Abstract: Apparatus for detecting the presence of combustible vapors, e.g., natural gas, in the air utilizes an aluminum heat sink having a sealed reference chamber and a sensing chamber through which samples of air are continuously drawn. Separate unsupported platinum filaments that change in resistive value as a function of combustible gas which comes in contact therewith are located within the two chambers. The bottom of the sensing chamber has an input passage and an output passage with a wire mesh screen covering both passages. A solid non-porous plate having an outer perimeter slightly smaller than the inner perimeter of the sensing chamber is located above the wire mesh screen. The electrical circuitry of the apparatus includes a filament burn-out detection circuit and an unbalanced wheatstone bridge that goes through balance to the reverse unbalanced condition only if a combustible vapor comes into contact with the sensing filament.