Patent Description:
From the state of the art, an air-gas mixture burning appliance having a burning unit, an air-gas mixing unit, and a gas valve is known. More specifically, known air-gas mixture burning appliances usually mix 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. 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.

Therefore, during the ignition phase of the burning unit, the maximum allowed opening time of the gas valve without a flame being detected is limited to a short time period, which is sometimes also referred to as the ignition safety time. The ignition safety time is usually a fixed duration of time that is predetermined.

Prior art document <CIT> discloses a locally powered intermittent pilot combustion controller including an igniter, a thermal electric and/or photoelectric device that produces an electrical signal having power when exposed to a flame, and a local power source for providing power when the thermal electric and/or photoelectric device is not exposed to a flame. In some cases, the intermittent pilot combustion controller may include a memory for storing information about an ignition sequence for igniting a pilot flame, and a controller coupled to the memory. The controller may be configured to initiate the ignition sequence of the pilot flame using information stored in the memory, determine whether the ignition was successful by monitoring the electrical signal produced by the thermal electric and/or photoelectric device, and adjust the information stored in the memory based on whether the ignition sequence completed successfully.

<CIT> discloses a burner automat for controlling a burner, which is fed from a fuel line provided with a solenoid valve and which is provided with an ignition and flame indication device, the burner automat having a timer for the margin of safety, the timer being designed as electronic up-and-down counter.

According to <CIT>, in the production of a synthesis gas from a fuel, combustion in a burner is regulated so that the fuel/air mixture is adjusted to lambda greater than <NUM> at the start and lambda less than <NUM> after the start, so that the fuel is partly oxidized.

In the remainder of this description, the term "gas" refers to 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. Moreover, the term "air" refers to any suitable oxidizer that may be mixed with a fuel gas to form a combustible air-gas mixture.

The present invention relates to an air-gas mixture burning appliance, comprising a burning unit for burning a combustible air-gas mixture, an air-gas mixing unit that is arranged upstream of the burning unit and is adapted for mixing of air and gas to form the combustible air-gas mixture, a gas valve that is arranged upstream of the air-gas mixing unit and is adapted for regulating a flow of the gas to the air-gas mixing unit, a flame detector for sensing a presence of a flame in the burning unit; and a controller that is adapted for determining a variable ignition safety time for an ignition phase of the burning unit by selecting from at least two different durations of time, the ignition safety time being the maximum allowed opening time of the gas valve without a flame being detected, wherein the controller determines the variable ignition safety time as an anticipated maximum duration of time that elapses between an opening of the gas valve and a closing of the gas valve for avoiding a hazardous accumulation of the combustible air-gas mixture in the burning unit; and wherein the controller directs the gas valve to shut off if, during the ignition phase of the burning unit, the gas valve is open and the flame detector fails to sense a flame in the burning unit during the variable ignition safety time.

Accordingly, the inventive air-gas mixture burning appliance may be able to adjust the ignition safety time and adapt to different modes of operation.

Optionally, the air-gas mixture burning appliance further comprises a secured gas pathway that is arranged between the gas valve and the air-gas mixing unit and is adapted for providing the gas from the gas valve to the air-gas mixing unit, and wherein the controller determines the variable ignition safety time based on an inferred concentration of the gas in the secured gas pathway.

Thus, the controller can select the variable ignition safety time based on whether the secured gas pathway is already filled with a gas or whether the secured gas pathway is filled with air at the beginning of the ignition phase.

Preferably, the controller determines the variable ignition safety time based at least on the type of the gas.

Accordingly, the inventive air-gas mixture burning appliance may be operated with different gas types.

Optionally, the controller determines the variable ignition safety time by selecting from a predetermined number of at least three different discrete durations of time, wherein all values of the at least three different discrete durations of time are smaller than or equal to a predetermined maximum ignition safety time.

Thus, the controller may have a simple design and be low cost.

According to one aspect, the controller determines the variable ignition safety time by selecting from a continuous range of values, wherein all values in the continuous range of values are smaller than or equal to a predetermined maximum ignition safety time.

Accordingly, the controller may fine-tune the ignition safety time and react to the combination of multiple parameters that may have an influence on the ignition safety time.

Preferably, the air-gas mixture burning appliance further comprises a flame detector for sensing a presence of a flame in the burning unit, wherein the controller determines the variable ignition safety time based at least on an elapsed time since the flame detector previously sensed a flame in the burning unit.

Thus, the inventive air-gas mixture burning appliance may reliably predict the concentration of the gas in the secured gas pathway at the beginning of the ignition phase.

The controller directs the gas valve to shut off if, during the ignition phase of the burning unit, the gas valve is open and the flame detector fails to sense a flame in the burning unit during the variable ignition safety time.

Accordingly, the controller ensures the prevention of a dangerous build-up of the combustible air-gas mixture after the burning unit and a related delayed ignition.

Preferably, the controller triggers a warning and/or puts the air-gas mixture burning appliance into one or more of a safe state, a restricted state, or a non-operational state, if during a predetermined number of consecutive ignition phases of the burning unit, the gas valve is open and the flame detector fails to sense a flame in the burning unit during the variable ignition safety time.

Accordingly, an operator may be alerted and/or the risks associated with a malfunction of the air-gas mixture burning appliance may be reduced.

Furthermore, a method of operating an air-gas mixture burning appliance that comprises a controller, a burning unit for burning a combustible air-gas mixture, a flame detector for sensing a presence of a flame in the burning unit, an air-gas mixing unit that is arranged upstream of the burning unit and is adapted for mixing of air and gas to form the combustible air-gas mixture, and a gas valve that is arranged upstream of the air-gas mixing unit and is adapted for regulating a flow of the gas to the air-gas mixing unit, comprises determining with the controller, a variable ignition safety time for an ignition phase of the burning unit by determining an anticipated maximum duration of time that can safely elapse between an opening of the gas valve and a closing of the gas valve for avoiding a hazardous accumulation of the combustible air-gas mixture in the burning unit.

Accordingly, the inventive method may operate an air-gas mixture burning appliance by adjusting the ignition safety time and adapt to different modes of operation.

The method further comprises directing with the controller, the gas valve to shut off if, during the ignition phase of the burning unit, the gas valve is open and no flame is detected in the burning unit during the variable ignition safety time.

Thus, the inventive method may prevent a hazardous build-up of the combustible air-gas mixture in the burning unit of the air-gas mixture burning appliance.

Preferably, the air-gas mixture burning appliance further comprises a secured gas pathway that is arranged between the gas valve and the air-gas mixing unit and is adapted for providing the gas from the gas valve to the air-gas mixing unit, and determining the variable ignition safety time further comprises determining the variable ignition safety time based at least on the type of the gas, and/or on an inferred concentration of the gas in the secured gas pathway, and/or on an elapsed time since a flame detector previously sensed a flame in the burning unit.

Thus, the inventive method may consider different parameters when determining the ignition safety time.

Optionally, the method further comprises triggering with the controller, a warning and/or putting, with the controller, the air-gas mixture burning appliance into one or more of a safe state, a restricted state or a non-operational state, if, during a pre-determined number of consecutive ignition phases of the burning unit, the gas valve is open and no flame is detected in the burning unit during the variable ignition safety time.

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 a burning unit <NUM> for burning a combustible air-gas mixture <NUM>, an air-gas mixing unit <NUM>, a gas valve <NUM>, a flame detector <NUM>, and a controller <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.

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

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

As shown in <FIG>, the fan <NUM> may push air <NUM> into the air-gas mixing unit <NUM>. If desired, and as shown in <FIG>, the fan <NUM> may draw the combustible air-gas mixture <NUM> from the air-gas mixing unit <NUM>.

The gas supply unit may include the gas valve <NUM>. The gas valve <NUM> is arranged upstream of the air-gas mixing unit <NUM> and is adapted for regulating a flow of the gas <NUM> to the air-gas mixing unit <NUM>. A secured gas pathway <NUM> is arranged between the gas valve <NUM> and the air-gas mixing unit <NUM> and is adapted for providing the gas <NUM> from the gas valve <NUM> to the air-gas mixing unit <NUM>.

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

During an ignition phase of the air-gas mixture burning appliance <NUM>, the combustible air-gas mixture <NUM> from the air-gas mixing unit <NUM> is ignited in the burning unit <NUM> at the burner surface <NUM>. The resulting flame <NUM> is illustratively stabilised against the burner surface <NUM>.

According to the invention, the flame detector <NUM> is provided for sensing presence of a flame <NUM> in the burning unit <NUM>. Thus, the flame detector <NUM> is suitable for determining whether a flame <NUM> is present at the burner surface <NUM> in the burning unit <NUM>. 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>.

The flame detector <NUM> is connected to the controller <NUM> and provides a flame detection signal <NUM> to the controller <NUM> 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 shown in <FIG>, the controller <NUM> is connected to the gas valve <NUM>. Illustratively, the controller <NUM> may receive a control signal <NUM> from the gas valve <NUM>. By way of example, the control signal <NUM> may be indicative of the status of the gas valve <NUM>. For example, the controller signal <NUM> may indicate whether the gas valve <NUM> is open or closed.

Illustratively, the controller <NUM> may send an actuator signal <NUM> to the gas valve <NUM>. If desired, the actuator signal <NUM> may direct the gas valve <NUM> to open and/or to close. According to the invention, at the beginning of an ignition phase, the controller <NUM> directs the gas valve <NUM> to open.

During the ignition phase, the controller <NUM> tracks the duration of time between the opening of the gas valve <NUM> and the detection of a flame signal <NUM> at the burning unit <NUM>. To avoid a delayed ignition, which refers to igniting a hazardous accumulation of the combustible air-gas mixture <NUM> and which usually leads to an explosion that may damage internal components of the air-gas mixture burning appliance <NUM> and endanger the surrounding environment, the maximum allowed opening time of the gas valve <NUM> without a flame <NUM> being detected is limited to a short period of time, which is sometimes also referred to as the ignition safety time.

In contrast thereto, the gas valve <NUM> needs to be open for a minimum opening time. The minimum opening time is defined as the time that is required for the gas <NUM> to flow from the gas valve <NUM> through the secured gas pathway <NUM> to the air-gas mixing unit <NUM> and from there, as a combustible air-gas mixture <NUM> to the burner surface <NUM>, where a sufficient quantity of the combustible air-gas mixture <NUM> is required to accumulate such that a reliable ignition of the flame <NUM> is ensured.

However, the time between the opening of the gas valve <NUM> and the accumulation of a quantity of the combustible air-gas mixture <NUM> in the burning unit <NUM> that is sufficient for a sustained ignition may depend on the concentration of the gas <NUM> in the secured gas pathway <NUM> at the opening of the gas valve <NUM>.

As an example, If the air-gas mixture burning appliance <NUM> were to stop burning gas <NUM>, but after a short period of time be re-lit, then the secured gas pathway <NUM> would still be filled with gas <NUM>, and the combustible air-gas pathway <NUM> would still be filled with the combustible air-gas mixture <NUM>, and, as a result, a flame <NUM> could be established comparatively quickly at the burner surface <NUM>.

As another example, in between periods of operation, the concentration of the gas <NUM> in the secured gas pathway <NUM> and in the combustible air-gas pathway <NUM> decays over time. Thus, if the air-gas mixture burning appliance <NUM> is re-lit after a sufficient period of time, the secured gas pathway <NUM> and the combustible air-gas pathway <NUM> is nominally filled with air and contains a negligible concentration of the gas <NUM> at the start of the ignition sequence. In this case, the nominally air that is present in the secured gas pathway <NUM> and in the combustible air-gas pathway <NUM> must be displaced by gas <NUM> that is streaming through the opened gas valve <NUM>.

This process of re-priming the secured gas pathway <NUM> and the combustible air-gas pathway <NUM> takes some time, which may depend, for example, on the flow rate of nominally air and the volume of the consecutive pathways <NUM>, <NUM>.

Consequently, a flame <NUM> appears at the burner surface <NUM> only after this priming process has sufficiently progressed, and takes more time in comparison to the ignition attempt made shortly after an end of a previous burn that was described in the previous example.

Thus, the controller <NUM> is adapted for determining a variable ignition safety time for an ignition phase of the burning unit <NUM> by selecting from at least two different durations of time. Thereby, the controller <NUM> determines the variable ignition safety time as an anticipated maximum duration of time that elapses between an opening of the gas valve <NUM> and a closing of the gas valve <NUM> for avoiding a hazardous accumulation of the combustible air-gas mixture <NUM> in the burning unit <NUM>.

Since the decay of the concentration of the gas <NUM> in the secured gas pathway <NUM> over time is driven by a diffusion process that is relatively repeatable and predictable, the controller <NUM> may determine the variable ignition safety time based on an inferred concentration of the gas <NUM> in the secured gas pathway <NUM>.

Moreover, the decay of the concentration of the gas <NUM> and/or the time to reprime the secured gas pathway <NUM> and the combustible air-gas pathway <NUM> may depend on the type of gas <NUM>. For example, the difference in density of hydrogen or any other fuel gas that is less dense than air may result in an exacerbated delay in the appearance of a flame <NUM> at the burner surface <NUM>. Therefore, the controller <NUM> may determine the variable ignition safety time based at least on the type of the gas <NUM>.

Illustratively, the controller <NUM> may determine the variable ignition safety time based at least on an elapsed time since the flame detector <NUM> previously sensed a flame <NUM> in the burning unit <NUM>.

By way of example, the controller <NUM> may determine the longest duration of time after the opening of the gas valve <NUM> that it is safe to wait for a flame <NUM> to appear in the burning unit <NUM> before the gas valve <NUM> must be closed to prevent an excessive accumulation of the combustible air-gas mixture <NUM> in the burning unit <NUM>. This longest duration of time is sometimes also referred to as a predetermined maximum ignition safety time.

Illustratively, the controller <NUM> may determine the variable ignition safety time by selecting from a predetermined number of at least three different discrete durations of time wherein all values of the at least three different discrete durations of time are smaller than or equal to a predetermined maximum ignition safety time. If desired, the controller <NUM> may determine the variable ignition safety time by selecting from a continuous range of values, wherein all values in the continuous range of values are smaller than or equal to a predetermined maximum ignition safety time.

According to the invention, the controller <NUM> directs the gas valve <NUM> to shut off if, during the ignition phase of the burning unit <NUM>, the gas valve <NUM> is open and the flame detector <NUM> fails to sense a flame <NUM> in the burning unit <NUM> during the variable ignition safety time. For example, the controller <NUM> may direct the gas valve <NUM> to close using actuator signal <NUM>.

By way of example, the controller <NUM> may trigger a warning, if during a predetermined number of consecutive ignition phases of the burning unit <NUM>, the gas valve <NUM> is open and the flame detector <NUM> fails to sense a flame <NUM> in the burning unit <NUM> during the variable ignition safety time.

The warning may be any signal that alerts an operator about the malfunction of the air-gas mixture burning appliance <NUM>. For example, the warning may be a visual signal (e.g., a flashing display, a color coded display, a message, etc.), an audio signal (e.g., a warning message, a beeping signal, etc.), a tactile signal (e.g., a vibration), or any combination thereof.

Illustratively, the controller <NUM> may put the air-gas mixture burning appliance <NUM> into one or more of a safe state, a restricted state, or a non-operational state, if during a predetermined number of consecutive ignition phases of the burning unit <NUM>, the gas valve <NUM> is open and the flame detector <NUM> fails to sense a flame <NUM> in the burning unit <NUM> during the variable ignition safety time.

If desired, the controller <NUM> may trigger a warning and put the air-gas mixture burning appliance <NUM> into one or more of a safe state, a restricted state, or a non-operational state, if during a predetermined number of consecutive ignition phases of the burning unit <NUM>, the gas valve <NUM> is open and the flame detector <NUM> fails to sense a flame <NUM> in the burning unit <NUM> during the variable ignition safety time.

<FIG> shows a schematic view of an air-gas mixture burning appliance <NUM> having a fan <NUM> downstream of an air-gas mixer <NUM> and an air flow channel <NUM> to a reference pressure port <NUM> of a gas valve <NUM>. As shown in <FIG>, the air-gas mixture burning appliance <NUM> may include an air inlet for providing air <NUM>, a gas inlet <NUM> for providing gas <NUM>, and a flue outlet <NUM> for the evacuation of exhaust gas <NUM>. Illustratively, the air-gas mixture burning appliance <NUM> may further include an air-gas mixing unit <NUM>, a burning unit <NUM>, and a heat exchanger <NUM>.

The air-gas mixing unit <NUM> is preferably adapted for mixing of air <NUM> and gas <NUM> to form a combustible air-gas mixture <NUM>. Preferentially, the combustible air-gas mixture <NUM> is a homogenous mixture of the air <NUM> and the gas <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> flows towards the burner surface <NUM>. If desired, the fan <NUM> may drive the combustible air-gas mixture <NUM> through the combustible air-gas pathway <NUM> 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>. The heat exchanger <NUM> may transfer the heat that is generated at the burner surface <NUM> to another medium. For example, the heat exchanger <NUM> may transfer the heat that is generated at the burner surface <NUM> to water in a water circuit. The flue outlet <NUM> may evacuate the exhaust gas <NUM> from the air-gas mixture burning appliance <NUM>.

The gas valve <NUM> may include a reference pressure port <NUM>. Preferably, the air flow channel <NUM> is adapted for providing the flow of the air <NUM> to the reference pressure port <NUM>, and the gas valve <NUM> is adapted for regulating the flow of the gas <NUM> based on the pressure of the flow of the air <NUM> at the reference pressure port <NUM>.

The controller <NUM> is adapted for determining a variable ignition safety time for an ignition phase of the burning unit <NUM> by selecting from at least two different durations of time. Thereby, the controller <NUM> determines the variable ignition safety time as an anticipated maximum duration of time that elapses between an opening of the gas valve <NUM> and a closing of the gas valve <NUM> for avoiding a hazardous accumulation of the combustible air-gas mixture <NUM> in the burning unit <NUM>. Illustratively, the controller <NUM> may send an actuator signal <NUM> to the gas valve <NUM>. If desired, the actuator signal <NUM> may direct the gas valve <NUM> to open and/or to close. For example, at the beginning of an ignition phase, the controller <NUM> may direct the gas valve <NUM> to open.

Thus, actuator signal <NUM> may override the reference pressure port <NUM> of the gas valve <NUM>.

<FIG> shows a flowchart illustrating a method <NUM> of operating an air-gas mixture burning appliance. The air-gas mixture burning appliance comprises a burning unit for burning a combustible air-gas mixture, a flame detector for sensing a presence of a flame in the burning unit, an air-gas mixing unit that is arranged upstream of the burning unit and is adapted for mixing of air and gas to form the combustible air-gas mixture, and a gas valve that is arranged upstream of the air-gas mixing unit and is adapted for regulating a flow of the gas to the air-gas mixing unit.

For example, the air-gas mixture burning appliance <NUM> of <FIG> or <FIG> comprises a burning unit <NUM> for burning a combustible air-gas mixture <NUM>, a flame detector <NUM> for sensing a presence of a flame <NUM> in the burning unit <NUM>, an air-gas mixing unit <NUM> that is arranged upstream of the burning unit <NUM> and is adapted for mixing of air <NUM> and gas <NUM> to form the combustible air-gas mixture <NUM>, and a gas valve <NUM> that is arranged upstream of the air-gas mixing unit <NUM> and is adapted for regulating a flow of the gas <NUM> to the air-gas mixing unit <NUM>. During operation <NUM>, the air-gas mixture burning appliance determines, with a controller, a variable ignition safety time for an ignition phase of the burning unit by determining an anticipated maximum duration of time that can safely elapse between an opening of the gas valve and a closing of the gas valve for avoiding a hazardous accumulation of the combustible air-gas mixture in the burning unit.

Air-gas mixture burning appliance <NUM> of <FIG> or <FIG> uses controller <NUM> to determine a variable ignition safety time for an ignition phase of the burning unit <NUM> by determining an anticipated maximum duration of time that can safely elapse between an opening of the gas valve <NUM> and a closing of the gas valve <NUM> for avoiding a hazardous accumulation of the combustible air-gas mixture <NUM> in the burning unit <NUM>.

During operation <NUM>, the air-gas mixture burning appliance directs, with the controller, the gas valve to shut off if, during the ignition phase of the burning unit, the gas valve is open and no flame is detected in the burning unit during the variable ignition safety time.

Air-gas mixture burning appliance <NUM> of <FIG> or <FIG> uses controller <NUM> to direct the gas valve <NUM> to shut off if, during the ignition phase of the burning unit <NUM>, the gas valve <NUM> is open and no flame <NUM> is detected in the burning unit <NUM> during the variable ignition safety time.

If desired, the air-gas mixture burning appliance may further comprise a secured gas pathway that is arranged between the gas valve and the air-gas mixing unit and is adapted for providing the gas from the gas valve to the air-gas mixing unit.

For example, the air-gas mixture burning appliance <NUM> of <FIG> or <FIG> may comprise a secured gas pathway <NUM> that is arranged between the gas valve <NUM> and the air-gas mixing unit <NUM> and is adapted for providing the gas <NUM> from the gas valve <NUM> to the air-gas mixing unit <NUM>.

In such an air-gas mixture burning appliance, the operation of determining <NUM> the variable ignition safety time may further comprise the operation of determining the variable ignition safety time based at least on the type of the gas, and/or on an inferred concentration of the gas in the secured gas pathway, and/or on an elapsed time since a flame detector previously sensed a flame in the burning unit.

As an example, the air-gas mixture burning appliance <NUM> of <FIG> or <FIG> may determine the variable ignition safety time based at least on the type of the gas <NUM>.

As another example, the air-gas mixture burning appliance <NUM> of <FIG> or <FIG> may determine the variable ignition safety time based at least on an inferred concentration of the gas <NUM> in the secured gas pathway <NUM>.

As yet another example, the air-gas mixture burning appliance <NUM> of <FIG> or <FIG> may determine the variable ignition safety time based at least on an elapsed time since a flame detector <NUM> previously sensed a flame <NUM> in the burning unit <NUM>.

As yet another example, the air-gas mixture burning appliance <NUM> of <FIG> or <FIG> may determine the variable ignition safety time based at least on the type of the gas <NUM>, and an inferred concentration of the gas <NUM> in the secured gas pathway <NUM>.

As yet another example, the air-gas mixture burning appliance <NUM> of <FIG> or <FIG> may determine the variable ignition safety time based at least on the type of the gas <NUM> and an elapsed time since a flame detector <NUM> previously sensed a flame <NUM> in the burning unit <NUM>.

As yet another example, the air-gas mixture burning appliance <NUM> of <FIG> or <FIG> may determine the variable ignition safety time based at least on an inferred concentration of the gas <NUM> in the secured gas pathway <NUM>, and an elapsed time since a flame detector <NUM> previously sensed a flame <NUM> in the burning unit <NUM>.

As yet another example, the air-gas mixture burning appliance <NUM> of <FIG> or <FIG> may determine the variable ignition safety time based at least on the type of the gas <NUM>, and on an inferred concentration of the gas <NUM> in the secured gas pathway <NUM>, and on an elapsed time since a flame detector <NUM> previously sensed a flame <NUM> in the burning unit <NUM>.

Illustratively, the controller of the air-gas mixture burning appliance may further trigger a warning and/or put the air-gas mixture burning appliance into one or more of a safe state, a restricted state or a non-operational state, if, during a pre-determined number of consecutive ignition phases of the burning unit, the gas valve is open and no flame is detected in the burning unit during the variable ignition safety time.

As an example, the controller <NUM> of the air-gas mixture burning appliance <NUM> of <FIG> or <FIG> may trigger a warning, if, during a predetermined number of consecutive ignition phases of the burning unit <NUM>, the gas valve <NUM> is open and no flame <NUM> is detected in the burning unit <NUM> during the variable ignition safety time.

As another example, the controller <NUM> of the air-gas mixture burning appliance <NUM> of <FIG> or <FIG> may put, the air-gas mixture burning appliance <NUM> into one or more of a safe state, a restricted state or a non-operational state, if, during a predetermined number of consecutive ignition phases of the burning unit <NUM>, the gas valve <NUM> is open and no flame <NUM> is detected in the burning unit <NUM> during the variable ignition safety time.

Claim 1:
An air-gas mixture burning appliance (<NUM>), comprising:
a burning unit (<NUM>) for burning a combustible air-gas mixture (<NUM>);
an air-gas mixing unit (<NUM>) that is arranged upstream of the burning unit (<NUM>) and is adapted for mixing of air (<NUM>) and gas (<NUM>) to form the combustible air-gas mixture (<NUM>);
a gas valve (<NUM>) that is arranged upstream of the air-gas mixing unit (<NUM>) and is adapted for regulating a flow of the gas (<NUM>) to the air-gas mixing unit (<NUM>);
a flame detector (<NUM>) for sensing a presence of a flame (<NUM>) in the burning unit (<NUM>); and
a controller (<NUM>) that is adapted for determining a variable ignition safety time for an ignition phase of the burning unit (<NUM>) by selecting from at least two different durations of time, the ignition safety time being the maximum allowed opening time of the gas valve (<NUM>) without a flame (<NUM>) being detected, wherein the controller (<NUM>) determines the variable ignition safety time as an anticipated maximum duration of time that elapses between an opening of the gas valve (<NUM>) and a closing of the gas valve (<NUM>) for avoiding a hazardous accumulation of the combustible air-gas mixture (<NUM>) in the burning unit (<NUM>); and wherein the controller (<NUM>) directs the gas valve (<NUM>) to shut off if, during the ignition phase of the burning unit (<NUM>), the gas valve (<NUM>) is open and the flame detector (<NUM>) fails to sense a flame (<NUM>) in the burning unit (<NUM>) during the variable ignition safety time.