Changes of air temperature and air pressure cause air temperature and air pressure-dependent fluctuations of the air/fuel ratio λ to occur. Combustion devices are therefore typically set with an air surplus to avoid unhygienic combustion. Setting combustion devices to an air surplus, however, results in a lower efficiency of the system.
Typically, rotational speed sensors and air pressure switches are used for measuring the amount of air. Rotational speed sensors are not sensitive to fluctuations in air temperature and air pressure. Air pressure switches are optimized for a specific pressure, so often a number of switches are used to monitor across a number of pressures. Despite this, adjusting the entire operating range of the combustion device is typically ineffective. Known solutions for adjustment require two units. The occurrence of turbulence makes the problem even more difficult, since the signal of a flow sensor is greatly influenced by its installed position in the middle of a turbulent flow. As well as this, the turbulence causes the measurement signal to be very noisy.
For example, European patent EP1236957B1 describes adaptation of a burner-operated heating device to an air exhaust system, including a pressure sensor/air mass sensor 28 arranged in the air feed 14 or exhaust gas venting system of a heating device. A regulating device 30 regulates a fan 26, starting from the signal of the sensor 28. To adapt the instantaneous air volume flow to a required air volume flow, an operating characteristic curve 40 is stored. To improve the regulation behavior with large differences in temperature and with respect to emergency operating characteristics a temperature sensor 35 is provided.
Other known systems include a pneumatic composite having mass balancing, a venturi nozzle which creates a vacuum, and a mass flow sensor in an additional duct. An open-loop or closed-loop controller regulates the speed of a fan as a function of the signal of the sensor.