Abstract: A sensor of inadequate combustion conditions at a main burner, comprising a small auxiliary gas burner the primary air-gas flow to which consists of a predetermined fraction of the flow of primary air and gas to the main burner plus a predetermined fraction of the flow of secondary air supplied to the main burner, with no secondary air supply of its own. The total combustion air thus supplied to the auxiliary burner is made such that the flame at the auxiliary burner extinguishes when the combustion air at the main burner flame becomes inadequate for satisfactory combustion but is still sufficient to maintain a flame at the main burner. A flame sensor at the auxiliary burner produces indications of absence of flame at the auxiliary burner, which in turn indicates inadequate combustion conditions at the main burner.