This is very intriguing!
Can he try something else? Have the secondary rise higher above the fuel load before it enters the combustion chamber/chimney. I think this will "supercharge" the burn as was shown at the end of the last video with the fire roaring. Time had lapsed, the fuel level dropped as it was consumed which led to the secondary air being exposed longer and thus heated more. I got this idea from (abogart?) when he mentioned his work with the furnace at work where the temp increased rapidly.
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TomS
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