In a burner flame, the hottest part is just above the "inner cone". But for domestic heating purposes,

Question

heat is extracted from the cooler exhaust gas completely above the flame, not from a location within the flame. Why is the hottest part of the flame not used as the site of heat extraction?

Answer 1

I suspect (though without proof!) that intercepting the reacting gases in the flame to extract their heat would quench the flame process, resulting in incomplete combustion, and thus generation of carbon monoxide and small oxygenated organic compounds. (You can sample these materials by running your finger quickly through a wax candle flame, several times, then smelling it.) Those would be air pollutants if released untreated. Possibly there might be soot buildup as well. So, although some of the energy of the flame, just above the reducing-atmosphere cone, may be siphoned off for specific purposes such as melting glass tubing, extracting all of it is not sustainable. However -- could some of it be siphoned off (via a heat exchanger), as a higher-temperature economically profitable stream, as long as the rest was allowed to completely react for (downstream) conventional heat-extraction? Don't know the answer to that.

-- Cheers, --Mr. d.