Fire is tough to do right. Here is my advice for doing flame (it might give you an idea for what to do with your furnace).
My best stab at doing flame is to first paint it yellow, then paint half way down from the top with dark red. Wash dark red over the entire "flame", then dry brush a few coats of yellow near the base and lightly hit the tips with red, and you might even add a pinch of gold to the very tips. The effect should be yellow in the middle, with red flames at the top with some transition. More often than not new painters will paint flame red to yellow (yellow on top), but flame just doesn't look like that.