<p>The heating device has a flashback protecting thermal element (14) that is arranged in a combustion air inlet (3) for monitoring flashback flame in the combustion air inlet. The flashback protecting thermal element is heated by the flashback flame in the combustion air inlet in a distance. An electrical flashback voltage is generated by the flashback protecting thermal element. A flame protecting voltage and the flashback voltage are merged by polarity. A solenoid valve device (13) automatically takes a closing position takes and interrupts gaseous fuel supply (8).</p>
