METHODS OF HEATING 347 



389. Gas for Heating Purposes. 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. 



390. Hot- Water Heating. A hot-water heating system 

 (Fig. 175) operates by the movement of hot water from the 



