Information Centre

Gas Fire and Chimney Flue Types

If you are purchasing a new gas fire, then you need to know what type you need to buy, this will depend on the type of chimney you have (or don't have). Below is a quick summary of the different kinds of fire and flue types that you will come across. Although this should give you enough information to have an idea about the nature of flue/gas fire you require, you will always need to take the advice of a gas safe registered engineer.


There are three main flue Types.

Chimney Flue Types, Class 1 chimney, Class 2 chimney, traditional chimney

• Class 1 Flue (Traditional chimney with terminal pot). This can accept a class 1 or 2 fire

• Prefabricated Flues (7’’ or 5’’ diameter), has a Stainless steel pipe that goes all the way through the house and terminates on the room with a GC1 terminal

• Pre-Cast Flue has a concrete block going through the house, in the roof it converts to a stainless steel pipe and terminates with GC1 terminal or ridge tile


If your property does not have a flue, then you have four options

  1. Flueless Gas Fire
  2. Balanced Flue Fire
  3. Power Flue Gas Fire
  4. Bi Ethanol Fireplace


Common Chimney, Flues Terms 

Any work that you are getting done with your fire and chimney needs to be done by a professional. Below are some common terms that will help you when discussing your fire and chimney options

  • Flue: The tube made from twin wall stainless steel removes the products of combustion from the back of your fire to the atmosphere.
  • Flue Liner: The material inside the chimney, this may be concrete, stainless steel or ceramic
  • Flue Pipe: This is the pipe made from twin wall stainless steel that connects your fire or stove to the flue.
  • Chimney: This is the part that goes around a flue (all chimneys have flues)
  • Chimney Terminal: This is the bit that sits on the very top of your stack, often called a pot or terminal.


Pre Fabricated Flues 

Depending on your flue diameter these will accept either a Class 1, Class 2 or Multi-Flue gas fire.

A Pre Fabricated Flue consists of a twin wall stainless steel pipe that is interlocked and runs through the house to the roof. On the roof, you will either see a flue pipe with a terminal or a terminal ridge tile. Alternatively, if you have a chimney with a stainless steel pipe running up it to your roof, then you have a prefabricated flue.

If your stainless steel pipe is 7’’ then you can use a class 1, or class 2  fire, if it is 5’’ in diameter then you can only use class 2 fires. Multi flue fires will also work for both 5’’ and 7’’ diameter flues.

If you chimney has failed the smoke bomb test you have the option of putting a flue liner down the chimney, either 5’’ or 7’’. Some builders will not want to put down a 7’’ flue liner as they can be more difficult to install, so check which diameter your stove or gas fire requires.


Chimney - Class 1

You can only use Class 1 or Multi-Flue gas fire with this flue type

Chimneys are easily recognisable as you will have a traditional chimney stack with a gas or pot terminal on the roof or running up the side of your house to the roof. These are quite common on homes built up until the 1960s but not on modern houses. A traditional chimney expels the combustion materials using the natural circulation of through your room. 

If you have this type of chimney, you will need to ensure it is in good working condition and this is done with a smoke test. If this fails then the bad news is that your chimney will need lining, this is where you need to decide the diameter of the flue liner, if you go for 7'' then you can fit a class 1 or 2 fire, if you can go for 5'' then you can only fit a Class 2. If you are going to use a multi flue fire then it doesn't matter which diameter tube you go for.


Pre-Cast Flue - Class 2

You can only use Class 2 or Multi-Flue gas fire with this flue type

Pre Cast Flues are 5'' in diameter and are found often in houses built from the 1960s onwards. The Flue is made up of blocks made from concrete or clay. These are fitted in the cavity wall of the house and end on the roof with a ridge vent or metal terminal.


Common Gas Fire Types and Terms 

Power Flue Gas Fires

Ideal when you have no chimney

If you have a home without a chimney or flue then a power flue fire may be right for you. These have an electronically driven fan that goes on the outside of the wall, and this is used to expel the fumes from the fire. This type of fire gets its air to burn from inside the room and then discharges the products of combustion through the fan to the outside atmosphere. 

These fires are quite expensive, and the fans can make them noisy. So if you have a quiet room or are sensitive to noise, you may be annoyed by the fan.


Balanced Flue Fire

Ideal when you have no chimney

These are glass fronted fires that expel the combustion gases out through the back wall. They do not use a fan and are highly efficient; they also get very hot, so upgrades to fire surrounds and back panels may be necessary. The running costs are generally lower than open fronted multi-flue fires however they are expensive to buy and install. This type of fire takes the air from outside and then expels the products of combustion back outside, this is a room sealed product.


Flueless Gas Fires

Ideal for simple installations when you have no flue.

These are easy to use and install, they are 100% efficient as there is no wasted heat up the chimney and so they are ideal for secondary heat in a room they are not designed to be a sole source of heat, they have a lower heat rating than other fires, going up to around 2.5kw.  Flueless fires cannot be used in tiny rooms as they must have sufficient air to burn, and they do need an air vent.


Bio Ethanol Fires

No gas required instead you add liquid Bio-Ethanol

These are very easy to fit, are silent and do not cause smells or smoke. They don't need a chimney or flue; however, you do have to keep topping them up with liquid between 3 - 5 hours of usage. This type of fire is cheap to instal but expensive to buy, and the fluid is costly too, typically 5 litres will burn for around 5 hours, and 10 litres of fuel will cost around £40!