All furnaces must be dried out every day. Why? Because wood fuel is very high in moisture. Even the most well-seasoned wood contains about 20% moisture and that is a lot of water.
This moisture will create creosote; it will also drip off your chimney cap and stain your roof.
What should happen to the moisture in your wood? We want it to burn off and exit your furnace harmlessly in the form of steam. But that steam coats your entire furnace walls, tubes, door seals, chimney, and your ash with MOISTURE! Even if you burn your furnace perfectly every day, you will still produce a LOT of steam that puts a layer of moisture on your entire boiler.
This is bad. Moisture is bad for steel. Trust us on this - we won't bore you with the technical reasons, but moisture is just bad for steel. Not to mention the fact that this moisture creates nasty creosote.
So HOW can I dry out my furnace? It's easy! Your furnace MUST burn at least several hours each day with only hot dry coals in it.
Yep, the moisture in wood is burning OUT of the wood during MOST of the burn process. It is ONLY after the wood is burned down to black coals when there is no more moisture left in the wood and it can then burn in a very DRY state.
Every day, you must allow your wood to burn down COMPLETELY to a bed of coals. ONLY THEN CAN YOUR FURNACE DRY OUT!
So what most people do is they only put enough wood in their furnace so that it will be completely burned down to ashes and only a few coals when they refill.
What is the worst thing to do? Good question!
NEVER "Top Off" your furnace with wood fuel. If you go out to check on your furnace, and it is still half full of wood, DO NOT "Top It Off" with wood...here's why:
When you first fill your furnace with wood fuel, the water in the wood will start to boil off. When your wood is only half burned, THERE IS STILL a lot of moisture in the wood. If you then add more fresh wood (full of moisture), your furnace NEVER has a chance to dry out. If you always do this, then your furnace walls, tubes, door, and seals will continually be coated with steam, and never have a chance to dry out.
Again, the solution is simple: Please only put enough wood in your furnace so that it will be completely burned down to ashes and only a few coals when you refill.