My apologies if you already knew this, but I’ve been playing for months and just figured it out:
Pile some wood in your fireplace (I used a stone fireplace but that probably doesn’t matter) and apply a lighter (or mini-flamethrower CBM) to get a small flame (yellow ‘4’). Now light it again. You get a hotter flame (a pink ‘4’). Now light it again! A raging inferno (red ‘4’)! Now you’ll have to take off some winter clothes or you will become so overheated you hallucinate.
Doesn’t work that way for random piles-of-sticks-on-the-ground fires, just fireplaces.