There are conflicting opinions when it comes to closing glass doors on fireplaces. Some say they should be closed to retain heat, while others say if you close them the glass can shatter. I wanted to dive into the details behind this and figure out which one is the real answer. Here’s what I found.
Most fireplaces (especially prefab fireplaces) are made with tempered glass, which is less heat resistant and should be open when a fire is burning. Other fireplaces (such as masonry fireplaces) are made with ceramic glass doors which have a higher temperature threshold and can be closed while a fire is burning.
So why do most fireplaces have tempered glass, and what happens when you close your fireplace’s doors? Let’s take a closer look.
Need a fire starter for your fireplace? Check out these top-rated fire starters on Amazon.
Generally, Open Tempered Doors and Close Ceramic Doors
Tempered Glass Doors
The most common type of fireplace glass door is tempered glass. Tempered glass is strengthened against physical and heat damage, making it heat resistant to about 400-500ºF.
Just about every prefab fireplace is made with tempered glass.
Prefabricated fireplaces are fireplaces that are assembled in a factory and later installed in a home. On the other hand, masonry fireplaces are built into the home itself.
When glass manufacturers create tempered glass, they heat the glass and then quickly cool it down. This makes it stronger than non-tempered types of glass, also known as annealed or untreated glass.
The tempering process causes the outer surfaces of the glass to compress and the inner surfaces of the glass to tense. Aside from increasing temperature and break resistance, this glass will also break into smaller pieces that stick together, instead of large sharp shards that could cause injury.
All of the fireplaces sold at home improvement stores and on Amazon have tempered glass and need to have the doors open while a fire is burning.
Tempered glass is the right choice if your fire stays below 400-500ºF. However, anything above that likely causes too much thermal expansion.
Most tempered glass doors have a thickness of 1/8″ to 3/16″, so it’ll expand in its metal frame and shatter from excess heat (usually above 500ºF).
However, some manufacturers offer glass doors with a thickness of 1/4″, making them fully tempered safety glass that can be used when closed. Still, it’s not recommended with a large fire.
Ceramic Glass Doors
Another, less common form of glass for fireplaces and wood stoves is ceramic glass.
Ceramic glass looks like glass, but it’s actually a hardened ceramic material that is transparent.
This is not “glass” in the traditional sense, but rather is a composite of various oxide crystals in a silicon glass matrix.
Ceramic glass is significantly stronger and harder than tempered glass. Due to the closer proximity that wood stove fires have to their doors, and their much higher heat, ceramic glass is commonly used on these appliances.
Aside from strength and durability, the main difference between ceramic and tempered glass is that ceramic glass will break into chunks while tempered glass shatters.
If you’re not sure which type of glass your fireplace has, it’s safe to assume it’s tempered glass. Check your fireplace’s manual for more information about its glass doors and safety information.
What Happens When You Close Fireplace Glass Doors?
Many gas fireplaces have a single sheet of glass that’s not able to be opened (see above). This has several advantages including safety, cleaner burning, and preventing fumes from entering the home.
This is common for small-to-medium gas fireplaces that don’t generate as much heat, or gas fireplaces that have thicker and stronger tempered glass.
Recommended: 5 Reasons Gas Fireplaces Won’t Light (& How to Fix It)
Other gas fireplaces have glass doors that open, allowing access to the flame. The glass doors should be open at all times as they often get hot enough to impact the integrity of the tempered glass, causing it to shatter.
Generally, if your gas fireplace has glass doors, they should always be left open when a fire is burning.
There are two primary types of wood-burning fireplaces. Those that are prefabricated (made in factories), and those that are built into the home (masonry).
Prefab wood-burning fireplaces are almost always made with tempered glass.
Masonry wood-burning fireplaces are commonly built as open fireplaces (without any glass) or have glass doors that are either tempered or ceramic.
Prefab fireplaces generally last about 20-30 years while masonry fireplaces can last well over 100 years.
The best way to tell which type of glass doors your fireplace has is by checking your fireplace’s manual or checking with the glass door’s manufacturer.
If you find your fireplace’s glass doors can be closed while a fire is burning, it has some advantages such as protection against jumping embers, more controlled combustion, and reduction of heat loss.
As a result of better combustion and heat, firewood lasts much longer and has a cleaner burn (less smoke and creosote buildup).
Closing fireplace glass doors also solves the problem of chimneys creating a draft in your home.
Drafts occur from open fireplaces burning the oxygen inside the home. This sucks in the outside cold air from the spaces between windows and doors and sends the fireplace’s warm air up the chimney.
Drafts make open, wood-burning fireplaces extremely inefficient and can even make your house colder than it would be without using the fireplace.
Closing fireplace glass doors while the fire is burning prevents drafts from happening. But again, this is usually only possible with ceramic glass doors, or other doors that can be closed with a fire burning.
If you aren’t sure what kind of fireplace glass doors you have, leave your fireplace’s doors open to be on the safe side.
A yearly inspection and cleaning are recommended for all types of fireplaces and their components, including glass doors and the chimney or vent.
If you have a prefab fireplace, keep the doors open while a fire is burning.
On the other hand, if you can confirm your fireplace’s glass doors are ceramic (and not tempered), then you should be fine closing them while burning. Still, check your fireplace’s manual to double-check if it’s okay to do so.
The manual should specifically say if this is recommended or not for your model.
When in doubt, keep your fireplace’s doors open while burning a fire. This helps prevent issues such as the fireplace’s glass doors breaking from overheating.
If you have an open, wood-burning fireplace, and you have an issue with drafts and you don’t have fireplace doors you can close, consider switching to a gas fireplace.
Most people I talk to who switched don’t miss the mess and smoke of firewood and like the ability to start and stop a fire with the flick of a switch. Not to mention the boost in how efficiently the room or house is heated.
To learn more about how chimney drafts work, check out this helpful post by Action Chimneys.