Abstract: Method of determining discontinuites in the heat exchanger of a warm air furnace directed to spraying the burner within the furnace with a non-corrosive solution of sodium carbonate. The solution is sprayed on the burner after it is heated and in operation but with the blower fan inoperative. Thereafter a blue flame from a gas torch is played adjacent to or into a register or opening in the plenum of the furnace or an opening in the piping or duct work leading from the plenum. If the blue flame turns yellow this indicates there is a deleterious leakage of combustion gases from the heat exchanger of the furnace.