Hello red51,
Don't know if anyone already reports it or not but if it's the case sorry, I don't see it anywhere.
There's i little bug which is only visual with the big furnace only (not with the 2 others)...
If your big furnace is turned off and you unload the chunks (going far away) and you come back it will be burning. But, this is only visual and don't consume fuel or smelt ores...
You have just to turn it on/off again and it's alright again 
