What is Creosote?

Creosote is a flammable and corrosive substance that can build up on the walls of your fireplace and chimney. Creosote is formed when unburned wood particles, fly ash and other volatile gases combine as they exit the chimney. If there’s a poor draft, these unburned particles and gases can condense and build up on the walls of your chimney. Over time, these creosote or soot buildups can become a danger as they can potentially result in a chimney fire.

Causes of Chimney Creosote Problems

Smoke, or better termed, flue gas, is released by the initial fire, or primary combustion. Burning wood, no matter how it is done, releases pollutants in the form of gases and particulate matter. Flue gas, comprised of steam and vaporized but unburned carbon based byproducts (vaporized creosote). If smoke exiting the chimney is cooled to below 250 degrees Fahrenheit, the gases liquefy, combine, and solidify to form creosote. A flue too large for the woodburning appliance may increase the likelihood that creosote will build up. Restricted air supply, unseasoned or rain-logged wood, and cool surface flue temperatures also encourage creosote to build up.

Creosote Stages:

1. Condensation: In an attempt to save fuel, many people reduce the amount of air entering the combustion chamber, causing the appliance to smolder. This has serious side effects. Smoke then cools as it rises in the chimney, not having enough heat energy to escape the stack. The flue gas then condenses on the inside of the chimney and the sap/resin turns into creosote.
2. Liquidation: This sap/resin turns liquid and can seep into mortar joints or cracks in the flue tiles. The corrosive property held by creosote destroys masonry work and jeopardizes the life of your chimney.
3. Solidification: As temperatures fall and rise within the chimney, causing the heating and cooling of the creosote. This quickly enables build-up, reducing the amount of space the flue gas has to exit the chimney and, in extreme cases, blocking the flue completely. This build up can look like tar and is frequently termed “tar build-up.”
4. Friable State Creosote: When the solid creosote is burned, all volatile oils are removed leaving a residue that appears very similar to honeycomb which is crisp and easy to sweep from chimneys.

Creosote Glazing

Glaze is formed in the solidification stage and occurs when new layers of creosote are being added so quickly that the layers below it have no time to dry. These fresh layers then insulate previous deposits so it eventually solidifies creating the rock-like substance known as glaze.

How to Remove Creosote Glaze

To remove this hard-as-a-rock glaze, it must first be broken down chemically. The two best products to use are Anti-Creo-Soot (ACS) Liquid spray or ACS Powder. ACS in liquid form is a chimney creosote removal spray that can be applied directly onto the fire or used to pre-treat the wood before burning. The vapor goes up the flue and attaches to the glazed creosote and chemically modifies it, converting it into a harmless ash that can easily be swept out with a chimney sweep brush. ACS in powder form is a little bit more powerful. This powdered chimney creosote remover is applied to the chimney walls and up the flue. When you start a fire, the temperature needs to reach 300 degrees F. Once this happens, the ACS Powder causes the creosote to expand and contract at a different rate than the flue it’s attached to, causing it to peel away from the flue liner. Both of these products contain unique chemical catalysts that breaks down the creosote over time. They are the most powerful when used in combination. Use the ACS Powder for the first 2 weeks to break down the really heavy duty creosote. Then use the regular ACS liquid spray every time you have a fire. Give it 5-6 sprays each fire to reduce creosote buildup and keep your chimney creosote-free.

After you solve your creosote problem, it’s a good idea to check the rest of your chimney and make sure it’s in good shape. Keeping your bricks waterproofed and performing chimney crown repair if needed are essential steps to take to keep your chimney in proper repair.

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