Fireplace Grate

How Fireplaces Work

In Indoor Fireplaces, Wood Burning Fireplaces by James O'KellyLeave a Comment

Open fireplaces have been integrated into homes across the world for hundreds of years for heating and cooking purposes.

Fireplaces don’t have any mechanical, electrical or moving parts, and are designed to allow you to have a fire in your home safely and efficiently.

So how do fireplaces work?

Fireplaces work by creating a non-combustible open environment where a fire can be built, started and maintained to provide heat and aesthetic appeal to a home. A fireplace works in tandem with a chimney to provide a passageway for byproducts from a fire to leave a home safely.

This guide explains in detail how traditional open fireplaces work. For our complete guide on how wood burning stoves work click here.

How Fireplaces Work

A traditional open fireplace has two main components:

  • The fireplace
  • The chimney

Open fireplaces are designed into the structure of a building, meaning that they can be expensive to retrofit.

Open fireplaces also work in conjunction with chimneys to provide an environment where fires can be safely started and operated in a home.

The diagram below shows how an open fireplace in connected to the chimney.

Fireplace & Chimney Diagram
A typical fireplace and chimney

The labeled images below show what each component of a typical open fireplace can be referred to as.

Fireplace Labeled Diagram
Our open fireplace in our living room
Fireplace Insert Labelled Diagram
Our open fireplace in our kitchen with a gas fireplace insert

For our complete guide to each component of an open fireplace and what their purposes are click here.

Open fireplaces are typically made from non-combustible masonry materials such as brick, concrete and stone. Our living room fireplace is made from bricks within the firebox, and has a concrete hearth and surround.

Firebox Labeled Diagram

Where the fireplace meets the room is known as the fireplace opening. The fireplace opening allows the user to build and light a fire from the safety of the room. The opening also allows heat from the fire to be radiated into the room.

A fire can be built on the platform located at the base of the fireplace, known as the hearth. The hearth needs to be made from a non-combustible material to withstand the heat from a fire. For more information click here for our in-depth guide on fireplace hearths.

For safety reasons, the hearth sticks out from the fireplace into the room to help prevent any hot ashes or embers from interacting with the floor of the room.

To help with air supply and holding the fuel in place, many fireplaces can be found with a grate to allow the fire to be built above the hearth.

Fireplace Grate
The grate that was located in our living room fireplace when we bought the house

A fire needs both fuel and oxygen to survive. Common fuels to be used within open fireplaces include wood and coal, while the air supply to the fire must be fed from the room the fireplace is heating.

Oxygen within the room may not be enough to sustain the fire for it’s entire duration, and so many rooms with open fireplaces will have external air vents to ensure that the fire is always fed with a constant supply of oxygen.

Our living room has an external air vent that we open before having any fire in our open fireplace. The vent can also be closed after a fire to prevent cold air from entering the house when the fireplace isn’t in use.

Fireplace Vent
The external air vent located in the same room as our open fireplace that helps to supply the fire with a constant supply of fresh air

An air vent can help prevent an open fireplace from drawing warm air from the rest of a home, which in some cases can cause the house to become colder as a result of a fire.

A lack of air from getting to the fire can cause it to burn the wood inefficiently, leading to more smoke and more creosote (tar) being produced as a result of incomplete combustion. Cracking open a window within the room can also help if an open fireplace fire is smoldering due to a lack of sufficient air supply.

Newer homes are generally built to much higher standards and so this can cause problems with air supply to a fireplace due to the air tightness of a home. Opening an external air vent or window can become more important in these situations.

How A Chimney Works

As a fire in an open fireplace can create harmful gases and smoke, and in potentially greater proportions when the fire is burning poorly, it’s important for a fireplace to be able to vent these from your home.

A chimney provides the required route for byproducts from a fire to safely leave your home. The chimney will be located just above the fireplace, and will rise vertically, or near vertically through the roof of your home.

At the top of every fireplace firebox will be the throat of the chimney, where the fireplace meets the chimney. The images below show what our chimney looks like from inside our open fireplace.

The throat of our chimney located just above the open fireplace
Chimney From Fireplace
Looking up the chimney from the fireplace

Chimneys work on the simple concept that hot air rises and cold air falls. Hot air from a fire will rise up the chimney and out of your home.

The rising hot air will create a pressure difference within the fireplace known as the ‘draft’ (or ‘draught’ in British English).

The draft is essentially the word used to describe the pulling power of the chimney.

This pressure difference also in turn sucks air from the room into the fireplace. This cycle allows the fire to burn continuously as long as there is sufficient air supply from within the room, and enough fuel to burn.

Fireplace & Chimney Airflow
As hot air (red arrows) from the fire rise up the chimney, colder air (green arrows) is sucked into the fireplace from the room

The greater the pressure difference the stronger the draft.

The two main factors that can influence how much draft is produced by a chimney can be:

  • The temperature of the air being generated by the fire. The hotter the air is compared to the outside, the greater the pressure difference and therefore the greater the draft.
  • The height of the chimney. Typically, the higher the chimney the greater the draft, but other factors do play a role.

As the majority of homes are designed and constructed differently, there will always be different combinations of open fireplace and chimney sizes. Both the fireplace and chimney must work together to be able to provide enough draft to remove byproducts from a fire in home.

There are a number of structural things that can influence how well a chimney draws on a fireplace, including:

  • The size of the fireplace firebox
  • The size of the fireplace opening
  • The internal diameter of the chimney
  • The height of the chimney
  • How far the chimney extends above the roofline.

A draft helps to prevent smoke and gases from a fire from pouring out into your home. The arrangement of the chimney and fireplaces can dictate how well the chimney draws on the fireplace.

There are also a few other things that can affect the draft, including:

  • Whether the chimney is dirty or blocked
  • The weather conditions

Burning wood can release creosote, a black tar-like substance that can line the internal walls of a chimney. Through build up of creosote over time, the chimney can become less efficient at providing a draft on the fireplace.

It’s therefore important that a chimney is swept at least once per year to ensure that it’s safe to have a fire in an open fireplace.

Windy weather can cause a backdraft down the chimney, which can affect how well you can maintain a fire in your home during these conditions.

If you want to lean more the Chimney Safety Institute of America provides more information on how a chimney works.

Having A Fire

A fireplace needs a chimney to be able to draw byproducts from a fire out of a home, while also drawing more air into the fireplace from the room in order to feed the fire. As a fire needs oxygen to survive, this pulling effect created by a fireplace and chimney helps to feed the fire with a continuous supply of air.

The other requirement for having a fire in an open fireplace is fuel.

Wood can be the most common source of fuel for an open fireplace fire because of its abundance of supply, ability to gather from your own property or land, relatively cheap price when buying and the ability to give off lots of heat when burnt.

Much like how a chimney needs to provide sufficient draft to remove waste gases and pull fresh oxygen into the fire, a fire also needs a source of fuel that is in a good enough condition to burn efficiently.

Wood that is freshly cut (and therefore high in moisture content) will not burn effectively in a fire.

Wet wood therefore needs to be dried out through a process known as seasoning before it can be used as firewood. The time it takes for logs to dry out can be influenced by the time of year it was cut and the type of tree it was cut from.

The seasoning process. It’s important to ensure that wood is dry enough to be used as firewood so that it burns efficiently

Wood from coniferous trees, such as Pine, is known as softwood, and typically takes a short amount of time to grow than hardwood trees. As a result the wood is generally less dense than hardwoods and so the seasoning process typically doesn’t take as long. The drying out process for softwoods can typically take anywhere from 6 to 12 months.

Wood from deciduous trees, such as Oak, is known as hardwood, and is generally denser than softwoods because they can take a longer time to grow. Hardwood logs therefore can take a longer time to dry out compared to softwoods, taking typically 1-2 years.

The time of year the wood is cut will also dictate how long it takes for it to season. Wood cut during the winter months will typically have a lower moisture content than wood cut in the spring, and so may not take as long to dry out.

Wood can also be kiln dried to vastly increase the seasoning process.

Whichever type of wood is burnt, to allow a fireplace to work as efficiently as possible the wood must be dry enough. It’s recommended that firewood should have no greater than 20% moisture content. You can use a moisture meter to accurately read the moisture content of any logs before being used on a fire.

Dry firewood, along with a clean chimney producing enough draft, and a good air supply from the room will give a fire what is needs to burn effectively and to produce the most amount of heat.

It’s very hard to get a log to catch alight with simply a naked flame. Fires in open fireplaces are therefore built up to allow heat to be transferred to larger pieces of wood more easily.

Fires are typically built up with some sort of high combustible material, such as newspaper or firelighters, at the base of the fire, while small bits of wood known as kindling are laid on top. Finally, smaller sized logs can be added on top of the kindling to complete the fire. Lighting the newspaper or firelighters helps to transfer the flames to the kindling, which in turn causes the logs to start burning with more ease.

Burning wood that is too wet will hinder the whole process, as the wood will be harder to light and harder to burn. More energy is required by the fire to burn off any excess moisture content before it can start burning the wood effectively.

An inefficiently burning fire can lead to incomplete combustion of the wood, which in turn can cause the fire to smolder and more smoke and creosote to be produced.

Ash from a wood fire will fall through the fireplace grate (if one is being used) onto the hearth into an ash pan. Many fireplaces have an ash dump located beneath the firebox to allow ash to fall through and collect.

Ensuring that using wood that is dry enough is therefore an important aspect of having a fire in an open fireplace. If you don’t have a moisture meter then look to get one for your fireplace as it’s an essential item.

You can find our current favorite moisture meter over on Amazon here.

Upgrading A Fireplace

Open fireplaces aren’t known for their effectiveness at heating a room or home, and so many are upgraded to improve the heat output. Options for upgrading a fireplace include:

  • Wood burning fireplace inserts
  • Gas fireplaces inserts
  • Electric fireplace inserts
  • Wood burning stoves
  • Multi fuel stoves

Wood burning fireplace, gas and electric inserts can all be installed within the entire opening of a fireplace.

Wood burning stoves and multi fuel stoves can be installed within existing open fireplaces, but require the chimney to be lined with a flue for safety reasons. Wood stoves don’t take up all of the space within an open fireplace, and examples are shown below:

Our wood burning stove in an open fireplace
Wood Burning Stove
Our multi fuel stove in an open fireplace

Further Reading

Parts Of A Fireplace Explained (With Labeled Pictures & Diagrams)

How To Get More Heat From Your Fireplace

Reasons Why Your Fireplace Is Smoking

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