Gas, both natural and manufactured, is used extensively for heating. It burns with either a blue flame or yellow, luminous flame, depending upon the type of flame device or burner which is used. The yellow flame is suitable only for fireplaces or portable heaters and its burner must be kept cleaned and regulated so that no smoke or soot is given off. Since blue-flame gas heating appliances do give off smoke and soot they are usually connected with a flue or set in a fireplace that has an effective flue.
The blue flame is hotter than the yellow flame because it is the product of perfect combustion, while particles of un-burned carbon are floating about in the yellow flame. The burners of blue-flame heating appliances are usually provided with an air shutter by which the quantity of air which mixes with the gas within the burner can be regulated. If a large amount of air is admitted the number of carbon particles is increased and the result is imperfect combustion. The shutter should be opened sufficiently so that the flame above each burner opening will have a sharply defined inner blue or bluish green cone. This indicates that an adequate amount of air is mixing with the gas in the burner.
If the air shutter is too wide open the gas may "fire back" and burn within the burner itself. When the gas burns inside the burner, combustion is incomplete and dangerous products of partial burning are given off. The improper burning of the gas within the burner is sometimes called "lighting back" and is accompanied by a roaring noise. When this unusual noise is heard the gas should be turned out at once and, after a moment, lighted again.