How much ash, if any, should be left in the firebox?

Roughly 1 inch of ash is the acceptable amount to leave in the firebox. Anything beyond that should be removed once the fire is completely out. The 1 inch layer of ash acts as an insulation and can allow the fire to burn hotter. At the end of the season, all ash should be removed to prevent acidic damage to your unit. If you have any further questions please feel free to give us a call! Thanks.