Patent Description:
From the state of the art, an air-gas mixture burning appliance with an air-gas mixing unit, a burning unit, a flame detector, and a gas supply unit is known. In this air-gas mixture burning appliance, hydrogen may be used as gas and mixed with air to form a combustible air-gas mixture.

More specifically, such an air-gas mixture burning appliance usually mixes air and gas directly before the burning unit. During the ignition phase, the combustible air-gas mixture enters the burning unit where it is ignited at a low heat input to assist with stability and acoustics upon start up. However, sometimes the combustible air-gas mixture is not ignited immediately, which can lead to a build-up of the combustible air-gas mixture after the burning unit. A delayed ignition, which refers to igniting the built-up combustible air-gas mixture, usually leads to an explosion that may damage internal components of the air-gas mixture burning appliance and endanger the surrounding environment.

Delayed ignition is unproblematic for current natural gas burning appliances. However, delayed ignition may have severe consequences for appliances that burn a combustible air-hydrogen mixture. For example, the explosion caused by a delayed ignition of a combustible air-hydrogen mixture may not only damage internal components of the appliance, but damaged internal components may be ejected from the boiler case of the appliance. Moreover, the high sound levels that such an explosion produces, could potentially lead to hearing damage of people who are in the vicinity of such an appliance.

Current appliances include a controller that activates a spark electrode and then opens the gas valve for a pre-set ignition safety time. The gas valve remains open If ignition and a stable flame is achieved during the pre-set ignition safety time. Current natural gas burning appliances light a combustible air-natural gas mixture that has a fixed concentration.

In the remainder of this description, the term "gas" refers as any fuel in gaseous form that when mixed with air forms a combustible air-gas mixture. Examples for such a gas include hydrogen, propane, butane, methane, liquefied petroleum gas, etc..

The concentration of the combustible air-gas mixture, which is sometimes also referred to as the air-gas ratio or the air to gas ratio, is the mass of air per mass of gas in the air-gas mixture. A complete combustion takes place when all the gas of the combustible air-gas mixture is burned. In other words, the exhaust gas is free of unburned gas. The air-gas ratio of a complete combustion is referred to as the stoichiometric air-gas ratio, and the ideal gas-air ratio is called stoichiometric gas-air ratio.

The equivalence ratio between gas and air is defined as the ratio of the actual gas-air ratio to the stoichiometric gas-air ratio. The equivalence ratio between gas and air is sometimes also referred to as the equivalence gas-air ratio and denoted by the symbol φ. The inverse of the equivalence gas-air ratio is sometimes referred to as the equivalence air-gas ratio, which is also denoted by the symbol λ. Thus, φ = <NUM>/λ. The equivalence air-gas ratio is also defined as the ratio of the actual air-gas ratio to the stoichiometric air-gas ratio.

Thus, the equivalence gas-air ratio is equal to the equivalence air-gas ratio and equal to one if the combustion is stoichiometric (i.e., φ = λ = <NUM>). If the combustion is lean with excess air, the equivalence gas-air ratio is smaller than one (i.e., φ < <NUM>). and the equivalence air-gas ratio greater than one (i.e., λ><NUM>). Similarly, if the combustion is rich with incomplete combustion, the equivalence gas-air ratio is greater than one (i.e., φ > <NUM>) and the equivalence air-gas ratio smaller than one (i.e., λ<<NUM>).

Current natural gas burning appliances light a combustible air-natural gas mixture that has a fixed, rich concentration slightly below that of the stoichiometric air-gas ratio (i.e., λ<<NUM>). However, a delayed ignition of a combustible air-hydrogen mixture with an air-hydrogen ratio slightly below that of the stoichiometric air-hydrogen ratio would cause serious damage to the air-gas mixture burning appliance and to people who are in the vicinity of such an appliance during the occurrence of such a delayed ignition.

Document <CIT> describes a burner for the combustion of hydrogen and oxygen gas. The connection between the burner and a hydrogen gas storage tank and an oxygen gas storage tank is provided with flow control valves to control the gas flow rate of hydrogen gas and oxygen gas such that <NUM><NUM> of oxygen gas are mixed with <NUM><NUM> of hydrogen gas for combustion, whereby <NUM>% of the total volume percentage (%) of oxygen is supplied from the oxygen storage tank, and the remaining <NUM>% of the total volume percentage of oxygen is supplied from the combustion air supplied by external combustion air suction means. Document <CIT> describes a burner pin structure of a gas boiler that is designed to improve ignition performance, reduce ignition noise, and reduce ignition failure due to delayed ignition by allowing a weak flame to be injected in a diagonal direction.

Document <CIT> describes a proportional control valve with a valve body, displaced by the spring load of a spring and an electromagnetic force depending on the amount of conduction for a coil, and controls the supplying amount of fuel in accordance with the amount of conduction for the coil. When thermal power is to be changed, the amount of power conduction for the coil is set once at a value A by a pulse generating circuit and the opening degree of the proportional control valve is set at the opening degree upon the maximum combustion, for example. Thereafter, the amount of power conduction, corresponding to the amount of combustion set by a thermal power regulating switch, is supplied to the coil through a thermal power regulation processing circuit, a proportional valve electric current switching circuit and a proportional valve driving circuit. According to this method, the opening degree of the proportional control valve is set in a predetermined opening degree in accordance with the setting condition of the thermal power regulating switch at all times regardless of the hysteresis characteristics. Further, when the opening degree of the proportional control valve is changed, an objective amount of conduction may be set after setting once the amount of conduction upon the minimum combustion.

Document <CIT> describes an electromagnetic type safety valve that is interposed in a gas supply path and an ignition plug connected to a piezo-electric element that is mounted near a burner. A microswitch is turned on in response to an operation of ignition and a controller detects the operation of ignition. A latch type solenoid valve is electrically energized in the direction of moving a plunger to the position of opening the valve. At the moment of electric energization, the latch type solenoid valve remains unchanged if opened. When made for ignition, the electric energization is continued for about two seconds. As a result, even when the piezo-electric element is struck by the hammer to generate vibration, the vibration occurs during the continuation of the electric energization. During the period, the plunger is attracted by a magnetic force of a permanent magnet or the like, thereby keeping the valve open.

The present invention relates to a method of operating an air-gas mixture burning appliance that comprises an air-gas mixing unit, a burning unit that is arranged downstream of the air-gas mixing unit, a flame detector, and a gas supply unit that is arranged upstream of the air-gas mixing unit, the gas supply unit comprising a first gas flow channel with a gas flow restrictor, a second gas flow channel that is hydraulically parallel to the first gas flow channel and comprises a first gas valve, and a second gas valve that is arranged upstream of the first and second gas flow channels. The method comprises closing the first gas valve, opening the second gas valve, with the gas flow restrictor, restricting flow of gas through the first gas flow channel to the air-gas mixing unit, with the air-gas mixing unit, mixing air with the gas from the first gas flow channel to form a combustible air-gas mixture, igniting the combustible air-gas mixture in the burning unit, with the flame detector, sensing for presence of a flame in the burning unit, and in response to failing to sense the flame in the burning unit, maintaining the first gas valve in the closed position.

Advantageously, the inventive method may prevent the build-up of a damaging concentration of the combustible air-gas mixture in the burning unit of the air-gas mixture burning appliance, thereby eliminating the risks associated with a delayed ignition of such a damaging amount of the combustible air-gas mixture.

Preferably, the combustible air-gas mixture has a first equivalence ratio between gas and air.

Thus, the inventive method may adjust the equivalence ratio between gas and air to a first value for as long as no flame is detected in the burning unit.

Preferably, the first equivalence ratio between gas and air is smaller than <NUM>.

Accordingly, the actual gas-air ratio is lean and below the stoichiometric gas-air ratio, which may prevent an explosion in case of a delayed ignition.

According to one aspect, the method may further comprise in response to sensing presence of the flame in the burning unit, opening the first gas valve.

Thus, the gas-air mixture burning unit may adjust the equivalence ratio between gas and air to a different value after ignition of the combustible air-gas mixture, thereby ensuring a clean and efficient combustion after the ignition of the combustible air-gas mixture.

Preferably, the method may further comprise waiting for a predetermined duration between sensing presence of the flame in the burning unit and opening the first gas valve.

Accordingly, the burning unit may ensure the establishment of a stable flame during the predetermined delay.

Preferably, the predetermined duration is between <NUM> seconds and <NUM> seconds.

Thus, the predetermined delay may allow for the establishment of a stable flame across the entire burner surface.

According to one aspect, the method may further comprise with the air-gas mixing unit, mixing air with the gas from the first and second gas flow channels to form another combustible air-gas mixture.

Thus, the air-gas mixing unit may vary the equivalence ratio between gas and air of the combustible gas-air mixture.

Preferably, the other combustible air-gas mixture has a second equivalence ratio between gas and air that is different than the first equivalence ratio.

Thus, the air-gas mixing unit may set the equivalence ratio between gas and air of the combustible gas-air mixture to a different value.

Preferably, the second equivalence ratio between gas and air is greater than the first equivalence ratio.

Thus, the air-gas mixture burning appliance may perform a more efficient combustion of the combustible air-gas mixture.

Exemplary embodiments of the present invention are described in detail hereinafter with reference to the attached drawings. In these attached drawings, identical or identically functioning components and elements are labelled with identical reference signs and they are generally only described once in the following description.

<FIG> shows an exemplary air-gas mixture burning appliance <NUM> with an air-gas mixing unit <NUM>, a burning unit <NUM>, and a flame detector <NUM>. By way of example, the air-gas mixture burning appliance <NUM> may be used in a boiler or, more generally, in a building heating system. Preferably, the gas used is hydrogen such that the air-gas mixture burning appliance <NUM> forms an air-hydrogen mixture burning appliance.

The air-gas mixing unit <NUM> is preferably adapted for mixing of air and gas to form a combustible air-gas mixture <NUM>. Preferentially, the combustible air-gas mixture <NUM> is a homogenous mixture of the air and the gas.

By way of example, the air-gas mixing unit <NUM> includes an air supply unit <NUM> and a gas supply unit <NUM>. Illustratively, the air supply unit <NUM> includes a fan <NUM> that may be operated with an adaptable fan speed and/or within predetermined ranges of fan speeds to draw air into the air-gas mixing unit <NUM>.

The air supply unit <NUM> and the gas supply unit <NUM> may be interconnected via a mixer <NUM> which forms a corresponding discrete point of mixing <NUM>. Preferably, the combustible air-gas mixture <NUM> is formed at the discrete point of mixing <NUM> and guided via the mixer <NUM> to the burning unit <NUM>.

Illustratively, the burning unit <NUM> is provided with a burner surface <NUM> that is arranged downstream of the air-gas mixing unit <NUM> such that the combustible air-gas mixture <NUM> that is formed at the discrete point of mixing <NUM> flows towards the burner surface <NUM>. The combustible air-gas mixture <NUM> is burned by the burning unit <NUM> and, more specifically, at the burner surface <NUM>.

By way of example, the burner surface <NUM> is illustrated with a comparatively small flame <NUM> which occurs e.g. during an ignition phase of the air-gas mixture burning appliance <NUM>. As an example, during such an ignition phase, the air-gas mixing unit <NUM> may have a low firing rate, i.e. a comparatively low rate at which feed of the combustible air-gas mixture <NUM> from the air-gas mixing unit <NUM> to the burning unit <NUM> occurs, in terms of volume, heat units, or weight per unit time. As another example, during such an ignition phase, the air-gas mixing unit <NUM> may provide a combustible air-gas mixture with a first equivalence rate between gas and air. If desired, the combustible air-gas mixture may be a lean combustible air-gas mixture with an equivalence ratio between gas and air that is below the stoichiometric ratio between gas and air. The comparatively small flame <NUM> is illustratively stabilised against the burner surface <NUM> and detected by means of the flame detector <NUM>.

According to one aspect, the flame detector <NUM> is provided for sensing presence of a flame <NUM> in the burning unit <NUM>. By way of example, the flame detector <NUM> detects a flame signal <NUM> in the burning unit <NUM>. Thus, the flame detector <NUM> is suitable for determining whether a flame <NUM> is present in the burning unit <NUM>, or not. However, it should be noted that suitable flame detection techniques that may be used with the flame detector <NUM> are well-known to the person skilled in the art and are, therefore, not described in more detail, for brevity and conciseness. For instance, the flame detector <NUM> may use any suitable sensing element for sensing presence of the flame <NUM> in the burning unit <NUM>.

Illustratively, the flame detector <NUM> is connected to a controller <NUM>. Preferably, the controller <NUM> is adapted to control supply of gas to the air-gas mixing unit <NUM>, in particular to control the gas supply unit <NUM>, on the basis of a detection signal <NUM> provided by the flame detector <NUM>. If desired, the controller <NUM> may control a gas valve of the gas supply unit <NUM> on the basis of the detection signal <NUM>.

The detection signal <NUM> may be created and/or provided by the flame detector <NUM>, or alternatively by the controller <NUM>, by comparing the detected flame signal <NUM> with a predetermined flame detection threshold. Thus, the controller <NUM> may create a control signal <NUM> on the basis of the detection signal <NUM>. If desired, the gas supply unit <NUM> may use the detection signal <NUM> e.g. to regulate the flow of gas to the air-gas mixing unit <NUM> such that the combustible air-gas mixture <NUM> has a variable equivalence ratio between gas and air based on the detection signal <NUM>, i.e., based on whether the flame detector <NUM> senses the presence of a flame <NUM> in the burning unit <NUM> or fails to sense the presence of a flame <NUM> in the burning unit <NUM>.

As an example, the gas supply unit <NUM> may supply a first flow of gas to the air-gas mixing unit <NUM> such that the combustible air-gas mixture <NUM> has a first equivalence ratio between gas and air when the flame detector <NUM> fails to sense the presence of the flame <NUM>. If desired, the first equivalence ratio between gas and air may be smaller than <NUM>. In other words, the combustible air-gas mixture <NUM> may be lean.

As another example, the gas supply unit <NUM> may supply a second flow of gas to the air-gas mixing unit <NUM> such that the combustible air-gas mixture <NUM> has a second equivalence ratio between gas and air that is different than the first equivalence ratio between gas and air when the flame detector <NUM> senses the presence of the flame <NUM>. If desired, the second equivalence ratio between gas and air may greater than the first equivalence ratio between gas and air. Thus, the gas supply unit <NUM> may regulate the equivalence ratio between gas and air by providing a richer combustible air-gas mixture <NUM> when the flame detector <NUM> senses the presence of the flame <NUM>.

In some embodiments, the gas valve <NUM> may open with a predetermined delay after the flame detector <NUM> senses the presence of the flame <NUM>. For example, the gas valve <NUM> may open with a predetermined delay that is between <NUM> seconds and <NUM> seconds. Preferably, the predetermined delay is between <NUM> seconds and <NUM> seconds.

Illustratively, the control circuit <NUM> may include a timer. Upon receipt of the detection signal <NUM> from the flame detector <NUM>, the control circuit <NUM> may trigger the timer. When the timer has timed the predetermined delay, the control circuit <NUM> may send control signal <NUM> to the gas supply unit <NUM>.

<FIG> shows the air-gas mixture burning appliance <NUM> of <FIG> with the air-gas mixing unit <NUM>, the burning unit <NUM>, the controller <NUM>, and the flame detector <NUM>. However, in contrast to <FIG>, the air-gas mixture burning appliance <NUM> is shown with a greater flame <NUM> after the ignition phase. As an example, the air-gas mixing unit <NUM> may be operated at a high firing rate, i.e. a comparatively high rate at which feed of the combustible air-gas mixture <NUM> from the air-gas mixing unit <NUM> arrives at the burning unit <NUM>, which may lead to the greater flame <NUM>. The high firing rate may be associated with a normal operating range of the burning unit <NUM> compared to the low firing rate that is associated with the ignition phase of the air-gas mixture burning appliance <NUM>, as described in <FIG>.

As another example, after the ignition phase, the air-gas mixing unit <NUM> may provide a combustible air-gas mixture <NUM> with a second equivalence rate between gas and air. If desired, after the ignition phase, the combustible air-gas mixture <NUM> may be a rich combustible air-gas mixture with an equivalence ratio between gas and air that is above the stoichiometric ratio between gas and air. The comparatively great flame <NUM> may emit toward the flame detector <NUM> a flame signal <NUM> having a higher intensity than the flame signal <NUM> emitted by the flame <NUM> of <FIG>.

<FIG> shows an illustrative gas supply unit <NUM>. Gas supply unit <NUM> may be arranged upstream of an air-gas mixing unit (e.g., air-gas mixing unit <NUM> of <FIG>), which is arranged upstream of a burning unit (e.g., burning unit <NUM> of <FIG>).

Illustratively, gas supply unit <NUM> may be adapted to regulating the flow of gas to the air-gas mixing unit such that the combustible air-gas mixture produced by the air-gas mixing unit has a variable equivalence ratio. By way of example, the illustrative gas supply unit <NUM> may regulate the flow of gas depending on whether the presence of a flame or the absence of a flame is detected in the associated burning unit.

As an example, the gas supply unit <NUM> may supply a first flow of gas to the air-gas mixing unit such that the combustible air-gas mixture has a first equivalence ratio between gas and air when the absence of a flame is detected in the associated burning unit (e.g., using flame detector <NUM> of <FIG>). If desired, the first equivalence ratio between gas and air may be smaller than one. In other words, the gas supply unit <NUM> may supply the first flow of gas to the air-gas mixing unit such that the air-gas mixing unit produces a lean mixture of air and gas as long as the absence of a flame is detected in the burning unit.

As another example, the gas supply unit <NUM> may supply a second flow of gas to the air-gas mixing unit such that the combustible air-gas mixture has a second equivalence ratio between gas and air when the presence of a flame is detected in the associated burning unit. The second equivalence ratio may be different than the first equivalence ratio between gas and air. If desired, the second equivalence ratio may be greater than the first equivalence ratio between gas and air. For example, the second equivalence ratio between gas and air may be greater than one. In other words, the gas supply unit <NUM> may supply the second flow of gas to the air-gas mixing unit such that the air-gas mixing unit produces a rich mixture of air and gas as long as the presence of a flame is detected in the burning unit.

If desired, the gas supply unit <NUM> may be adapted to supplying more than two discrete flows of gas to the air-gas mixing unit. For example, the gas supply unit <NUM> may be adapted to supplying any amount of gas to the air-gas mixing unit selected from a continuous range. The continuous range may be selected such that the variable equivalence ratio between gas and air is between <NUM> and <NUM>. Preferably, the continuous range may be selected such that the variable equivalence ratio between gas and air is between <NUM> and <NUM>.

Gas supply unit <NUM> may have at least two hydraulically parallel gas flow channels. For example, gas supply unit <NUM> may have two, three, four, or more hydraulically parallel gas flow channels. As shown in <FIG>, gas supply unit <NUM> may have two hydraulically parallel gas flow channels <NUM>, <NUM>.

Each gas flow channel of the at least two hydraulically parallel gas flow channels may be adapted to supplying a flow of gas to the air-gas mixing unit. The flow of gas may be regulated in at least one gas flow channel of the at least two hydraulically parallel gas flow channels. If desired, the flow of gas may be interrupted in the at least one gas flow channel of the at least two hydraulically parallel gas flow channels.

A controller may control the at least two hydraulically parallel gas flow channels. Preferably, the controller may control and/or regulate the flow of gas in the at least two hydraulically parallel gas flow channels.

Illustratively, the controller may control only a subset of the at least two hydraulically parallel gas flow channels. In some embodiments, the controller may control each gas flow channel of the at least two hydraulically parallel gas flow channels independently of the other gas flow channels. If desired, the controller may control at least two gas flow channels of the at least two hydraulically parallel gas flow channels together.

By way of example, gas flow channel <NUM> of gas supply unit <NUM> may include a gas flow restrictor <NUM> that restricts flow of gas through gas flow channel <NUM> to the air-gas mixing unit. Gas flow channel <NUM> may be hydraulically parallel to gas flow channel <NUM>.

Illustratively, gas flow channel <NUM> may include a gas flow restrictor <NUM> that restricts flow of gas through gas flow channel <NUM> to the air-gas mixing unit. If desired, gas flow channel <NUM> may include a gas valve <NUM>. The gas valve <NUM> may be either closed and prevent the flow of gas through gas flow channel <NUM> or opened and allow the flow of gas through gas flow channel <NUM>.

Gas valve <NUM> may be activated electrically or pneumatically. As shown in <FIG>, a solenoid <NUM> may control gas valve <NUM>. Thus, solenoid <NUM> may open or close gas valve <NUM>. If desired, solenoid <NUM> may be controlled electrically. For example, an electrical control signal from a controller circuit (e.g., control signal <NUM> from controller <NUM> of <FIG>) may control solenoid <NUM>.

Illustratively, the control signal may be indicative of the presence or absence of a flame in the burning unit. As an example, consider the scenario in which the control signal is indicative of the absence of a flame in the burning unit. In this scenario, the solenoid <NUM> may control the gas valve <NUM> such that the gas valve <NUM> remains closed. Thus, only gas flow channel <NUM> may provide a flow of gas to the air-gas mixing unit such that the combustible air-gas mixture has a first equivalence ratio between gas and air when the control signal is indicative of the absence of a flame in the burning unit.

If desired, the gas flow restrictor <NUM> may be selected such that the first equivalence ratio between gas and air is smaller than one when only the gas flow channel <NUM> provides a flow of gas to the air-gas mixing unit. Thus, the combustible air-gas mixture may be lean. In case of a delayed ignition, the concentration of gas in the air-gas mixture may be low enough to prevent an explosion which may prevent damage to the air-gas mixture burning appliance.

As another example, consider the scenario in which the control signal is indicative of the presence of a flame in the burning unit. In this scenario, the solenoid <NUM> may control the gas valve <NUM> such that the gas valve <NUM> opens. If desired, the control signal may direct the solenoid <NUM> to open the gas valve <NUM> with a predetermined delay. For example, the predetermined delay may be selected to be between <NUM> and <NUM> seconds.

When the gas valve <NUM> is open, both gas flow channels <NUM>, <NUM> may provide a flow of gas to the air-gas mixing unit such that the combustible air-gas mixture has a second equivalence ratio between gas and air when the control signal is indicative of the presence of a flame in the burning unit.

If desired, the gas flow restrictors <NUM>, <NUM> may be selected such that the second equivalence ratio between gas and air is greater than the first equivalence ratio. For example, the second equivalence ratio between gas and air may be greater than one when both gas flow channels <NUM>, <NUM> provide a flow of gas to the air-gas mixing unit. Thus, the combustible air-gas mixture may be rich and provide for an improved running efficiency of the air-gas mixture burning appliance once a stable flame has been established in the burning unit.

Illustratively, gas supply unit <NUM> may include an additional gas valve <NUM>. The additional gas valve <NUM> may be arranged upstream of the first and second gas flow channels <NUM>, <NUM>. The additional gas valve <NUM> may be adapted to completely shutting off the flow of gas to the air-gas mixing unit.

Preferably, the additional gas valve <NUM> is a zero governor gas valve <NUM>. If desired, the zero governor gas valve <NUM> may include at least one gas regulator <NUM>. The at least one gas regulator <NUM> may be adapted to maintaining zero pressure at the outlet of the zero governor gas valve <NUM>.

Illustratively, the additional gas valve <NUM> may include at least one of a pressure-controlled valve or an electronically-controlled valve. As shown in <FIG>, the additional gas valve includes two electronically-controlled valves <NUM> that are controlled independently by solenoids <NUM>, <NUM>.

<FIG> shows a functional diagram for illustrating operation of the air-gas mixture burning appliance of <FIG>. As shown in <FIG>, the air-gas mixture burning appliance <NUM> may include an air-gas mixing unit <NUM>, a burning unit <NUM> that is arranged downstream of the air-gas mixing unit <NUM>, a flame detector <NUM>, and a gas supply unit <NUM> that is arranged upstream of the air-gas mixing unit <NUM>. The gas supply unit <NUM> may include a first gas flow channel <NUM> with a gas restrictor <NUM>, a second gas flow channel <NUM> that is hydraulically parallel to the first gas flow channel <NUM> and comprises a first gas valve <NUM>, and a second gas valve <NUM> that is arranged upstream of the first and second gas flow channels <NUM>, <NUM>.

During operation <NUM>, the air-gas mixture burning appliance may close the first gas valve. For example, the air-gas mixture burning appliance <NUM> of <FIG> may close gas valve <NUM> during the ignition phase of the air-gas mixture burning appliance <NUM>.

During operation <NUM>, the air-gas mixture burning appliance may open the second gas valve. For example, the air-gas mixture burning appliance <NUM> of <FIG> may open the second gas valve <NUM>, thereby enabling a flow of gas to the first and second gas flow channels <NUM>, <NUM>.

During operation <NUM>, the air-gas mixture burning appliance may, with the gas restrictor, restrict flow of gas through the first gas flow channel to the air-gas mixing unit. For example, the air-gas mixture burning appliance <NUM> of <FIG> may, with the gas restrictor <NUM>, restrict flow of gas through the first gas flow channel <NUM> to the air-gas mixing unit <NUM>.

During operation <NUM>, the air-gas mixture burning appliance may, with the air-gas mixing unit, mix air with the gas from the first gas flow channel to form a combustible air-gas mixture. For example, the air-gas mixture burning appliance <NUM> of <FIG> may, with the air-gas mixing unit <NUM>, mix air with the gas from the first gas flow channel <NUM> to form a combustible air-gas mixture <NUM>.

During operation <NUM>, the air-gas mixture burning appliance may ignite the combustible air-gas mixture in the burning unit. For example, the air-gas mixture burning appliance <NUM> of <FIG> may ignite the combustible air-gas mixture <NUM> in the burning unit <NUM>.

During operation <NUM>, the air-gas mixture burning appliance may, with the flame detector, sense for presence of a flame in the burning unit. For example, the air-gas mixture burning appliance <NUM> of <FIG> may, with the flame detector <NUM>, sense for presence of a flame <NUM> in the burning unit <NUM>.

During operation <NUM>, the air-gas mixture burning appliance may, in response to failing to sense the flame in the burning unit, maintain the first gas valve in the closed position. For example, the air-gas mixture burning appliance <NUM> of <FIG> may, in response to failing to sense the flame <NUM> in the burning unit <NUM>, maintain the first gas valve <NUM> in the closed position.

During operation <NUM>, the air-gas mixture burning appliance may, in response to sensing presence of the flame in the burning unit, open the first gas valve. For example, the air-gas mixture burning appliance <NUM> of <FIG> may, in response to sensing presence of the flame <NUM> in the burning unit <NUM>, open the first gas valve <NUM>.

If desired, the air-gas mixture burning appliance may wait for a predetermined duration between sensing presence of the flame in the burning unit and opening the first gas valve.

Claim 1:
A method (<NUM>) of operating an air-gas mixture burning appliance (<NUM>) that comprises an air-gas mixing unit (<NUM>), a burning unit (<NUM>) that is arranged downstream of the air-gas mixing unit (<NUM>), a flame detector (<NUM>), and a gas supply unit (<NUM>) that is arranged upstream of the air-gas mixing unit (<NUM>), the gas supply unit (<NUM>) comprising a first gas flow channel (<NUM>) with a gas flow restrictor (<NUM>), a second gas flow channel (<NUM>) that is hydraulically parallel to the first gas flow channel (<NUM>) and comprises a first gas valve (<NUM>), and a second gas valve (<NUM>) that is arranged upstream of the first and second gas flow channels (<NUM>, <NUM>), the method comprising:
closing (<NUM>) the first gas valve (<NUM>),
opening (<NUM>) the second gas valve (<NUM>),
with the gas flow restrictor (<NUM>), restricting (<NUM>) flow of gas through the first gas flow channel (<NUM>) to the air-gas mixing unit (<NUM>),
with the air-gas mixing unit (<NUM>), mixing (<NUM>) air with the gas from the first gas flow channel (<NUM>) to form a combustible air-gas mixture (<NUM>),
igniting (<NUM>) the combustible air-gas mixture (<NUM>) in the burning unit (<NUM>),
with the flame detector (<NUM>), sensing (<NUM>) for presence of a flame (<NUM>) in the burning unit (<NUM>), and
in response to failing to sense the flame (<NUM>) in the burning unit (<NUM>), maintaining (<NUM>) the first gas valve (<NUM>) in the closed position.