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Wood Burning Furnaces: Best Burn Practices

October 9th, 2023 | 5 min. read

By Louis Greubel

A close-up of logs that are nearly burnt to ash. They are engulfed in flames.

It’s easy to get lost in a sea of details when thinking about wood furnaces. People tend to wonder, “How do I know which one to get? When I do find one, how am I going to install it? How do I connect it to my ductwork? How do I clean and maintain it over time?”

And those are all important questions. But perhaps the most important question that you should keep front and center in your head is, “How should I be burning wood in my furnace?

Wood burning is as much an art as it is a science. We should know; we manufacture an EPA-approved furnace, and the process of getting a wood burning appliance approved by the Environmental Protection Agency includes plenty of burn tests. From the kind of wood you should use to the process of keeping a fire burning, there’s a lot that goes into good burn practices. And that’s exactly what we want to teach you about.

By the end of this guide, you’ll know exactly what you need to do to burn wood in your wood burning furnace the right way. We’ll teach you about moisture content, firewood storage, and starting and maintaining a fire so you can get the best out of your wood furnace.

How to Season Firewood for Your Wood Furnace

A close-up of a stack of seasoned firewood

Before even putting wood into your furnace, you have to make sure you have the right kind of firewood. In some cases, 50% or more of the weight of a piece of firewood may be water, which doesn’t exactly translate into a good burn.

Firewood burned in a furnace should contain 20% moisture or less — a measurement you can ascertain from a moisture meter. But how can you reduce the amount of moisture in your firewood? The answer is a process called seasoning, and it involves four steps:

  1. Splitting firewood
  2. Stacking firewood
  3. Covering firewood
  4. Storing firewood

Splitting Firewood

Picture a cylindrical piece of a tree trunk or branch that’s about 16” high and 12” in diameter. It’s not a good idea to try to burn this piece of wood. It’s too bulky, and it may contain too much moisture. It’s best to split that wood into four (or more) pieces to help it dry out quickly and burn more easily.

In most cases, when you buy firewood it will already be split. But whether you’re buying wood or you plan to split it yourself, you should be looking to wind up with logs that are about 12” to 16” long and about 4” to 6” in width.

Stacking Firewood

The best and most efficient way to organize your firewood is to stack it. But it’s not a good idea to stack firewood on the ground; it will gather moisture, which defeats the purpose of the seasoning process.

Instead, buy (or make) a log rack. They allow you to store firewood neatly, and the bottom of the rack rests a few inches above the ground, preventing moisture from finding its way to the logs you’re trying to season.

Covering Firewood

In order to season your firewood effectively, you need to cover it with either a sheet of metal, some plywood, or a tarp to protect it from rain or snow. This may sound counterintuitive, but it’s best only to cover the top of the logs, leaving the sides uncovered.

Covering the sides of the logs on your rack may actually cause moisture build up on them, slowing the seasoning process. Instead, leave the sides open to promote airflow to your firewood. Also, be sure to angle your cover in such a way that allows precipitation to run off and fall to the ground (instead of pooling on top of the cover).

Storing Firewood

After you’ve split, stacked, and covered your wood, the rest is simply a waiting game. Over time, the moisture will evaporate from the wood, bringing you below the 20% threshold. Then your firewood will be ready to burn.

Generally, there are two types of wood you can season and burn in a furnace: softwoods (like cedar, pine, spruce, or juniper) and hardwoods (like oak, hickory, maple, or birch). The length of time it takes to season each type of wood varies:

  • Softwoods need to be stored for at least 6 months
  • Hardwoods need to be stored for 12 months or more

Plan accordingly and start seasoning your winter firewood in the spring. The longer the wood is allowed to season, the more cleanly and efficiently it will burn.

How to Start a Fire in Your Wood Furnace

Close-up of a smoldering fire that's starting to die down

After you have a good supply of well-seasoned firewood whose moisture content is under 20%, you’ll be ready to fire up your wood burning furnace.

If you’ve ever made a campfire, the process of starting a fire in a furnace is similar. Start with some kindling (e.g., dry twigs, newspaper, etc.) and get a small fire going. Starting small allows your furnace’s flue to warm up, which promotes a good draft and helps to alleviate creosote buildup.

As the kindling burns, add some medium-sized pieces of seasoned wood to get the fire growing bigger. It’s helpful to keep the furnace’s door open to allow in a bit of air to get the fire going (or if your furnace has a draft blower, you can turn that on to promote airflow). If you do open the door to allow air in, do not leave the open door unattended.

Keep adding bigger and bigger pieces of seasoned wood until the fire takes hold and encompasses the furnace’s firebox.

How to Maintain a Fire in a Wood Furnace

Close-up of a healthily burning, roaring fire

After you get a fire going and the wood fuel burns up, the fire will inevitably diminish (a process that depends on the size of your furnace’s firebox, the type of wood you use, how well-seasoned it is, how much air is getting to the fire, and other factors). As the flames begin to dim, it’s important to stoke them to make sure the furnace continues to put off heat.

To start with, when adding more wood to the fire, open your furnace door slowly to prevent smoke from spilling out. From there, use appropriate hearth tools (like a shovel or fire poker) to spread the coals around.

Don’t put new firewood directly on top of the coals. Instead, push the coals to the perimeter of the firebox and put new wood in the middle. Doing this will promote better airflow and speed up the combustion process. Put the split end of the firewood (i.e., the end opposite the bark) toward the coals. It will catch fire more easily.

Be sure not to overload the firebox with firewood. If there’s too much wood, the fire will smolder, provide less heat, and create too much smoke due to a lack of airflow. Instead, spread logs out, allowing the oxygen coming into the furnace to move freely and feed the fire. If your fire needs a boost, turn your draft blower on to supply it with a bit more air.

How to Remove Ashes from Your Wood Furnace

A bucket full of ashes from an extinguished fire with a small black shovel in it

Wood burning furnaces come with two doors on the front of them — a larger door that leads to the firebox, and a smaller door that leads to the ash pan. Firewood burned in the firebox disintegrates into ashes, which fall through the grate and into the ash pan. After using your furnace for a while, this ash pan will fill up and need to be cleaned.

Wait for your fire to simmer down a bit and open the door to the ash pan. Scoop the ashes out and dump them into a metal container away from flammable material. After closing and sealing the ash pan door, allow the ashes in the metal container to cool off. Be absolutely certain the ashes have cooled off completely before you dispose of them!

Why You Should Follow Good Burn Practices

Wood burning furnaces are complex machines. Like engines, their designs are intricate and intentional, and they rely on a combination of engineering, chemistry, and physics to work their magic. Also like an engine, improper operation of a furnace poses a very real risk of injury or damage to property.

Not following good burn practices can lead to excess smoke in your home, the buildup of ultra-flammable creosote in your chimney flue, or even something as simple (yet dangerous) as a hot coal falling out of the furnace door and starting a fire.

It’s not hard to prevent these dangers, either. Use seasoned wood, be cautious when your furnace’s door is open, and dispose of ashes properly. If you follow good burn practices, you’ll be able to heat your home while keeping everyone in it safe.

Louis Greubel

Louis earned a bachelor's degree in English with a focus in rhetoric and composition from St. Louis University in 2017. He has worked in marketing as a content writer for over 5 years. Currently, he oversees the HY-C Learning Center, helping HY-C subject matter experts to share their decades of home solution products experience with homeowners and sales partners across the country.