Too Much Moisture in Your Firebox Will Damage Your Outdoor Furnace

Too Much Moisture in Your Firebox Will Damage Your Outdoor Furnace

How to Achieve a DRY BURN

Your outdoor wood boiler works perfectly and saves you a lot of money on your heating bills each year. Until it stops running well. Then it wasn't your best investment ever.

Don’t assume that just because it runs great, it will keep running great without you taking care of it. 

So since the EPA banned outdoor wood burning boilers, you can’t replace your boiler once it reaches its end of life.

This means you have to do your normal maintenance, ash management, use water treatment, get the water tested, and all that important stuff… 

But the thing is ALL outdoor wood furnace owners – especially those whose furnaces start to have problems – wish they had started doing SOONER, which is what we call the DRY BURN. 

What is the DRY BURN?

All wood contains lots of moisture. Even the most seasoned hardwood contains 20% moisture. When the wood burns, that water has to burn off; it evaporates into steam and mostly leaves the boiler out of the stack.

However, part of that moisture condenses inside your boiler. This is terrible because corrosion occurs when water comes into contact with steel. When moisture condenses on steel surfaces and persists, a lot of corrosion occurs.

How do you get a DRY BURN?

The moisture in wood is slowly burnt out as it burns. Unfortunately, that moisture is pouring out of the wood during most of the burn cycle — only at the very end of the burn cycle, when the wood is just dry hot coals, is all of the moisture out of the wood.

This is the condition of DRY BURN. If you fire your boiler for several hours in the DRY BURN condition, you will obtain a DRY BURN!

But many outdoor wood burner owners load up their boiler in the morning with a huge load of wood, and by the end of the day, the wood is only half-burned. 

This wood has NEVER reached a DRY BURN state and is still giving the inside of your boiler a moisture bath. Then they “TOP OFF” their firebox burn chamber with more moisture-filled wood, adding even more moisture to the process. 

Even the most seasoned hardwood contains about 20% moisture. That is a lot of moisture, and you gotta deal with it before it deals with you.

Unfortunately, outdoor boilers that don’t get a DRY BURN every day, will begin to experience damaging corrosion. The parts of the outdoor wood stove that are cooler (such as the perimeter of the door) start to show corrosion damage.

The ASH will absorb this moisture and if not removed often, will form an acid paste that eats through steel. Steel in the corners can begin to show pitting damage that, if left unchecked, will lead to a leak in your steel.

One other problem you may experience is a tremendous amount of moisture will drip off your chimney cap and create rust corrosion on your roof. This will require that you sand your roof every year, and repaint it. If not repainted annually, then over time, this corrosion can eat holes through the roof!


Only load into your firebox the amount of wood needed to get you to your next fill. You want your wood to burn down completely to coals and burn in that DRY BURN state for the last few hours of every wood load. 

This will dry out all the components that may have become moist from condensation. This will protect your heating system – and your INVESTMENT! 

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