Wood Stove Fire

Why Does Firewood Hiss?

In Firewood by James O'KellyLeave a Comment

When firewood burns it can produce a number of different noises depending on the properties of the wood. One of the noises you may sometimes hear from a fire can be a hiss.

Firewood doesn’t always make hissing sounds when burning, and so why does firewood hiss?

Hiss sounds from burning firewood is a sign that the wood is too high in moisture or sap content. Unseasoned firewood that is still too wet to burn efficiently can make hissing noises as the excess moisture within the wood is burnt off.

Burning unseasoned wood can lead to a number of issues with your fires and fireplace, both in the short term and in the long term, and so It’s important to understand why your firewood is making hissing noises.

We’ve discussed in more detail below why your firewood would hiss, what the issues are with hissing fires and how you can remove the hissing from your fires for a better burning experience.

Why Does Firewood Hiss?

All wood has a certain amount of moisture contained within it, and the amount of moisture naturally contained within trees at any particular time can depend on the type of wood and time of year.

Once cut, unseasoned ‘green’ wood will be naturally high in moisture.

Green Wood Moisture Content
Unseasoned wood can produce hissing noises when used in fires because of the high moisture content

As a result of high moisture content in unseasoned firewood it would struggle to burn on a fire.

Before high moisture content wood can be properly combusted, more energy is required by a fire to burn off the excess moisture.

When wood can’t be combusted effectively the fire can struggle to burn the wood, leading to a fire that is hard to get going and hard to keep going. More smoke can also be produced as a result of incomplete combustion of the wood.

Another byproduct of burning wood that is too wet can be hissing noises coming from the fire.

Firewood can produce a hiss noise as the excess moisture within the wood heats up and tries to escape. The higher the moisture content of the wood the more the firewood can hiss when used in a fire.

Another reason why firewood may hiss can be a result of high sap content within the wood.

Sap content can be particular high in softwoods such Pine, Cedar and Douglas Fir.

Sap contained within wood can lead to your firewood bubbling when used on a fire, creating hissing noises as it oozes out the ends of the logs.

If firewood has got wet, such as being left in the rain, then it may also produce hissing noises if the moisture hasn’t completely dried out.

If already seasoned firewood gets wet then the wood should be left out to dry for up to a week in an area that is sheltered from the wet weather, but also open to allow the air to circulate.

Don’t use firewood that has got wet as it can act in the same way as unseasoned firewood and struggle to burn in your fires.

Understanding Whether Your Firewood Will Hiss

Before burning a new batch of firewood there are a few things you can do to check whether your firewood has been fully seasoned, and therefore dry enough to burn without any issues.

This will help you to understand whether your firewood is still too wet or has been properly seasoned to a point where the moisture content is low enough that it won’t hiss when used on your fires.

Unseasoned firewood will be higher in moisture content and can therefore produce more hissing sounds when burnt.

Typical characteristics of unseasoned firewood include:

  • A greenish tint to it.
  • Visible moisture, most commonly found under the bark.
  • The bark or wood will peel away in strands.
Green Wood
An example of green unseasoned wood

Seasoned firewood will be lower in moisture content than unseasoned firewood and will typically produce fewer hiss noises throughout a fire.

Typically characteristics of seasoned firewood include:

  • Lighter in weight.
  • No visible green color in the wood.
  • Weathered looking or course.
  • Can be splitting or cracking at the ends.
  • The bark can be more easily removed and in bigger pieces.
Seasoned Firewood
An example of a properly seasoned log that won’t hiss on fires

The best way to check the moisture content, and therefore understand how your firewood will react when burnt, is to use a moisture meter.

A moisture meter is small and inexpensive (but essential) tool that allows you to see the exact moisture content of your firewood.

Moisture Meter
A moisture meter helps you to identify how your wood will burn

A moisture meter provides a moisture reading of your firewood as a percentage. Simply pushing the prongs into the wood gives a reading.

The Environmental Protection Agency (EPA) states that firewood with a moisture content of between 15% and 20% will burn the most efficiently. Within this moisture range the firewood will have a low enough moisture content that it doesn’t hiss and struggle to burn, but also high enough that it doesn’t burn too quickly to be an efficient heat source.

If your firewood is still too wet it will have a reading of over 20%. Firewood over 20% moisture will therefore burn ineffectively in a fire and can produce hissing sounds as the excess moisture is burnt off.

As the moisture content of firewood increases above 20% it becomes progressively harder to catch fire and burn. You can also expect to hear an increasing number of hissing noises as the fire attempts to burn off the excess moisture.

As hissing noises are a sign of a poorly burning fire you should therefore look to ensure that you’re always burning properly seasoned or kiln dried firewood with less than 20% moisture content.

Under 20%, a fire will be able to combust firewood much more effectively, without needing to use extra energy to burn off the excess moisture within the wood.

Moisture Firewood
Low moisture firewood is less likely to hiss on your fires

Moisture meters are an important tool for any fireplace or stove and help you to understand how well and hot your fires will burn before they’re even started

See our list of recommended moisture meters here.

Should Seasoned Firewood Hiss?

If firewood has been properly seasoned to below 20% moisture content it shouldn’t hiss when used on a fire.

Firewood can sometimes be miss-sold as ‘seasoned’ when in reality is hasn’t been ‘fully seasoned’ down to recommended moisture levels.

Firewood with a moisture level of below 20% is generally recognized as being properly seasoned firewood.

If you’re buying in your firewood use a moisture meter to check the actual moisture content of the wood before taking it home or when it’s delivered.

If you want to be burning firewood that doesn’t struggle to catch fire, struggle to burn, produce more smoke and make hissing noises as a result of excess moisture within the wood, then it’s important to ensure that you’re only burning low moisture content firewood.

Properly seasoned firewood is therefore the perfect choice for your fires.

If you’re struggling to buy properly seasoned firewood then look to get kiln dried firewood, which can be more expensive to buy but can typically have much lower moisture levels that what is possible through air drying firewood.

Further Reading

How To Season Firewood

A Complete Guide To Seasoned Firewood

A Complete Guide To Kiln Dried Firewood

Why Does Firewood Pop And Crackle?

What Is A Cord Of Firewood?

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