Why Do Logs Hiss When Burning?

Does your firewood make a weird hissing noise when you burn it? It’s a common phenomenon when burning freshly cut wood, but what causes it? Why do logs hiss when burning them?

Hiss sounds in burning wood are a telltale sign that your firewood has a high moisture or sap content. If you don’t give your wood enough time to season it will make a wide range of noises as the water burns out of the wood and releases into the air. Make sure your firewood is covered, up off the ground, and has plenty of airflow to speed up the seasoning process.

If your firewood is hissing at you then you definitely need to change how you’re handling the seasoning process. Burning unseasoned wood can lead to a wide number of issues. Fires will be hard to light, smolder at a low temperature and give off lots of smoke.

In the rest of this article I’ll describe how to get a handle on your high moisture level and teach you a few easy ways to speed up the seasoning process and dry up your firewood fast. You’ll end up with a far better burning experience and fires that light so much easier. By the end of this post your firewood should never hiss at you again.

Why Does Firewood Hiss? (I Apologize For The Science Talk)

I will start off with the simple answer before I get into all the science talk. Firewood will hiss because there’s too much moisture in the wood for it to effectively burn. That moisture burns off on the inside of the log and gets trapped in steam pockets/bubbles.

Once those pockets burst the wood will pop, crackle or hiss depending on how fast the pressure is released. Now let me get into a little bit of the science behind combustion. You might want to skip the next section because it’s kind of dry.

Moisture Causes Incomplete Combustion

During the burning process firewood goes through a chemical reaction known as combustion. Combustion occurs as the wood material heats up and is converted into heat. When a fire has poor combustion, caused by excess moisture, only a small amount of the matter is converted into heat and all the rest is released into the air.

Most of that failed combustion particulate matter will release as black smoke, but you’ll occasionally a little bit of noise caused by gas trapped under the wood surface. As the gas bubble bursts you’ll usually hear a crackle or pop, but sometimes you end up with a slow leak.

That’s when you hear the hiss as waters burned off and the gas is slowly released through the leak. Smaller leaks will produces a long hiss and larger holes make a crackle or pop. The hiss is kind of like a steam kettle making noise on the stove as it releases steam.

So why are we experiencing incomplete combustion? In this case the incomplete combustion is caused by having way too much moisture in the wood. Freshly cut wood or wood that’s heavily saturated in water will restrict the combustion process.

The outside of the wood will be burning hot and the inside will have steam pockets that form as the water boils off. Steam will remain trapped until the outside surfaces burns down and gets weak enough for the steam to escape.

How Can I Tell If There’s Enough Moisture in My Wood For It To Hiss?

Every piece of wood starts with a lot of moisture in it. How much moisture each piece of wood starts with will depend on the type of wood and the humidity level of its surroundings.

Most pieces of green freshly cut wood will start off with a moisture level in the 35-45+ percent range. Heavily saturated wood might even be higher. The optimal burn range is about 10-15 percent so it needs to shed a lot of water during the seasoning process.

You can actually buy a cheap moisture meter to test the water content in your firewood. They’re really not all that expensive so you might want to check them out. So why does a high moisture level matter?

You will have a very hard time burning unseasoned firewood. It’s really hard to light, it smolders at a low temperature, and it will release loads of black smoke. That’s because it takes way more energy to boil water than to burn wood. You end up with incomplete combustion and all kinds of black smoke as the lightweight particulate matter gets released into the air.

Your Fire Will Be Terrible

Wet wood results in a fire that’s hard to light, impossible to keep going, and will smoke out everybody in a 30 ft range. Throw in a few cracks and pops that throw hot sparks at your guest. I’ve been in this situation so many times in my life, and it’s not what I would consider to be a relaxing night.

The sad thing is all of those problems can easily be avoided. All you need to do is properly season your firewood. It’s a really easy process that I’ll go over farther down in this post.

Watch Out For Sappy Wood

Another reason why firewood might hiss is because of excessive sap content. Sap is usually found in soft woods like pine, cedar, etc. Extremely high sap content results in the same problem as you have with wet wood.

The outside burns faster than the inside and the sap gets trapped in little bubbles. As the sap burns off and those bubbles pop you may end up with a hissing noise.

Hardwoods Have Less Water

Hardwood varieties are less likely to snap, crackle and hiss than softwoods. There are a few reasons for this, but the biggest difference is hardwoods have a lower moisture content. They won’t produce as many steam pockets and unseasoned wood will burn way cleaner.

There will also be less resin, sap, and tar in the wood. These organic substances are known to heat up and seal off sections inside the wood creating gas chambers. If there’s any water back behind the seal it will cause large bursts and you’ll probably end up with red hot wood jumping out at you.

Bigger Fires Will Improve Combustion

Don’t just toss a couple logs in the fire if you’re using green wet wood. You want to use a bunch of wood, bigger pieces work better so the fire can really get going. You might need to use propane torch (Flame King Torch) or small benzomatic torch (if you’re inside) to start a fire with wet wood.

The bigger and hotter your fire is the faster the water will burn off. Generally speaking, bigger fires will be less likely to make noise, because they’re hot enough to completely burn the wood/water. More heat means better combustion, less smoke, and better longer lasting fires.

The Fastest Way To Season Firewood

If you’ve determined that the hissing noise is caused by wet wood then you’ll have to season it. Just so you know, firewood can’t be seasoned indoors. So don’t even bother trying to dry firewood in the garage or basement. There’s not enough heat to dry out the sun. You need lots of sunshine and a nice summer breeze to speed up the seasoning process.

So how do you season firewood? As the name implies, you have to wait a season for the firewood to dry. It should take about 6 months of summer sun to get your moisture level down to the optimal 10-15 percent range.

Seasoning wood is surprisingly easy once you understand a few simple rules. Firewood needs to be stacked outside off the ground, cover it up, leaves the sides exposed to promote airflow, and it should get lots of sun. You’re firewood will be seasoned in no time if you can supply all those things.

I recommend either buying or building a firewood rack to store your wood on. I’m not a carpenter so I like to use the firewood rack brackets that you use with pressure treated 2x4s. You can screw together a rack in under 5 minutes and I like that you can replace the boards after they start to look ugly in 5-10 years.

You can buy a commercial firewood cover, but most people use a tarp. I bought the REDCAMP firewood cover a few years ago for my cabin and I really like it. It’s still holding up strong and it’s so much easier to use than a tarp. Just unzip the sides, grab your wood and zip it back up. You don’t have to mess around with bungie cords and ropes that hold the tarp down.

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