vapor level does not return to an acceptable and/or safe vapor level within the period of time.

Abstract: An improved system and operating method for a smoke generator machine and control room to be enclosed into a single trailer for the training of personnel in the art of visible smoke opacity compliance monitoring of industrial smoke discharge from smoke stacks by distinguishing the variations of smoke opacity emission based on the requirements set forth by the U.S. Environmental Protection Agency EPA said smoke generator is principally used for production of white and black smoke for simulating smoke discharge from industrial smoke stacks by heating various fuels to the flash point in burn boxes to create white or black smoke and venting said smoke through a duct system to a smoke stack by creating a artificial draft by a speed control fan.

Abstract: A fan modulated premix burner control system includes a regulating control valve for regulating the pressure of gas fed to a premix chamber. The pressure of the gas is based on the pressure of air also fed into the chamber to produce a constant air/fuel ratio. The system also includes an air restrictor for reducing the pressure of the air to permit operation of the burner at low fan speeds.

Abstract: A direct gas-fired industrial air heater low fire start control, includes circuitry for simulating a resistant circuit which bypasses the discharge temperature sensors, the remote temperature selector, and a space temperature controls, that has the effect of driving the modulating valve to a fixed open setting which in turn changes the valve voltage in order to obtain desired gas flow. In a further embodiment, an isolated DC voltage source which normally bypasses the voltage input to the system of the modulating valve can be inserted into the circuitry to effectively drive the modulating valve to a fixed open setting to obtain the desired gas flow rate. Various circuitry, and bypass gas flow arrangements, are further shown to increase the efficiency of ignition of the burner assembly, when started.

Abstract: A reactant supply and control system supplies a regenerative burner assembly with streams of pilot fuel and pilot air. The system can maintain a pilot flame throughout regenerative cycles in which a main flame is turned on and off, and supplies either or both of the pilot streams with an increased flow rate for shifting from a regenerative exhaust condition to a regenerative firing condition.

Abstract: An electronic gas control apparatus including a variable flow valve, an igniter, a flame sensor, and an electronic controller is provided. The valve has a flow range that equals or exceeds a required flow range for any one of the types of gas suitably combusted by the burner. The electronic controller is operably coupled to each of the valve, the igniter, and the sensor. The electronic controller is configured to activate the igniter, to receive a flame status signal from the sensor, and to recognize a position of the valve when the flame status signal indicates a flame at the burner. As such, the electronic controller is calibrated for the gas, line-pressure, jet and burner configuration being used.

Abstract: A gas regulating fitting that, in addition to enabling an electronic ignition of the gas flow, also permits a manual ignition. A covering element (17) is displaceably mounted on the housing (1) of the gas regulating fitting and, in a first position, covers a tappet (10; 14), which is provided for actuating a thermoelectric ignition safety valve (26) and a main valve (19), and covers a control switch (13) of a piezoelectric igniting element. When the covering element (17) is in a second position, an actuation of the tappet (10; 14), which inevitably occurs when the covering element (17) is displaced, ensures that the main valve (19) is located in the closed position. In addition, the control switch (13) and the tappet (10, 14) are released in this position in such a manner that an ignition of the gas flow is made possible by a manual actuation thereof.

Abstract: An apparatus for operating a gas valve in a gas-fired heating system includes a solenoid having a pick coil and a hold coil connected to the pick coil and to a thermo-generator. A power supply powers the pick coil to open the gas valve. The thermo-generator powers the hold coil to hold the gas valve open. The apparatus is small and inexpensive compared to systems that use DC-DC converters and/or stepper motors to operate a millivolt valve. A millivolt valve can be operated via power from a heater thermostat, without AC power having to be wired to the heater.

Abstract: A linearly regulated fuel valve system is provided that includes a slotted conical rotatable plug component that is received in a conical cavity formed in a valve body which can be remotely controlled. The slotted plug serves to establish multiple flow paths through the valve body when rotated so as to align an opening formed in the cavity wall with an orifice formed in the side of the plug. The rotational orientation of the plug with respect to the wall and the orifice in the plug is set by a latch element permitting graduated stepwise adjustment settings such as high, medium, low and shut off of gas flow through the valve. Accordingly, the unique system may be adjusted by remote controlled to a wide range of settings by a single cone having a constant channel for fuel flow.

Abstract: While ensuring technical safety and a short start-up time the invention permits the operation of autothermic partial oxidation of fuels processed into pulverized fuel such as lignite and bituminous coals, petroleum cokes, solid grindable carbon-containing residues, as well as solid-liquid suspensions or slurries, with a gasification agent containing oxygen at operating pressures of up to 8 MPa (80 bar). The selective configuration of the fail-safe monitoring of the gasification process only the supply of the main fuel is cut off. Through the continued operation of the pilot and ignition burner the reactor is kept at operating pressure and after the fault has been rectified fuel gasification can be restarted with the pilot and ignition burner without a complicated placement and pulling of a starter burner and subsequent pressurization of the reactors.

Abstract: A cooking stove that prevents the execution of a process such as ignition of a burner when a touch switch changes from a non-sensing state to a sensing state. In STEP201, when a first touch switch, which is one of the touch switches provided in an operation, is turned on from off, the process advances to STEP202, where a controller provided in a cooking stove main body starts a 1-second timer. The controller executes a loop composed of STEP203 and STEP210. When a second touch switch, which is one of the touch switches and is different from the first touch switch, is turned on from off in STEP210 before the 1-second timer times up in STEP203, the process advances from STEP210 to STEP211. The controller performs “error reporting” by blinking a display portion and activating a buzzer.

Abstract: An air quality sensor and interrupter system includes an air quality sensor to measure the air quality around a gas appliance and an appliance shut-off device to turn off the gas appliance in response to a signal from the sensor indicating unsafe air. An exhaust can also be activated to remove the unsafe air. Unsafe air can be indicated by a measurement of 18.5% or less oxygen in the air. If the sensor fails, an alarm can alert the user and the appliance shut off until the sensor is replaced or repaired. A bypass allows the user to keep the appliance turned on, but only for a limited amount of time. The user can keep utilizing the bypass to keep the appliance turned on for extended periods of time.

Abstract: A safety system for connection to a toxic gas detector and a toxic gas producing engine, and its operating method, includes an interrupt device coupled to connections to the detector and the engine. The interrupt device includes a toxic gas signal detecting circuit and a circuit for permitting starting of the engine. The detecting circuit activates the permitting circuit if the toxic gas signal represents a toxic gas concentration below a predetermined level. In one embodiment, the permitting circuit is activated if the safety system is healthy and fully functional. Similarly, a safety system for connection to a toxic gas detector and a toxic gas supply, and its operating method, includes an interrupt device coupled to connections to the detector and the toxic gas supply. The interrupt device includes a toxic gas signal detecting circuit and a circuit for permitting of flow from the toxic gas supply.

Abstract: Systems, methods, and controllers for controlling gas-fired appliances such as warm air furnaces are disclosed. An illustrative furnace system can include a burner unit in communication with a combustion air flow conduit and heat exchanger, a variable speed inducer fan or blower adapted to provide a flow of combustion air to the burner unit, a furnace controller and motor speed control unit adapted to regulate the speed of the inducer fan or blower, and a pneumatically modulated gas valve adapted to variably output gas pressure to the burner unit based at least in part on the combustion air flow.

Abstract: Systems and methods for controlling gas to gas-fired appliances are disclosed. An illustrative system can include a modulating gas valve adapted to supply gas to a burner unit, an inducer fan adapted to produce a combustion air flow at the burner unit, a pressure reducing element in fluid communication with the gas valve and adapted to output at least one pressure signal for sensing the combustion air flow outputted by the inducer fan, and a controller for controlling the speed of the inducer fan. The pressure reducing element can include a venturi tube, flow nozzle, or other suitable device capable of producing pneumatic signals that can be used by the gas valve to modulate the gas supplied to the burner unit. By pneumatically linking the gas valve to the actual combustion air flow, the gas valve can be operated over a wide range of firing rates by adjusting the speed of the inducer fan.

Abstract: A furnace system responsive to a thermostat includes a pollutant sensor for sensing a pollutant concentration in the furnace system. The pollutant sensor is configured to open when the pollutant concentration reaches a pollutant threshold and close when the pollutant concentration falls below the pollutant threshold. When the thermostat is calling for heat, a furnace controller monitors the pollutant sensor and disables the furnace system for a lockout period if a lockout criterion related to the pollutant sensor is met.

Abstract: The current invention provides a method and apparatus, which uses symbol sequence techniques, temporal irreversibility, and/or cluster analysis to monitor the operating state of individual burner flames on a appropriate time scale. Both the method and apparatus of the present invention may be used optimize the performance of burner flames.

Abstract: In certain embodiments, a pressure regulating device for selectively coupling with a first source or a second source includes a housing defining at least a portion of a first input channel, a second input channel, and an output channel. In some embodiments, the device further includes a first connector configured to couple with the first source such that the first source is in communication with the first input channel. The device also includes a second connector configured to couple with the second source such that the second source is in communication with the second input channel. The device further includes a first regulator configured to regulate flow from the first input channel to the output channel and a second regulator configured to regulate flow from the second input channel to the output channel.

Abstract: An interactive system for controlling the operation of an HVAC system is provide that comprises a thermostat for initiating the operation of the HVAC system in either a full capacity mode of operation or at least one reduced capacity mode of operation, and a controller for an outside condenser unit having a condenser fan motor and a compressor motor, the controller being capable of operating the compressor in a full capacity mode and at least one reduced capacity mode. The system also comprises a controller for an indoor blower unit having a blower fan motor, the controller being capable of operating the blower fan motor in a full capacity mode an at least one reduced capacity mode. The system further includes a communication means for transmitting information between the outside condenser unit controller and at least the indoor blower controller, where the information relates to the operation of the indoor blower and the outdoor condenser unit.

Abstract: An apparatus includes a combustor and a thermal recovery device that is integrated with the combustor. The combustor produces an exhaust flow in response to an exhaust flow from an electrochemical cell.

Abstract: For adjustment of a desired burner gas-air ratio over the broadest possible load range without additional pressure tapping of the burner, combustion chamber, or air lines, a ratio controller is provided that permits adjustment of the gas flow as a function of counterpressure. For adjustment, the ratio controller has at least one position-variable measurement site, or at least two measurement sites, connected directly or indirectly via a pilot valve, to an actuating diaphragm via a valve block or throttle valve block. Depending on whether the control pressure is picked up more from one or more from the other measurement site, the gas flow and therefore the gas-air ratio can be adjusted to be larger or smaller.

Abstract: A blower for combustion air in a wall/floor furnace is provided that includes a blower housing (26), and blower wheel (27), with an air inlet (28) and an air outlet (29), and with a fuel feeder line (1) for fuel, wherein a mass current sensor (13) for determining the air mass current is located on the air inlet (28), which is functionally connected with a data processing device (4) and sends signals to the data processing device for calculation of a ratio of combustion medium to combustion air in dependence on a desired heating capacity.

Abstract: A control unit for controlling operation of a fuel-fired burner includes a housing having an air inlet which is open to an interior chamber of the housing. An air pump is positioned in the interior chamber of the housing and has an air inlet which is open to the interior chamber of the housing. The air pump generates reduced air pressure in the interior chamber which draws air into the housing and into the pump's inlet. This flow of air cools an electronic controller along with other components position in the housing. The air pump may be operated to draw air from the interior chamber of the housing and supplies the air to a combustion chamber of the fuel-fired burner to facilitate operation of the burner. An associated method of advancing air to a fuel-fired burner is also disclosed.

Abstract: A gas furnace controller for disrupting the flow of gas to a gas furnace upon detection of excessive levels of carbon monoxide comprises an air sample collector in fluid communication with a carbon monoxide detector, a gas supply shutoff circuit for controlling a valve in a gas supply line to the gas furnace; and a controller in electrical communication with the carbon monoxide detector and the gas supply shutoff circuit. The controller is adapted to actuate the gas supply shutoff circuit and sound an audible alarm in response to detection by the carbon monoxide detector of a predetermined level of carbon monoxide.

Abstract: A safety device for an industrial boiler (CHA) including electromechanical relays (REn, RS), which are connected in series in order to form an electromechanical safety chain (CH1), and at least one shunt relay (RTn) which is normally open and mounted in parallel to one of the electromechanical relays (REN). The shunt relay is controlled by an automaton (AP1) so that it is closed during a time delay. The shunt relay (RTn) is provided with a contact that is connected to an electronic circuit (CEn) and said electronic circuit (CEn) opens an electromechanical relay (RS) which is mounted in series in the safety chain when the shunt relay (RTn) is closed for longer than the time delay. Given the arrangement, in the event of an automaton failure, the time delay is ensured by the electronic circuit so that a satisfactory level of safety is guaranteed in relation to the installation.

Abstract: A combustion apparatus 2 has a fuel spraying nozzle 12, a feed canal 16 and a return canal 17, both the canals connected to the nozzle, with the former canal 16 feeding a fuel to the nozzle and with the later canal 17 allowing an unsprayed portion of the fuel to flow back. An electro-magnetic pump 18 disposed in the feed canal 16 serves to compress the fuel towards the nozzle 12, and an injector valve 25 is disposed in the return canal 17. A low rate at which the fuel is sprayed out from the nozzle is controlled by a controlling means 40 by regulating the mode of opening and closing the valve body 33 of injector valve 25.

Abstract: Combustion control systems for maintaining a selected fuel-air ratio for a combustion apparatus supplied with air at a substantially constant volumetric rate are improved by means for adjusting the flow rate of fuel in relation to temperature fluctuations of the air. A temperature sensor positioned in the air stream before it mixes with the fuel is connected to a converter which transmits converted temperature signals from the sensor as adjustments of a remote control-flow regulator in the fuel line or in the air stream. Thus, if the flow regulator is in the fuel line, a drop in air temperature will cause an increase of the fuel flow rate. Conversely, a rise in air temperature will result in a reduced fuel flow rate. If the flow regulator is in the air stream, a drop in air temperature will cause a decrease in the air flow rate, while a rise in air temperature will result in increased air flow rate.

Abstract: Automatic operation of a blower burner (5) suitable for liquid and/or gaseous fuels for the generation of heat energy in a furnace (1) is monitored by an automatic regulating device (35). The automatic regulating device (35) comprises an automatic firing device array (26) and an actuating drive (27) located in a common housing (29). The automatic regulating device (35) is connected to a furnace control device (1A), monitoring probes (39) and a fuel feeding and dosing device (12) by means of a terminal sleeve (36) in the housing (29). Based on the signals from the monitoring probes (39), the automatic regulating device (35) regulates the amount of air flowing into the combustion chamber (7) depending on the amount of fuel required for heat output so that optimal fuel combustion takes place in the combustion chamber (7) (for instance combined electronic control).

Abstract: A method for setting an air ratio of a fuel/air mixture made from a gaseous fuel and air for the operation of a burner, including the steps of: a) determining the mass current of the fuel as a first measured value, b) determining a value for the determination of the Wobbe index of the fuel as a second measured value, c) determining a mass current of the air as a third measured value, and d) controlling the mass current of the air and/or the fuel in dependence on at least two of the measured values so that a specified air ratio ? of the fuel/air mixture is set.

Abstract: The combustion vibration estimating apparatus comprises an inputting unit for inputting limiting values of plant data, weather data and internal pressure variation, an internal pressure variation characteristic grasping unit for making internal pressure variation of a combustor into a mathematical model from the input plant data and weather data, a combustion vibration region estimating unit for applying a limiting value of the internal pressure variation to the mathematical model obtained by the internal pressure variation characteristic grasping unit to obtain combustion vibration-prone to be generated region, and an outputting unit for outputting a combustion vibration region estimation result by the combustion vibration region estimating unit.

Abstract: A method and apparatus for continuing the operation of a fuel fired furnace when the air supply from the blower to the furnace is interrupted by employing a combination of a ventilator and drop door, the ventilator having a power source independent of the blower.

Abstract: A gas furnace responsive to a thermostat includes a thermal switch wired in series between the furnace power supply and the thermostat. A microprocessor connected to the thermal switch detects when the switch opens and closes, carrying out prearranged programs in response thereto. The thermal switch is mounted so that it opens when an over-pressure in the furnace draft system is detected, as evidenced by hot flue gasses passing over the thermal switch probe. The switch is allowed to cycle at least one time before the furnace is disabled. After a certain period of the time, the combustion cycle is reinitiated and the above steps are repeated if the thermal switch again resets.

Abstract: The present invention aims at reducing the adverse effect of exhaust gas to the global environment and saving the fuel resources. The combustion temperature is raised by providing means for reducing the air pressure in a combustion equipment of an internal combustion engine.

Abstract: An adjusting device for a gas cooking appliance for adjusting the gas flow rate includes a distribution inlet conduit in fluid communication with a gas supply line of the gas cooking appliance, and a distribution outlet conduit in fluid communication with a gas burner line of the gas cooking appliance. A fluid communication can be established between the distribution inlet and outlet conduits via a plurality of parallel flow paths. A flow restrictor and a closing means that is operable by an electric or electronic control system of the gas cooking appliance are arranged in series in each individual flow path.

Abstract: The invention relates to a safety device for an industrial boiler (CHA) comprising relays (REm) which are connected to form an electromechanical safety chain (CH1). According to the invention, each relay (REm) is mounted on a printed circuit board (CIA, CIB, CIC, CIP). As a result of said arrangement, the relays can be mounted in a compact box that may be sealed so that it is impossible for an operator to access a relay in order to block it.

Abstract: Device (10) and method to optimize the combustion of hydrocarbons able to be used as fuel and mixed with air containing oxygen, said device (10) comprising means (20, 23) to generate a magnetic field, advantageously of the pulsating type, in correspondence with a fuel feed pipe (13).

Abstract: The invention relates to a safety device for an industrial boiler (CHA) comprising electromechanical relays (REn, RS), which are connected in series in order to form an electromechanical safety chain (CHA), and at least one shunt relay (RTn) which is normally open and mounted in parallel to one of the electromechanical relays (REn). Said shunt relay is controlled by an automation (API) so that it is closed during a time delay. According to the invention, the shunt relay (RTn) is provided with a contact that is connected to an electronic circuit (CEn) and said electronic circuit (CEn) opens an electromechanical relay (RS) which is mounted in series in the safety chain when the shunt relay (RTn) is closed longer than the time delay. Given said arrangement. In the event of an automation failure, the time delay is ensured by the electronic circuit so that a satisfactory level of safety is guaranteed in relation to the installation.

Abstract: A fault-tolerant multi-point flame sense circuit utilizes a single electronic switch to signal the presence or absence of flame to an electronic controller. Multiple flame sense electrodes may be input to this circuit. By configuring these electrodes in accordance with the present invention, cross-contamination of a single failed flame sense electrode will not affect the other flame sense electrodes' ability to sense a flame at their associated burner. The circuit provides inputs for a number of flame sense electrodes via input channels that are capacitively coupled to the line voltage and resistively coupled to an RC network that controls the state of an electronic switch. When a flame is present at any one of the electrodes, the resulting unbalance current flow through the RC network turns the switch off to indicate the presence of flame. This operation is not affected by a short on any other electrode in the circuit.

Abstract: A combustion apparatus 2 has a fuel spraying nozzle 12, a feed canal 16 and a return canal 17, both the canals connected to the nozzle, with the former canal 16 feeding a fuel to the nozzle and with the later canal 17 allowing an unsprayed portion of the fuel to flow back. An electromagnetic pump 18 disposed in the feed canal 16 serves to compress the fuel towards the nozzle 12, and an injector valve 25 is disposed in the return canal 17. A low rate at which the fuel is sprayed out from the nozzle is controlled by a controlling means 40 by regulating the mode of opening and closing the valve body 33 of injector valve 25.

Abstract: The present invention relates to a method of cleaning a container having an amount of anhydrous ammonia contained therein. The container is first inspected thoroughly for leaks. Heated nitrogen gas is then fed into the container. The heated nitrogen gas may be transported from a nitrogen storage tank to the container via at least one pipe. Liquid nitrogen may be fed into a vaporizer for vaporizing the nitrogen. The liquid nitrogen gas may then be heated via a heater, such as a steamer, to expand the nitrogen gas and ensure that no liquid nitrogen enters the container. The heated nitrogen gas may vaporize any liquid anhydrous ammonia contained therein. Further, the heated nitrogen gas may transport the anhydrous ammonia to a flare for incineration. The heated nitrogen gas may be added any number of times to reduce the concentration of the anhydrous ammonia therein to a desired level. The container may then be steam cleaned and opened to enter and thoroughly clean the inside of the container.

Abstract: Automatic operation of a blower burner (5) suitable for liquid and/or gaseous fuels for the generation of heat energy in a furnace (1) is monitored by an automatic regulating device (35). The automatic regulating device (35) comprises an automatic firing device array (26) and an actuating drive (27) located in a common housing (29). The automatic regulating device (35) is connected to a furnace control device (1A), monitoring probes (39) and a fuel feeding and dosing device (12) by means of a terminal sleeve (36) in the housing (29). Based on the signals from the monitoring probes (39), the automatic regulating device (35) regulates the amount of air flowing into the combustion chamber (7) depending on the amount of fuel required for heat output so that optimal fuel combustion takes place in the combustion chamber (7) (for instance combined electronic control).

Abstract: The invention relates to a method for setting the air ratio of a fuel/air mixture made from a gaseous fuel and air for the operation of a burner, wherein

Abstract: The present invention is directed to an oil burner system having an electric cord set coupled between a controller and a valve associated with a pump. The electric cord set is operable to activate a solenoid valve associated with the pump for delivery of fuel oil to a nozzle of the burner. The electric cord set comprises a voltage or temperature independent timer circuit operable to activate the solenoid valve a predetermined period of time after a call for ignition signal is generated by the controller, wherein the predetermined time period is substantially constant with respect to variations in line voltage or in an ambient temperature in which the oil burner system resides.

Abstract: A furnace control system for controlling a gas-fired induced-draft furnace having a variable speed inducer blower. A control apparatus, responsive to a signal corresponding to the magnitude of a pressure difference between an inlet and outlet of the combustion chamber, controls blower motor speed to maintain the pressure difference at a predetermined magnitude corresponding to a selected gas flow rate. Inducer blower motor speed is varied directly and precisely to maintain an optimal pressure drop across the combustion chamber. The control system can be used in multi-stage and modulating furnace systems and in furnace systems utilizing pressure-assist modulating gas valves.

Abstract: A proximity warning system for a fireplace including a monitor module and an alarm module. The monitor module is configured to sense when an object, such as a person, enters a defined zone proximate to the fireplace. The monitor module may use a variety of techniques to sense when an object enters the defined zone. For example, the monitor module may use capacitance or pulse infrared light to detect when an object enters the defined zone. The alarm module is coupled to the monitor module to generate an alarm when the monitor module senses that the object has entered the defined zone. The alarm may be audible or visual.

Abstract: A gas lamp system and method for lighting the system which utilizes a sparkless flame starter in close proximity to the torch to create a flame. The system further includes a safety control circuit which activates the sparkless flame starter and subsequently actives a gas valve to deliver the gas to the torch after the sparkless flame starter has been energized. The sparkless flame starter is a low-voltage device such as a glow coil which is also arcless.

Abstract: The invention relates to a device for gas burners, comprising at least two air nozzles 10, 11 for combustion air. The air nozzles 10, 11 are arranged in parallel. According to the invention, a flow restrictor 15 is assigned to one or more of the air nozzles 10, 11, respectively. Each of these flow restrictor 15 is capable of selectively shutting off the flow of air through the respective air nozzle 10, 11. This increases the range of modulation of the device according to the invention.

Abstract: A draft safeguard apparatus for use in a multi-poised furnace having an inducer housing for receiving flue gases from a heat exchanger. The furnace flue pipe is attached to the inducer housing by an elbow that is rotatably connected to the inducer housing. An elongated sensor housing is mounted upon the elbow. The housing provides a flow channel so that the inlet section of the elbow communicates with the surrounding ambient. A limit switch is mounted in the housing to sense the temperature in the flow passing through the housing. When the furnace is operating normally, ambient air is drawn into the vent system through the sensor housing. If the vent system becomes restricted, the flow through the housing is reversed and the limit switch is opened when the reverse flow temperature exceeds a given limit.