I'm playing on 1.16.5 with IE and Iron Furnaces mod
my external heater works fine when I put it next to a vanilla furnace, but modded ones don't seem to work, I have checked configs... but nowhere to see if this is supported or an intentional choice

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image : external heater powered with energy (powah creative cell) next to a diamond furnace, the same setup works for a vanilla furnace

IE's external heater is designed to speed up furnaces in addition to powering them.

Iron Furnaces are already faster, so I wouldn't want them to be buffed by our heater. This is an intentional lack of compatability.