Black glass can happen when burning a wood heater, fireplace or inbuilt. By properly burning your unit, balancing heat, efficiency, and burn time throughout the day, you will be able to avoid most of the creosote buildup. If your wood fireplace is getting black glass from creosote buildup every day or very frequently, you need to look at what is missing from the combustion triangle - heat, air, or quality seasoned fuel.
Below are some of the most common issues, how to fix it, and an explanation of the airwash system that is designed to keep your glass clean. Refer to your owner’s manual for proper burning instructions.
Pro Tip: Every time you load your unit let it run with the air damper fully open for at least 20 minutes, this will go a long way to cleaning your glass and ensuring your unit is well primed to burn the rest of the day efficiently.
Black Glass on a Wood Fireplace, Freestanding or Inbuilt
There are several reasons why your glass may be black on your wood-burning fireplace. If your wood has not been properly seasoned, it will allow creosote buildup; however, the most likely reason is the unit is not being burned correctly. Typical high-efficiency wood freestanding heaters, inbuilts, and fireplaces burn using either a two-stage or three-stage burn. When the fire is burning too cold, not allowing the secondary burn to engage or “go-to-work,” you will see a buildup of creosote on the glass.
To stop your fireplace from sooting up, you should ensure you are burning your fire hot enough to activate the secondary burns. Also, it is recommended to burn your fire hot (with a wide-open air damper) for at least 20 minutes during your reloading period and before dampering down the air to the desired level. This process will allow plenty of time for the secondary burn to initiate and start an efficient fire/coal bed.
To clean your glass, either leave the damper open while reloading to burn it clean, using the built-in airwash, or wait for your stove to cool, and use damp newspaper dipped in ash to quickly and easily remove some of the creosote buildup.
Pro Tip: Ensure the gaskets on the door are part of your annual maintenance. Over time and as the units are used, the door gasket will compress, potentially compromising the seal. The simple solution is to adjust your door handle per the manual, which will give you many years of leak-proof seals and ensure your air wash functions true to its name.
How Airwash Systems Work
In addition to the fire burning too cold, your airflow could be too restricted to allow for the glass to clean itself. An airwash system is designed to force air down the front of the glass; however, if the air is damper is closed or set to low, there will not be enough airflow inside the unit to stop any buildup. Again, a hot fire with a wide-open air control each time you load should help keep the glass clean. Do this, especially after your long all day or overnight burns.
Dark Glass on Catalytic Wood Fires
Catalytic and Hybrid Catalytic wood units with longer burn times, requiring less airflow, can have dark glass during the fire. Dark glass is different from black glass because dark glass is a lack of light being expelled by the fire as opposed to the buildup of creosote on the glass. Dark glass happens because the catalytic combustor works on lower temperature from the fire down below while increasing it above away from the glass and drawing the heat from the smoke and gases, out over a longer period. This is nothing to worry about and is a feature of this type of wood stove. If the Catalyst is disengaged and the airflow adjusted, the fire will quickly roar back to life, and the glass will no longer be dark.