When Does ASH Eat Holes in Outdoor Furnaces?

When Does ASH Eat Holes in Outdoor Furnaces?

When Does ASH Eat Holes in Outdoor Boilers?

Ash is normally harmless when it is dry and powdery, and only about 2 inches deep in an outdoor boiler.

But, when ash levels get deep, the ash absorbs moisture. "What Moisture!!??" you might ask. Well, wood fuel contains lots of water - even the most seasoned hardwood contains about 20% moisture (unless kiln dried and stored in a climate-controlled environment, nobody does this!).

When the wood burns, the moisture leaves the wood in the form of steam. We want that steam to leave the boiler and not get absorbed by the ash.

Sediment Buildup

Because...  Moist ash is like CAUSTIC PASTE! It will eat holes in steel. If the ash layer in your outdoor Wood furnace is more than 2-3 inches, the ash will absorb the moisture in the steam, and form a CAUSTIC paste that eats holes in steel.

Most often, we see holes form in fireboxes in the bottom rear, and against the rear wall below the ash level. You MUST rake this ash forward and remove it frequently - we recommend removing at least a few shovels full EVERY DAY!

Solution? Simple! Keep your ash levels low - about 1-2 inches in the bottom of your outdoor wood burner. Also, remember to remove ALL the ash every two weeks, and scrape out the inside of your firebox to remove all ash. 

Pay close attention to the rear bottom seam weld area - again this is the most common location where deep layers of ash eat holes in steel.

Deep layers of ash not only destroy steel but also kill your efficiency! The heat from your wood burning needs to transfer through the steel of your firebox to get into your home. If a large portion of your firebox steel is buried in ash, much of the heat from your wood will not transfer into your water jacket, and that heat ends up going out the stack - wasted!

Remember, maintain your outdoor wood stove so it can continue to save you money for decades!

Visit our online store OutdoorBoiler.com for more outdoor wood boiler parts!

Operating Furnace FAQs:

  • Am I burning too much wood?

You may be. Is the wood properly seasoned? Have you cleared the firebox? High wood consumption can be caused by a number of factors.

  • Can I leave my Outdoor Wood Burner over the weekend?

You may have someone "feed" your furnace for you or you may lower the thermostat temperature.

  • Should I turn my thermostat down at night?

No, the lower the water temperature will cause the use of more wood to recover.

  • What are the critical items to operate and maintain my furnace efficiently?

1. Keep water full

2. Manage ash

3. Use water treatment

4. Use seasoned wood

5. Keep fire in the middle of the firebox

6. Clean tubes regularly

Maintenance FAQs

  • Why is outdoor boiler water treatment important?

The boiler chemical (corrosion inhibitor) that you would apply to your furnace water coats the inside of the furnace to reduce the effects of corrosion on the metal. 

Click HERE to order our Liquid Armor Water Treatment and other outdoor wood furnace parts. Every gallon of Liquid Armor comes with FREE 2 sample bottles. Then submit your water sample to Outdoor Boiler Lab and get Free Water Test!

  • How often do I need to Inspect my Chimney/Flue

Inspect chimney and flue monthly and clean as needed. Clean chimney and flue annually. Perform cleaning and maintenance only when no fire is present in the firebox and ashes are cooled completely.

  • Why is ash management important?

If the ash becomes wet it is corrosive to the metal of the furnace. If the level of ash is too thick in the water box it does not allow for efficient heat transfer to the water in the water jacket of the furnace. The temperature of the water in the jacket is used to heat your house.


  • Why am I not getting hot water?

Check that the hot water system is connected properly. Check if the heat exchanger is partially or totally clogged.

  • Why does my furnace continue to burn when it is not operating?

Check the door seal, door adjustment, and solenoid.

  • I can't find a leak in my furnace. Why am I losing water?

Outdoor Wood Burning Boilers use a non-pressurized system, this means that the existing heating system is exposed to the air. Since the water in the system can and will be warmer than the air, there will be evaporation of the water in the system. The water must be checked and adjusted at least once a month, sometimes more.