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What Is a Fireplace Flue?

By Heather Phillips
Updated May 16, 2024
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A fireplace flue is the part of a fireplace system that connects the actual opening to the outside. It runs through the chimney, and can be made of masonry or metal. Often, one is made of round or square terracotta pipe stacks, or round metal pipe. The purpose of the flue is to vent smoke and gases from the fire to the outdoors.

If it is connected to a freestanding fireplace, a fireplace flue sometimes includes a chimney thimble. This is a sleeve of pipe, usually metal, that connects the piping coming out of the fireplace to the flue opening. It often has a damper mechanism, commonly a piece of metal that fits in or just below it, which can be opened or closed. The purpose of a damper is to prevent heat loss when a fireplace is not in use. Opening a damper when a fire is burning should allow smoke to travel up the chimney.

If smoke begins to fill the room when a fire is lit, instead of going up the fireplace flue, the damper should be checked, to make sure it is open. If it is, there are some other common causes of a fireplace not drafting properly. One that is easy to fix is a blocked flue, often due to a bird, hornet, or other animal's nest. Having a chimney sweep remove the nest alleviates the problem. Sometimes, a chimney cap is helpful in preventing animals from making their nests inside a flue.

Another situation that can create a smoke problem can be that the fireplace flue must be primed, or pre-heated. Sometimes, a cold flue will not draw smoke up. To fix this problem, a rolled up piece of newspaper can be lit and held as far up into the flue, past the damper, as possible. This may need to be done more than once to warm the air adequately. This whole process is known as priming.

Another potential problem in a fireplace flue is the build-up of creosote — a sticky tar-like substance — that can accumulate due to the incomplete burning of wood. It is important to have an often-used fireplace flue cleaned regularly, so that creosote does not collect on the inside of it. If this substance is not removed, it can ignite, causing a dangerous chimney fire.

Do Gas Fireplaces Have a Flue?

Gas fireplaces remain popular options for meeting households' heating needs. They may or may nor be a primary source of heat, depending on the home's energy usage patterns and other heating methods present like gas furnaces or ductless mini-split systems. Understanding your gas fireplace's operation is key to getting the most out of it and maintaining it for years to come. That includes knowing how it works, plus whether your type has or needs a flue.

How Gas Fireplaces Work

A gas fireplace incorporates combustion and an exhaust system. Typically, it uses a dedicated gas supply for fuel and an electrical ignition system to light that fuel. The ignition system may consist of a pilot, spark ignitor and thermocoupler. The pilot must light the primary burner flames for the gas fireplace to work.

Once the gas is ignited and the flames are burning, a gas fireplace starts producing heat. You can use its controls to change the flame size and heat output as needed. Often, these fireplaces include faux logs for ambiance and aesthetics. The fire continues thanks to an internal or external air supply that brings oxygen into the fireplace chamber. Natural b-vent and ventless systems use an internal air supply, while direct-vent fireplaces use external ones.

Vented Vs. Ventless Systems

It's easy to conclude that all gas fireplaces lack a flue. However, this conclusion is rather simplistic. Natural vent systems don't have their own flues, but they can use flues with existing chimneys to release waste air. On the other hand, direct-vent systems need a flue to discharge waste gasses. Ventless systems do not require a flue at all. With their fuel-burning efficiency, they don't require external ventilation.

How To Open a Fireplace Flue

It's always a good idea to know how to open your fireplace flue. You may need to open yours to improve fire burning efficiency, but you must also release smoke and carbon dioxide from the fire to the outdoor air. Most flues have a pretty simple setup, so it's easy to open and close once you know the technique.

Your flue's construction depends on the style of fireplace in your home. If you live in a more modern-style home, the flue may have a handle placed right above or just inside the opening. To open the flue damper, rotate the handle until it's in a vertical position. Rotating it back to the horizontal position closes the damper. If your fireplace has a stainless steel or cast iron damper, it may have been installed with a push-button control system. You'll just need to operate these controls according to the manufacturer's instructions.

Older fireplaces may have traditional throat dampers. These are controlled via a knob near the fireplace or a metal rod inside the firebox. In these cases, you'll turn the knob clockwise or push up the metal rod to open the damper. Pull the rod down or turn the knob counterclockwise to close the damper.

Some dampers are operated by metal chains installed right beside or inside their fireplaces. The metal chain is usually on a hook. These top-mounted dampers can be opened by taking the chain off the hook and letting it go. The chain will lift up as the flue damper opens. Simply pull the chain and secure it to the hook to close the damper.

How To Clean a Fireplace Flue

Cleaning a fireplace flue means cleaning the damper. You'll likely have to remove the damper to properly clean its surfaces. Fortunately, that's probably the hardest part of the job. Once you have the plate out of the chimney, cleaning it should be relatively easy. Of course, this depends on the amount of soot and ash buildup since it was last cleaned.

Before getting started, you should prep first. Sweep out your fireplace and throw the ashes away. Lay down several old newspapers to catch any soot or debris that loosens when you take the damper out. Follow your manufacturer's instructions to uninstall this part, then clean it using a few simple steps:

  • Use a wire brush to scrape any large debris off the surface. 
  • In a gallon bucket, mix 1 cup bleach and 1 gallon of warm water with 6 tablespoons of trisodium phosphate.
  • Dip your scrub brush into the mixture and gently scrub the front and back of the damper plate.
  • Once you've removed all visible debris, rinse the plate off in clean warm water. 
  • Dry the damper with a clean cloth before reinstalling it inside your chimney.
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