Method for Controlling a Gas Burner, in particular in Heating Installations with Blower

0001The invention relates to a method for controlling a gas burner, especially in heating systems with blower.

0002In heating systems with blower according to the prior art, the amount of fuel gas is adapted to the amount of air by means of a gas fitting. For this purpose, the air mass flow is usually measured by means of the pressure drop across a diaphragm and controlled by this control pressure, the fuel gas amount. This method for mixing fuel gas and air has the disadvantage that due to changing fuel gas composition, the excess air can vary; This can lead to high pollutant emissions during operation and starting difficulties.

0003It is known from EP 770 824 B1 that the fuel gas / air mixture can be regulated by measuring the ionization current, which is dependent on the excess air and has its maximum in the case of stoichiometric combustion, and the mixture is changed as a function of the ionization current signal can. The problem here is that a relatively small signal must be measured very accurately.

0004In a method for controlling the fuel gas-air mixture according to EP 833 106, a flame sensor is located near a burner plate. Increasing the excess air causes the flame to lift off, giving a rough measure of the excess air.

0005Also known is a method in which the oxygen content in the exhaust gas of a gas burner is measured and the fuel gas-air mixture is controlled such that there is a certain proportion of oxygen in the exhaust gas. Such a method is described, for example, in US 5,190,454. It should be noted that the measurement signal changes only slightly and also almost linearly in the working area. Oxygen sensors, which measure excess oxygen over years, are not part of the state of the art. This has the consequence that when measuring the oxygen content, a very accurate measurement is necessary and the sensors must be replaced early.

0006In EP 1 331 444 a method is described in which by means of a carbon monoxide sensor in the exhaust pipe of a heater, the fuel gas-air ratio can be adjusted. For such a method, gas sensors made of gallium oxide (Ga₂O₃), which operate at a temperature in the range between 400 and 800 ° C depending on the application. In this case, the sensor element is electrically heated by a mounted on its back heater. Depending on the temperature, the sensor changes its sensitivity. One therefore tries to keep the sensor temperature as high as possible (eg 690 ° C).

0007In the method for adjusting the fuel gas to air ratio, the fuel gas to air ratio is enriched until a significant increase in the measured carbon monoxide emissions takes place, and then the mixture is defined as lean. As a result, a correction factor is determined which serves for the correction of the deposited fuel gas-air characteristic curve to the individual case during operation in the entire modulation range. An advantage of the method is the lower power consumption of the fan, since the pressure loss for the volume flow measurement becomes superfluous. This also reduces the fan noise and a smaller, usually cheaper fan can be used. It is not necessary to distinguish between natural gas H, natural gas L and LPG by different components; it suffices to have a corresponding default for the control so that the burner can be started. The fuel gas-air mixture can be enriched by increasing the amount of fuel gas or reducing the amount of air. For leaning of the mixture, the amount of fuel gas can be reduced or the amount of air can be increased. It is conceivable that in the method both the fuel gas, and air quantity is changed.

0008The calibration procedure may also be performed outside the scheduled cycles when there is an unusually high carbon monoxide or hydrocarbon concentration. So it is conceivable, for example, that shortly after calibration due to a liquid-air admixture, the fuel gas-air ratio changes significantly and a high carbon monoxide or hydrocarbon concentration in the exhaust gas is produced.

0009The object of the invention is to optimize the method known from EP 1 331 444 and to integrate further functions.

0010Thus, it is not known from EP 1 331 444 how, with unknown fuel gas-air mixtures, the burner can first of all be reliably started so that the calibration procedure can be carried out. Furthermore, it is unknown how self-excited vibrations can be recognized and eliminated by the system itself without the intellectual activity of a human being. Another hitherto unresolved problem is the change in frame parameters during operation by heating.

0011Also, it is desirable to enable certain characteristics over the stress modulation range. Especially with large differences between minimum and maximum load of the burner, it can come with long, thin gas lines to large connection pressure fluctuations. The thereby resulting at part load relatively high pressure leads to a relatively rich mixture in the electronic gas-air composite, while the relatively low pressure at full load leads to a relatively lean mixture. Even component tolerances or special installation conditions can lead to a no longer optimal operation on the edge of the operating field / tolerance field, both at rated load and at partial load.

0012Furthermore, the problem of relatively short-term disturbances, for example, due to wind exposure of the air-exhaust system to be solved.

0013In the case of electronic gas / air composite systems, it is customary in the changeover from the 2nd to the 3rd gas families, ie from natural gas to liquid gas or vice versa, to manually reprogram some parameters stored in the device electronics in addition to the nozzle change.

0014In practice, gas burners for heating systems are operated with about 20 to 40% excess air. In modern modulating heaters, the combustion air volume flow is variably adjusted by the delivery rate of a modulated blower. The amount of fuel gas is adjusted either via a fuel gas-air composite, for example by means of a gas fitting, in which by means of a membrane of the fuel gas volume flow of the air flow, or via an adjustable gas valve.

0015The fuel gas mass flow through a nozzle is determined by the nozzle cross-section Q, the pressure in front of the nozzle (fuel gas pressure p_{fuel gas}) and behind it, the nozzle outflow factor ψ, the density ρ and the isentropic exponent κ of the fuel gas.<maths id="math0001"><math display="block"><mrow><mover accent="true"><mrow><mtext>m</mtext></mrow><mo>̇</mo></mover><mtext> = ψQ</mtext><msqrt><mfrac><mrow><mtext>2K</mtext></mrow><mrow><mtext>K - 1</mtext></mrow></mfrac><msub><mrow><mtext> p</mtext></mrow><mrow><mtext>fuel gas</mtext></mrow></msub><msub><mrow><mtext> ρ</mtext></mrow><mrow><mtext>fuel gas</mtext></mrow></msub><msup><mrow><mtext> (β</mtext></mrow><mrow><mtext>2 / K</mtext></mrow></msup><msup><mrow><mtext> - β</mtext></mrow><mrow><mtext>(K + 1) / K</mtext></mrow></msup><mtext>)</mtext></msqrt></mrow></math><img file="EP1522790A2_D0001.tif" /></maths><maths id="math0002"><math display="block"><mrow><mtext>β = </mtext><mfrac><mrow><msub><mrow><mtext>P</mtext></mrow><mrow><mtext>behind nozzle</mtext></mrow></msub></mrow><mrow><msub><mrow><mtext>P</mtext></mrow><mrow><mtext>in front of nozzle</mtext></mrow></msub></mrow></mfrac></mrow></math><img file="EP1522790A2_D0002.tif" /></maths>

0016Heaters are supplied with natural gas at a nominal pressure of 20 * 10⁵ Pa operated; with LPG it is 50 * 10⁵ Pa. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1:</title><tgroup cols="6" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="26.25mm" /><colspec colnum="2" colname="col2" colwidth="26.25mm" /><colspec colnum="3" colname="col3" colwidth="26.25mm" /><colspec colnum="4" colname="col4" colwidth="26.25mm" /><colspec colnum="5" colname="col5" colwidth="26.25mm" /><colspec colnum="6" colname="col6" colwidth="26.25mm" /><thead valign="top"><row><entry namest="col1" nameend="col6" align="left">Differences methane to propane</entry></row><row><entry namest="col1" nameend="col1" rowsep="0" /><entry namest="col2" nameend="col2" align="left">calorific value</entry><entry namest="col3" nameend="col3" align="left">density</entry><entry namest="col4" nameend="col4" align="left">Minimum air requirement</entry><entry namest="col5" nameend="col5" align="left">Vol.str. Fuel gas per kW</entry><entry namest="col6" nameend="col6" align="left">Vol.str.air per kW</entry></row><row><entry namest="col1" nameend="col1" rowsep="0" /><entry namest="col2" nameend="col2" align="left">H_u</entry><entry namest="col3" nameend="col3" align="left">ρ</entry><entry namest="col4" nameend="col4" align="left">I_min</entry><entry namest="col5" nameend="col5" align="left">dV_L/ dt</entry><entry namest="col6" nameend="col6" align="left">dV_B/ dt</entry></row><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="left">kWh / m³</entry><entry namest="col3" nameend="col3" align="left">kg / m³</entry><entry namest="col4" nameend="col4" align="left">m³_L/ m³_B</entry><entry namest="col5" nameend="col5" align="left">m³/H</entry><entry namest="col6" nameend="col6" align="left">m³/H</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Methane CH₄</entry><entry namest="col2" nameend="col2" align="char" char=",">9.968</entry><entry namest="col3" nameend="col3" align="char" char=",">.7175</entry><entry namest="col4" nameend="col4" align="char" char=",">9.52</entry><entry namest="col5" nameend="col5" align="char" char=",">.1003</entry><entry namest="col6" nameend="col6" align="char" char=",">.9551</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Propane C₃H_8th</entry><entry namest="col2" nameend="col2" align="char" char=",">25,893</entry><entry namest="col3" nameend="col3" align="char" char=",">2.0110</entry><entry namest="col4" nameend="col4" align="char" char=",">23,80</entry><entry namest="col5" nameend="col5" align="char" char=",">0.0386</entry><entry namest="col6" nameend="col6" align="char" char=",">.9192</entry></row></tbody></tgroup></table></tables>

0017Natural gas consists mainly of methane. Propane is a typical LPG. It is clear from Table 1 that a heater can not be changed from natural gas to LPG or vice versa without changes. Although approximately the same amount of air is required per load unit, the necessary fuel gas flows differ considerably.

0018Therefore, in practice, when using LPG, a throttling orifice is additionally installed in the gas path to correspondingly reduce the fuel gas flow. However, since the outflow behavior of different gases is different for the reasons mentioned above, the fuel gas-air ratio can be adjusted by means of a nozzle only at one operating point. For the modulation range, ie the range from minimum to maximum load, the ratio must also be adjusted.

0019According to the state of the art, when converting from natural gas to liquefied petroleum gas, the throttle must be used and, in addition, parameters must be reprogrammed in the electronics. If the heating engineer forgets the reprogramming, it can lead to malfunctions, at least one can expect an unhygienic combustion. An object of the invention is therefore to be able to dispense with the manual process of reprogramming and still guarantee a hygienic combustion.

0020According to the features of independent claim 1, the gas burner can be started independently of the existing gas quality. An existing flame is a prerequisite for carrying out the calibration procedure.

0021According to the features of the dependent claim 2 can be avoided that - quite common - Commissioning error, such as a closed gas tap, leading to problematic operating conditions.

0022According to the features of claim 3 self-excited combustion vibrations can be avoided.

0023According to the features of claim 4, a calibration after a certain period of operation after the ignition of the burner or upon reaching a certain temperature in the system can be initiated. As a result, the effect caused for example by the heating of a vacuum chamber, be compensated. As temperature, for example, the temperature in the vacuum chamber, the flow or return temperature can serve here.

0024Conversely, after a certain downtime again calibration may be necessary. Other factors considered are: The ratio between operating and standstill times, water content of the device, outside temperature, chimney draft, exhaust system, etc.

0025According to the features of claim 5, the characteristic curve, which serves as the basis for the control, be designed such that the fuel gas-air ratio is changed defined over the modulation range. For example, a high excess of air at a high modulation rate is to be avoided in order to prevent the flame from dissipating.

0026According to the features of claim 6, the optimum characteristic can be determined more accurately by a plurality of calibration operations at different burner loads.

0027According to the features of dependent claim 7, a calibration is carried out in particular if increased pollutant emissions occur in the previously specified framework conditions.

0028According to the features of claim 8 and 9 short-term disturbances can be eliminated eg by wind attack.

0029According to the features of claim 10, according to the invention, when there is a change in the type of gas (natural gas - liquefied petroleum gas) at at least one predetermined position of the means for adjusting the combustion air volume flow, for example, a given fan speed, and the means for adjusting the fuel gas volume flow, for example, a certain opening stroke of a gas fitting, outside the calibration point, a measurement of a specific gas concentration in the exhaust path of the gas burner, for example, oxygen or carbon monoxide, is carried out and the measurement result is compared with at least one reference value or at least one further measured value. The control adjusts - based on the measurement results - the ratio of the fuel gas volume flow to the combustion air flow rate by adjusting the fuel gas-air characteristic accordingly. If a carbon monoxide-sensitive sensor is used in the exhaust path, an evaluable signal can be obtained particularly easily. While an oxygen sensor measures a linear signal during near-stoichiometric combustion, a CO sensor can measure an exponential signal.

0030According to the features of dependent claim 11, a nozzle is used in the conversion from natural gas to liquefied petroleum gas operation to meet the very different gas properties and to allow for a load specification identical controls.

0031According to the features of dependent claim 12, the method is performed after the operation of the main electrical switch. This ensures that an automatic adjustment takes place during initial startup, but also during maintenance with possible nozzle change.

0032The invention will be explained below with reference to Figures 1 to 8. Show it:<ul id="ul0001" list-style="none"><li>1 shows a heating system for carrying out the method according to the invention,</li><li>2 shows the relationship between excess air and carbon monoxide emission,</li><li>3 shows characteristic curves in which the ratio between the fan speed and the number of steps of the stepping motor of the gas valve is shown,</li><li>4 air figures over the load to the characteristics of Figure 3</li><li>5 characteristic curves with respect to the ratio between the fan speed and the step number of the stepping motor of the gas valve according to an advantageous embodiment of the invention,</li><li>FIG. 6 shows the course of the measured carbon monoxide emissions in the case of sudden wind incidence and the compensatory measure in steady-state operation, FIG.</li><li>Fig. 7 at the start and windfall the effect of the compensation measure and</li><li>Figure 8 each one for adjusting the fuel gas-air ratio for natural gas and LPG.</li></ul>

0033A heating system according to FIG. 1 has a burner 1 with a surrounding heat exchanger 10, to which an exhaust pipe 9, in which an exhaust gas sensor 6 is connected. The burner 1, a fan 2 is connected upstream. On the input side of the blower 2 is an air intake line 13, in which also a fuel gas line 12, which is separated by a gas valve 4 from the fuel gas supply 11, extends. In the fuel gas line is optionally a throttle 17 for LPG operation. The gas valve 4 has an actuator 5. The blower 2 has a drive motor 7 with speed detection 8. Actuator 5, drive motor 7, speed detection 8 and exhaust gas sensor 6 are connected to a controller 3, which has a memory module 31 and computing module 32. Also with the control is an ionization electrode 14, which is positioned just above the burner 1, connected.

0034When burner operation is of the control 3 eg due to a room thermostat, not shown, in conjunction with a flow temperature detection, also not shown in the computing module 32, a target power of the burner 1 calculated. In the memory module 31, a desired signal for the fuel gas and air quantity is stored to the target power. With these desired signals, the blower 2 is driven with its drive motor 7 and its speed detection and the gas valve 4 with its actuator 5, whereby a fuel gas-air mixture flows into the blower 2 and from there to the burner 1. The mixture is burned on the outer surface of the burner 1, flows through the heat exchanger 10 and then flows through the exhaust pipe 9 into the open air.

0035Fig. 2 shows the relationship between carbon monoxide concentration and combustion air ratio λ. To achieve complete combustion, theoretically, a combustion air ratio<b>λ</b> of 1.0 necessary.<maths id="math0003"><math display="block"><mrow><mtext>λ = </mtext><mfrac><mrow><msub><mrow><mtext>m</mtext></mrow><mrow><mtext>L</mtext></mrow></msub></mrow><mrow><msub><mrow><mtext>m</mtext></mrow><mrow><mtext>L, min</mtext></mrow></msub></mrow></mfrac></mrow></math><img file="EP1522790A2_D0003.tif" /></maths>

0036Here m is_L the actual amount of air and m_{L, min} the stoichiometric amount of air. The combustion of hydrocarbons into carbon dioxide always produces carbon monoxide as an intermediate. Due to the limited reaction time in the heat-affected zone and insufficient mixing of fuel gas and air, however, in practice, a certain excess air is necessary to ensure complete burnout. Therefore, a CO value of well over 1000 ppm is usually reached at just over-stoichiometric combustion. Only at an excess of air of approx. 10%, the carbon monoxide emissions in the exhaust gas reacted clearly and reach in conventional burners values ​​below 100 ppm. As the air ratio increases, however, the combustion temperature drops because of the proportion of inert gases; the combustion reaction is slowed down and the reaction at the heat exchanger stops. Therefore, from an excess of air of approx. 80 % a significant increase in carbon monoxide emissions.

0037Since in stoichiometric combustion (theoretically) the entire fuel is burned and no excess air is present, in this case the combustion temperature is maximum. With excess air, the proportion of inert gases is increased, whereby the combustion temperature decreases. This has the consequence that the nitrogen oxide emissions are maximum in stoichiometric combustion and decrease as the excess air is increased. The efficiency of a heating system is maximum in stoichiometric combustion and decreases when increasing the excess air, since the inert gases absorb heat losses and the residence time of the exhaust gas in the heat exchanger is reduced due to the increased flow rate, which is not compensated by the improved heat transfer. However, it must be taken into account that soot formation can occur both with near-stoichiometric combustion and with very large excess air. this deteriorates the heat transfer at the heat exchanger.

0038The above facts have the consequence that gas burners are preferably operated with a defined excess of air. In the embodiment, therefore, it is assumed that a target air ratio of about 1.25. In Fig. 2, this corresponds to the point D, which lies in a desired range C.

0039When burning:<maths id="math0004"><math display="block"><mrow><msub><mrow><mover accent="true"><mrow><mtext>V</mtext></mrow><mo>̇</mo></mover></mrow><mrow><mtext>air</mtext></mrow></msub><msub><mrow><mtext> = I</mtext></mrow><mrow><mtext>min</mtext></mrow></msub><mtext> * Λ * </mtext><msub><mrow><mover accent="true"><mrow><mtext>V</mtext></mrow><mo>̇</mo></mover></mrow><mrow><mtext>fuel gas</mtext></mrow></msub></mrow></math><img file="EP1522790A2_D0004.tif" /></maths>

0040Here, I is_min the minimum air requirement. Since the ratio of fuel gas to air over the entire modulation range does not have to be constant in a real burner system, a dependency results<maths id="math0005"><math display="block"><mrow><msub><mrow><mover accent="true"><mrow><mtext>m</mtext></mrow><mo>̇</mo></mover></mrow><mrow><mtext>air</mtext></mrow></msub><mtext> = f (P) * </mtext><msub><mrow><mover accent="true"><mrow><mtext>m</mtext></mrow><mo>̇</mo></mover></mrow><mrow><mtext>fuel gas</mtext></mrow></msub></mrow></math><img file="EP1522790A2_D0005.tif" /></maths> f (P) is the power-dependent ratio function, which is almost linear, between fuel gas and air.

0041When commissioning the fuel gas and the pressure loss of the entire heating system are unknown. The fuel gas-air mixture, which therefore sets with a predetermined default for the fan 2 and the gas valve 4, therefore, can be very different. If the mixture is too lean, there is no ignition of the burner during a starting process; the same applies to a too rich mixture. Ignitable lean mixtures tend to lift the flame from the burner, whereby the ionization electrode 14 can not detect a flame. Ignitable, rich mixtures can cause a loud ignition noise and deflagration.

0042Therefore, according to the invention, a first starting operation with a basic setting, which corresponds to a lean mixture under probable boundary conditions, is carried out during initial startup. If this starting process does not lead to a detection of a flame by means of ionization electrode 14, the starting process is aborted. After a rinsing phase by means of the blower 2, a new starting process is carried out with a richer mixture. This can be achieved both by the fact that the gas valve 4 is opened further or the speed of the blower 2 is lowered. This procedure can be repeated until a flame is detected.

0043When installing a heater for the first time, the installer forgets to open the gas tap and only opens the gas tap after the first unsuccessful start attempts. This makes it possible for the burner to be operated with such high emissions in the near-stoichiometric or substoichiometric range that the calibration method according to the invention can no longer be carried out. Therefore, if a flame only at 3. or a later ignition attempt is detected, the flame again immediately cleared (closing the gas valve) and the next start attempt again started with the original fuel gas-air ratio. This ensures that the gas tap is open. Successful flame detection again in an advanced ignition test, it can be assumed that there is no superfat mixture. An immediate extinction of the flame is then no longer.

0044After the successful ignition follows a short operating time, in which the flame burns with unchanged operating conditions, so that a stationary state of the heater occurs and the flame can stabilize. After this phase begins the calibration of the fuel gas-air ratio.

0045At the beginning of the calibration, there is any fuel gas / air ratio. The control 3 continuously controls the actuator 5 of the gas valve 4 in such a way that more and more fuel gas passes into the blower 2 at the same amount of air. As a result, the mixture is enriched; the air ratio drops. The exhaust gas sensor 6 measures the carbon monoxide emission in the exhaust pipe 9 and forwards the signal to the controller 3. If regulation 3 registers that the carbon monoxide emission has a threshold value of 500 ppm specified in memory module 31 (point A in FIG. 2), the mixture is not further enriched. It is known that such carbon monoxide emissions at an air ratio of about 1.08 can be achieved. The control 3 is now the speed of the blower 2 of the speed sensor 8 of the drive motor 7 and the position of the gas valve 4 (for example, via the timing of the actuator 5 in the form of pulse width modulation) known. These data are stored in the memory module 31. By comparing these data with reference values ​​likewise stored in the memory module 31 in the calculation module 32, a correction factor k is set. The reference values ​​correspond to the setting for a correction value k = 1.

0046The correction value k can be calculated, for example, according to<maths id="math0006"><math display="block"><mrow><mtext>k = </mtext><mfrac><mrow><msub><mrow><mtext>f (pulse signal</mtext></mrow><mrow><mtext>is</mtext></mrow></msub><mtext>)</mtext></mrow><mrow><msub><mrow><mtext>f (pulse signal</mtext></mrow><mrow><mtext>reference</mtext></mrow></msub><mtext>)</mtext></mrow></mfrac></mrow></math><img file="EP1522790A2_D0006.tif" /></maths>

0047The pulse signal_is this corresponds to the number of steps of the stepper motor. The correction value is stored in the memory module 31. During future start-up of the burner system, at the start conditions (speed of the fan 2 and position of the gas valve 4), the default setting is changed by the correction factor in order to be able to initiate an optimized burner start. If no flame detection is performed with this setting, the mixture can be further enriched in further start attempts.

0048For the heating operation it follows that the control 3 in the demand-dependent operation following the burner start the fuel gas-air ratio according to the relationship<maths id="math0007"><math display="block"><mrow><msub><mrow><mover accent="true"><mrow><mtext>m</mtext></mrow><mo>̇</mo></mover></mrow><mrow><mtext>air</mtext></mrow></msub><mtext> (P) = f (P) * </mtext><msub><mrow><mover accent="true"><mrow><mtext>m</mtext></mrow><mo>̇</mo></mover></mrow><mrow><mtext>fuel gas</mtext></mrow></msub><mtext> (P) * k</mtext></mrow></math><img file="EP1522790A2_D0007.tif" /></maths> certainly. In the memory module 31 of the control 3 is a basic characteristic a stored in accordance with Figure 3. The basic characteristic a defines the ratio of fan speed to the number of steps of the stepping motor 5 of the gas valve 4 at reference conditions. Accordingly, the controller 3 gives the blower a certain speed and the stepper motor 5 a corrected by the correction factor k step position from the basic characteristic a before.

0049If the correction value is determined, the setpoint air number can be set. In order to achieve the setpoint air ratio of 1.25, the air ratio must be increased by 0.17 according to the above conditions.

0050From the reference values ​​it is known to how many clocking of the actuator 5 in the form of pulse width modulation of the gas flow rate must be reduced in order to achieve an air ratio of about 1.25.

0051Due to the relatively large desired range (C in FIG. 2), the measurement and control need not satisfy a particular accuracy. So it is not a problem if, for example, 700 ppm instead of 500 ppm are measured, since the difference in the excess air for both carbon monoxide emissions is minimal. The emaciation of the mixture can be done in a relatively large tolerance band. It is known that commercially available burners, which are to be operated with lambda 1.25, can be operated without problems in a range between 1.20 and 1.30. It is again very easy to reduce the mixture in such a way that it controls this area with sufficient safety.

0052The calibration can be carried out in fixed cycles (eg operating time).

0053When operating with the fuel gas-air ratio, which is established by the calibration, acoustic combustion oscillations, usually humming or whistling, may occur. These are usually associated with an increase in carbon monoxide emissions. By changing the fuel gas to air ratio, these noises can be eliminated. If an increased CO concentration in the exhaust gas occurs immediately after a calibration, the mixture is slightly enriched or emaciated (typically λ ± 0.2).

0054Heaters often have a vacuum chamber in which the components are located. Combustion air is drawn in from the environment and passed through the vacuum chamber before being fed to the burner t. As a result, heat losses of the components are absorbed within the vacuum chamber and remain in the system. However, this has the consequence that after a start of a cold heater, the air heats up in the vacuum chamber; the air density and thus the air mass conveyed by the fan at the same speed are reduced, while the gas temperature and mass change only insignificantly. This reduces the air ratio during combustion. In an electronic gas air network, the influence of this heating effect is much greater than with a pneumatic gas air network, since there is no direct relationship between the dynamic pressure signal and gas quantity. This can lead to airborne deviations of up to 20%. Consequence: The combustion then does not take place in the optimum range. Therefore, the calibration process can optionally be performed when the vacuum chamber has an elevated temperature.

0055It may happen that, for example, by LPG-air admixture in winter, the fuel gas-air ratio changes within minutes. An improper combustion would be corrected until the next routine calibration, possibly. even completely disregarded, since at the time of the next calibration the original mixture would be available again. To prevent this, the exhaust gas sensor 6 also measures the carbon monoxide emission in the exhaust pipe 9 at certain intervals or even permanently outside of the routine calibration. If a certain limit (point B in FIG. 2), then a calibration of the control 3 is initiated.

0056It may be advantageous that the air ratio over the modulation range has a certain tendency. FIG. 3 shows three characteristic curves a, b and c, which define the ratio of the rotational speed of the blower 2 to the number of steps of the actuator 5 of the gas valve 4. FIG. 4 shows the associated profiles of the air ratio over the load.

0057FIG. 5 shows three characteristic curves d, e and f which define the ratio of the rotational speed of the blower 2 to the number of steps of the actuator 5 of the gas valve 4. To generate the characteristic curve d, the heater was operated during the calibration with a single specific fan speed and from this the calibration point A was determined. The calibration point A is compared with the corresponding reference point on the basic characteristic a (FIG. 3) and from this the correction factor k is determined. With the aid of the correction factor k, the adjusted characteristic curve d can be determined from the basic characteristic curve a.

0058The characteristic curve e is based on the characteristic curve d and takes into account further calibration processes in which the calibration points D and E were determined. In an iteration or extrapolation method, the characteristic curve e is determined from the characteristic curve d and the calibration points D and E. The same applies to the characteristic f with the calibration points B and C.

0059It is possible to create a characteristic curve only with a calibration procedure and to carry out further calibration procedures only if there are apparently unfavorable combustion conditions (high carbon monoxide or hydrocarbon emissions). Alternatively, it is possible to take into account several calibration points during calibration.

0060FIG. 6 shows in steady-state operation the course of the measured carbon monoxide emissions in the event of sudden wind incidence and the compensatory measure. Initially, the device runs below the limit value G₁, By applying wind to the exhaust-air system, the flow resistance increases; the air ratio decreases significantly and the carbon monoxide emissions rise significantly until at time t₁ the limit value G₁ exceed. Now a time recording starts to run. At a time t₂ becomes the predetermined limit value G₂ exceeded. Is the time difference t₂-t₁ smaller than a predetermined period Δt_v, so the fuel gas-air mixture is emaciated. This reduces carbon monoxide emissions. If the CO measured value falls below the limit value G₁ and if the CO measured value rises again later, this may be due to the fact that wind incidence no longer exists and therefore the fuel gas / air mixture is much too lean, which can also result in high CO emissions. Accordingly, information must be stored in the memory module 31, which states that an emaciation has taken place. In view of this fact, provision may be made for enrichment instead of slimming.

0061In such an operation, it makes sense that the lower modulation limit is raised, since otherwise very small burner loads could occur in which a stable burner operation is not ensured.

0062FIG. 7 shows the course of the measured carbon monoxide emissions in the event of wind incidence and the compensatory measure at the burner start. If a predetermined limit G3 is exceeded, the fuel gas-air mixture is emaciated and a waiting time .DELTA.t₁₂ awaited. Is the CO reading still greater G₃, so the fuel gas-air mixture is again emaciated. This procedure can be repeated several times. If the CO measured value falls below the limit value G₃ and if the CO measured value rises again later, enrichment may also be required here for the reasons stated above.

0063FIG. 8 shows a characteristic curve for liquefied gas C.₃H_8th using the additional throttle 17 and a natural gas CH characteristic₄,

0064In comparison between natural gases and liquid gases, the gas volume flows differ by a factor of 2.6 with the same unit performance. Therefore, a throttle element 17 is installed in the fuel gas supply 11 or fuel gas line 12 in front of or behind the gas valve 4. This ensures that at maximum device performance, the number of steps of the actuator 5 of the gas valve 4 despite low flow rates is the same as in natural gas operation.

0065However, the modulation bandwidth differs from the natural gas characteristic. It can be seen that the liquefied gas line runs much flatter than the natural gas line. In the case of systems according to the prior art, this means that one must "tell" the device which type of gas it is used with. This setting is usually via buttons on the control 3.

0066In contrast, according to the invention only above mentioned throttle element 17 must be installed, which ensures that at full load regardless of whether natural gas or LPG is the correct mixing ratio.

0067The heater is put into operation at full load. Thereafter, the power is reduced by adjusting the fan speed and the step number of the actuator 5 according to the natural gas characteristic. If the heater is operated with liquefied petroleum gas, the fuel gas / air mixture is superfilled, since the heater would have to be operated with liquefied gas in accordance with the liquefied gas characteristic, that is to say the heater with less opening of the gas valve 4. Since with near-stoichiometric combustion, the CO content increases abruptly in the exhaust gas and this increase is detected by the CO sensor 6, the controller 3, which receives the signal of the CO sensor 6, switch from the natural gas characteristic to the liquid gas characteristic.

0068Since there are differences in the composition of both natural gas and LPG, fine tuning can also be done. For this purpose, at a predetermined fan speed, the gas valve 4 - starting from a lean mixture - as long as the direction of a richer mixture adjusted until a certain carbon monoxide emission occurs. The same is done at a different blower speed. The two settings that result in this way are significant for the gas type. As a result, the device can be set individually to the fuel gas composition by the fuel gas quantity is reduced by 20% for example, compared to the determined reference points for the desired operating point and the modulation curve is interpolated along these two target operating points.

0069As a result, it is also possible in principle to completely dispense with the nozzle for liquefied gas operation. In the process for switching from natural gas to liquefied petroleum gas without throttle nozzle 17, however, care must be taken that at times the composition of the fuel gas-air mixture could be outside the ignition range and possibly lead to deflagration if the ignition limit is exceeded. Therefore, break times with pure Luftspülpausen would be advisable.

0070It should also be noted that there is a strong increase in carbon monoxide emissions in both near-stoichiometric and super-stoichiometric combustion. In order to detect whether the current combustion is near-stoichiometric or strongly superstoichiometric, the fuel gas-air ratio can be changed. If the combustion is strongly superstoichiometric and the mixture is enriched, the carbon monoxide emissions decrease; At near-stoichiometric combustion, carbon monoxide emissions would continue to increase.

0071Optionally, in the calibration to change the mixture in the direction of fuel-rich composition instead of increasing the fuel gas amount and the amount of air can be reduced, while the amount of gas remains constant. Also, instead of an absolute carbon monoxide signal a gradient (eg CO change per speed change of the blower) are measured. The threshold does not have to correspond to a certain CO-equivalent signal, but can eg also according to background noise without CO (eg 20 mV) plus cut-off value (eg 0.5V). In this case, one would assume that the measured signal at carbon monoxide concentrations in the desired operating range are well below this threshold and the threshold is an indication that a certain fuel gas to air ratio was exceeded in the direction of fuel-rich mixture.

0072A further variant of the method according to the invention is that the calibration does not take place by an enrichment of the mixture up to a threshold value and subsequent emaciation, but rather by a leaning of the mixture up to a threshold value and subsequent enrichment. It is considered that - as shown in FIG. 2 visible - even with very low-fuel mixtures increase the carbon monoxide emissions. While in the carbon monoxide increase in fuel-rich mixtures the onset of the steep rise in almost all burners in the same <b>λ</b>Range, the steep increase in fuel-lean mixtures is very burner-specific. This applies both to the beginning of the ascent and to the gradient (ΔCO / Δλ).

0073It is also known that the emissions of unburned hydrocarbons behave in the same way in the exhaust gas as the carbon monoxide emissions. Therefore, in the method according to the invention also a sensor can be used, which generates a signal equivalent to the unburned hydrocarbons.