Gas Fireplace Insert

How Gas Fireplaces Work (Explained With Real Pictures)

In Gas Fireplaces by James O'KellyLeave a Comment

Gas fireplaces differ from all other types of fireplaces in that they burn gas as the source of fuel.

This makes the workings of a gas fireplace different to other forms of fireplace because gas can’t be burnt in the same way that wood or other forms of solid fuel can.

A gas fireplace works by:

  1. Using a dedicated gas supply provided to the fireplace to burn as fuel.
  2. Using an electrical supply for the ignition system to work (needed for many modern gas fireplaces that use electronic ignition system rather than standing pilot lights).
  3. Starting the pilot flame using an ignition system that typically consists of a pilot, a spark ignitor, and a thermocouple and/or thermopile.
  4. Using the pilot flame to light the main burner flames, which in turn are used to provide the main gas fireplace flame effects and the heat output.
  5. Allowing the size of the flames and therefore heat output from the fireplace to be adjusted using the controls.
  6. Providing a bed of faux fireplace media, such as logs or coals, to enhance the fire experience.
  7. Using an internal air supply (for natural b-vent and ventless gas fireplaces) or an external air supply (for direct vent types) to feed oxygen to the flames.
  8. Venting waste air externally (for natural and direct vent gas fireplaces) or burning the fuel efficiently enough to not need any form of external ventilation (for ventless gas fireplaces).
  9. Leaving the pilot flame on when the main burner flames are turned off (for standing pilot light gas fireplaces) or turning the pilot off when the fireplace is turned off (for electronic ignition types).

Our own gas fireplace is the natural type and so we’ve used this fireplace to explain how gas fireplaces work in general.

Gas Fireplace
Our gas fireplace insert that we’ve used to explain how gas fireplaces work

We’ve also had an old gas fireplace removed from our kitchen masonry fireplace when renovating and so we’ve used this to show more closely the internal working of a gas fireplace.

Gas Fireplace Burner Assembly
The burner assembly from our old gas fireplace

1) The Gas Supply

Gas fireplaces require a supply of gas in order to work.

This supply is found as a dedicated pipe providing gas to the fireplace, commonly from the mains gas from the house for natural gas but may also be from a propane tank outside.

Our gas fireplace uses natural gas from the mains house supply. The pipe supplying gas to the fireplace can be seen around the side of the chimney breast.

Gas Fireplace & Gas Line
The gas pipe (left) supplying gas to the fireplace (right)

It’s here where the shut-off valve is located and therefore where we can turn off the gas supply to the fireplace.

Gas Fireplace Main
The shut-off valve for the gas to the fireplace

In order for a gas fireplace to work a gas supply must be provided to the fireplace via a dedicated pipe, while also ensuring that any shut-off valves are turn on.

A gas supply pipe often enters a gas fireplace from the back.

As an example, for our current gas fireplace the gas supply pipe can be seen coming in from the back (behind the gas fireplace within the firebox of the masonry fireplace the gas fireplace is installed within), when looking through from the control area.

Gas Fireplace Inside
The gas supply entering the fireplace from the back

It’s also here where the gas supply pipe is connected to, or can be disconnected from, the fireplace.

This gas supply pipe extends straight to the controls, in the form of a dial, where the gas supply to the flames can be controlled.

Gas Fireplace Controls
The gas supply connecting up to the controls

As another example, the supply pipe connection to our old gas fireplace can be seen below.

Gas Fireplace Burner Assembly
The gas connection to our old gas fireplace can be seen in the top left

2) The Electricity Supply

Traditionally, gas fireplaces didn’t typically require the need for an electricity supply because standing pilot lights were used, where the pilot would be lit and remain on for the whole duration of the burning season.

The fireplace would be able to generate its own sparks required to ignite the pilot flame and therefore not require a dedicated electrical supply.

Our gas fireplace uses this more traditionally standing pilot light ignition system and didn’t need to be connected to the house electrics when installed.

Gas Fireplace Ignition
The ignition in our gas fireplace doesn’t require any sort of dedicated electricity supply

However, a standing flame that continuously burns gas even when the fireplace isn’t in use was never the most efficient part of a gas fireplace.

As a result, technology has developed to allow for intermittent pilot ignitions, or IPI, systems to be installed in more modern gas fireplaces where the pilot light turns off when the main burner flames also turn off.

This constant re-ignition of the pilot light in a gas fireplace means that many newer models of gas fireplace now require an electrical supply (from the house electrics) as well as a gas supply.

3) Ignition

One of the main parts inside a gas fireplace is the ignition system.

This ignition system is used to help light the main burner flames more efficiently and effectively and includes the use of a pilot flame in order to do so.

The main burner flames are the flames you see in a gas fireplace, while the pilot light is a small flame typically tucked away out of view.

There are typically two main types of gas fireplace ignition:

  • Standing pilot ignition
  • Intermittent pilot ignition

Standing pilot ignition is the more traditional version where the pilot flame remains constantly on, even between fires, albeit typically turned off when the fireplace isn’t used for an extended period of time such as through summer.

Intermittent pilot ignition (IPI) is more commonly found in newer models of gas fireplace and the pilot is only ignited when required and shuts off when the main flames are turned off. A gas fireplace with an IPI will require a dedicated electricity supply.

Our gas fireplace uses the standing pilot ignition, where we have to manually light the pilot light at the start of the burning season.

It can be found just to the left of the controls and the main burner, inside the burner assembly.

Gas Fireplace Ignition
The location of the ignition in our gas fireplace
Gas Fireplace Ignition
The ignition from above located next to the burner

The ignition system of a gas fireplace is typically made up of the following components:

  • Spark ignitor
  • Pilot
  • Thermocouple and/or thermopile

The actual components used inside a gas fireplace ignition will depend on the model of fireplace.

The ignition inside our current gas fireplace is made up of a spark ignitor, pilot and thermocouple. There’s no thermopile.

Gas Fireplace Parts Ignition Labelled

The ignition inside a standing pilot gas fireplace works as follows:

  • Gas is supplied to the pilot from the mains gas supply.
  • The controls are used to let gas through the pilot and to also provide the spark required to light the gas.
    • The controls often consist of a dial, with either the spark being provided as a separate button or on the dial itself.
  • Depressing the control dial lets gas through while turning the dial or pressing a separate button provides the spark required to light the pilot flame. The spark ignitor typically sits over or near to the pilot.
  • Once the pilot flame has ignited, the dial is typically held down for up to a minute to allow the thermocouple and/or thermopile to warm up and work.
  • The thermocouple/thermopile acts as a thermal generator and the heat from the pilot flame allows it to produce a constant voltage that in turn is used to help keep the pilot valve open, and therefore keep the gas to the pilot flame flowing.
  • If the pilot flame goes out, the thermocouple/thermopile cools and the pilot valve shuts off, thus preventing gas from escaping.

A thermopile typically consists of a number of thermocouples in one unit, often used to help generate a higher voltage required for more powerful gas fireplaces.

For more information see our complete in-depth guide on gas fireplace pilot lights, our guide to the parts of a gas fireplace explained and our guide to how to light and start a gas fireplace.

4) Igniting The Main Burner Flames

The pilot light can be seen as a steppingstone to lighting the main burner flames.

In order to be able to more effectively get the main burner flames to light, gas fireplaces incorporate a smaller, pilot flame that once itself is lit can be used to light the main gas burner.

This is because igniting the larger gas burner can be much more of a struggle if simply using a spark ignitor to start the main flames.

In order for the pilot to light the main burner, a gas fireplace pilot light is therefore typically found adjacent to the burner.

The pilot in our own gas fireplace can be found overhanging the burner tray, with the pilot flame pointed towards the burner.

Gas Fireplace Burner Tray
The pilot directed towards the burner

This allows the pilot flame to ignite the gas coming out of the burner tray when the main flames on the fireplace are started.

Lighting of the main burner flames on a gas fireplace using the pilot flame works as follows:

  • With the pilot flame lit and sustained, the controls on the gas fireplace can be used to start the main burner flames.
  • The control dial in a gas fireplace can be turned from the ‘Pilot’ position to the ‘On’ position.
  • The pilot flame remains on and gas is provided to the burner when the control dial is turned.
  • The pilot flame ignites the gas coming through the burner and the main flames on a gas fireplace are started.

For our particular model of gas fireplace, we simply need to turn the control dial further anticlockwise from the ‘Pilot’ position through to the ‘Gas’ position (indicated by the gas symbol shown below).

Gas Fireplace Control Dial
The main burner flame controls indicated by the gas flame symbols on the control dial

As the dial turns from Pilot to On, the pilot flame remains on.

At this ‘On’ position, gas is provided to the main burner tray and the pilot flame ignites the gas, thus starting the main flames.

This is how our particular model of gas fireplace works. Other models of gas fireplace commonly have an ‘On’ setting on the dial that can be turned to from the ‘Pilot’ position, with a separate dial to control the flame size.

5) Adjusting the Flames & Heat Output

With the main burner flames started, the controls on a gas fireplace can typically be used to adjust the size of the flames.

This in turn also affects the heat output from the fireplace.

Increasing the amount of gas getting to the burner increases the size of the flames and therefore increases the amount of heat a gas fireplace is generating.

For our particular model of gas fireplace, turning the control dial further anticlockwise from the small gas symbol to the larger gas symbol increases the amount of gas being supplied to the burner, and in turn increases the size of the flames.

Gas Fireplace Control Dial
The largest flames are produced when the dial it turned all the way to the larger of the two flame symbols

Other models of gas fireplaces can often have a separate dial for the flame size, with the main dial simply offering ‘Off’, ‘Pilot’ and ‘On’ positions.

Other models of gas fireplace may have a separate dial for adjusting the flames

The heat from a gas fireplace will be provided through the front, either in the form of radiating the heat out into the room and/or through convection of hot air using built-in blowers.

6) Enhancing the Fireplace Experience

Gas fireplaces burn gas as the sole source of fuel and no other source of fuel is combusted inside the firebox.

This means that without any accompanying fireplace effects a gas fireplace would simply be a set of flames.

The whole fireplace experience is therefore enhanced by using a set of faux fireplace media to help make the fire look more realistic.

This fireplace media can commonly be found in the form of logs, but may also come as coals or stones.

Clean Gas Fireplace
Many gas fireplaces use a log effect

Our own gas fireplace uses the coals as the media effect.

Gas Fireplace Coals
Our gas fireplace uses the coal effect

The media used in a gas fireplace isn’t real.

The logs or coals that you see in a gas fireplace is made from a non-combustible material, commonly ceramic.

Gas Fireplace Coal
Whatever the type of media used, it will be fake

These logs, coals etc are placed strategically within the firebox of a gas fireplace to ensure that the flames are flicking between them. It’s therefore always important that gas fireplace media is placed correctly before using the fireplace, or sooting and other more serious issues could potentially occur.

As an example, the coals for our gas fireplace must be placed in the correct arrangement as per the manufacturer’s guidelines in the owner’s manual.

Gas Fireplace
The coals arranged correctly in our gas fireplace

See our article on arranging gas fireplace media such as logs or coals for more information.

7) Air Supply

Gas fireplaces utilize a real fire inside the firebox to burn the gas.

As a fire requires both fuel and oxygen in order to burn, this means that gas fireplaces need to be supplied with a source of fresh air to work.

The way in which fresh air is delivered to a fire inside a gas fireplace can differ between the different types of gas fireplaces available.

Types of gas fireplaces that take fresh air from the room include:

  • Natural or B-vent gas fireplace
  • Ventless gas fireplaces

The type of gas fireplace that takes fresh air externally from a home (via a dedicated vent) includes:

  • Direct vent gas fireplaces

Natural or B-vent gas fireplaces (such as the one we own and use) typically have open fronts to the firebox where fresh air can be sucked in from the room.

Gas Fireplace Insert
The open front of our natural vent gas fireplace

Ventless gas fireplaces don’t typically have completely open fronts like natural vent types do but often have a mesh screen in front of the firebox. Air is still able to get to the fire from the room, however.

Direct vent gas fireplaces vent clean air externally from a home. They also have a sealed firebox using a glass-panelled front that keeps the air within the gas fireplace separate from the air within the room.

Direct Vent Gas Fireplace
The glass front of a direct vent gas fireplace

Direct vent gas fireplace types will therefore have a dedicated air vent/duct, typically sticking out the back of the unit, that must be connected up to a further vent installed through an external wall of the house.

Air will be sucked in through the vent to feed the fire and not from the room.

8) Waste Air Venting Arrangements

As gas fireplaces use a real fire to burn gas, by-products can be produced from this process and must typically be removed from the home for safety purposes.

Like for fresh air, the venting arrangements for waste air produced by a gas fireplace can also differ between the different types.

The types of gas fireplace that vent waste air externally include:

  • Natural or B-vent gas fireplaces
  • Direct vent gas fireplaces

The type of gas fireplace that doesn’t vent externally includes:

  • Ventless gas fireplaces

Natural/B-vent gas fireplaces are commonly found as insert form and typically installed within the firebox of existing masonry open fireplaces.

As a result, these types of gas fireplaces will often use the existing venting setup offered by the existing chimney in the form of the natural movement of air up the chimney known as the draft.

As an example, for our particular model of gas fireplace there’s simply an open vent at the top of the firebox in which waste air from the fire is sucked through and up the chimney.

Gas Fireplace Vent Hood
The vent located at the top of the firebox of our gas fireplace
Gas Fireplace Chimney
Looking through this vent to the chimney behind

There’s no flue or other form of positive venting system behind this vent apart from the chimney itself, but many other types of b-vent gas fireplace will have a flue that’s installed inside the chimney and connected up to the fireplace.

Direct vent gas fireplaces vent waste air externally in the same way that they vent fresh air.

There’ll often be a dual venting system out the back of a direct vent gas fireplace so that waste and fresh air are both served through the same overall vent installation.

Ventless gas fireplaces burn the gas to such a high efficiency (around 99%) that any waste air doesn’t need to be vented externally (see our guide on gas fireplace efficiency for more information).

The by-products produced are at such a low level that the waste air can be released back into the room.

For safety purposes, ventless gas fireplaces also come with oxygen and/or CO2 sensors to ensure that levels remain acceptable.

9) Standing Or Extinguished Pilot Flame

Once the fire in a gas fireplace is turned off, the pilot flame with either stay on or go off depending on the type of gas fireplace.

For gas fireplaces that use the traditional standing pilot ignition system, the pilot will remain on when the main burner flames are turned off.

This allows the main burner flames to be re-lit more effectively without having to go through the more onerous process of re-establishing the pilot light.

For gas fireplace that use the electronic intermittent polit ignition, the pilot flame with go off when the main burner flames are turned off.

As this pilot is electronically started, it can be re-lit at the touch of a button when either a wall switch or the remote is used to start the fireplace.

Further Reading

Parts Of A Gas Fireplace Explained

How To Light A Gas Fireplace

Gas Vs Electric Fireplaces Compared

A Complete Guide To Gas Fireplace Pilot Lights

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