Information and Advice

Stove Jargon and Terminology Explained

Cleanburn and Secondary Combustion

Cleanburn is a system where hot air is released into the stove just above the fire. This allows unburnt hydrocarbons within the smoke to be ignited, meaning you get more heat and therefore more efficiency, better environmental impact and also more flames. This burning of the smoke is referred to as 'secondary combustion'.


This is a system which directs outside air down the back of the stove glass. This keeps the flames and any smoke off the fire away from the glass therefore giving a clear flame picture.

Primary, Secondary and Tertiary Air

Primary air enters underneath or at the base of the fire box, the control for primary air will frequently be in the stove door underneath the glass. Generally primary air is used for burning solid fuels as it burns better with air coming from underneath.
Secondary air enters above the fire, sometimes it enters the stove somewhere near the base but is preheated within channels built into the stove and only once preheated is it released into the fire box itself. Frequently you will find the control for the secondary air just above the stove door but this isn't always the case. Generally secondary air is used more for burning wood as wood burns better with air coming from above.
Tertiary air is a third air inlet. Often used to assist the cleanburn system you will usually find tertiary air enters the stove at the back of the firebox just above the fire. Where you will find the tertiary air controls depends on the stove in question and not all stoves carry nor need this feature.

Smokeless Fuel

Smokeless fuel is solid fuel that is manufactured in such a way that it smokes up to 80% less than ordinary house coal, it is also more efficient to burn. Only smokeless fuel should be burnt on your stove and house coal should be avoided. If you are purchasing smokeless fuel you should specifically ask for a fuel that's suitable for burning on closed appliances as those that are designed to burn on open fires often contain binders that can damage stove glass.


The government department responsible for environmental protection, food production standards, agriculture, fisheries and rural communities in the United Kingdom and Northern Ireland. The Department for Environment, Food and Rural Affairs (Defra). A DEFRA Approved stove is a stove which has been approved to burn wood in Smoke Control Areas.

Smoke Control Areas

An area in the UK where it is illegal to emit smoke from the chimney. You can only burn authorised fuels or use DEFRA approved appliances to burn unauthorised fuels such as wood. Generally these are built up, urban areas. Check with your local council to see whether you live in a Smoke Control Area.


HETAS is the officially recognised government body to approve solid fuel domestic heating appliances and services. HETAS runs installer training schemes and a HETAS Registered installer is able to certify their own work.

Twin Wall, Single Wall Flue and Liner

A twin wall flue system is a flue pipe with 2 layers of steel with insulation in-between. This allows the flue to pass as close as 50mm to combustible materials, meaning a flue can be run through a house between joists or allow the flue to be run up the outside of the house, as the insulation means the flue will be warm enough to work. A twin wall flue system should be considered if you don’t have a masonry built chimney.
Single wall flue pipe is made of a single layer of enamelled steel or stainless steel. It is built to withstand the high temperatures near the outlet of the stove. Single wall flue is used to connect between the stove outlet and either the twin wall flue system or flue liner.
Flue liner is effectively a tube that connects between the single wall pipe where it passes into a masonry chimney and the top of the chimney stack. It can be made from single layer, flexible stainless steel, clay or pumice pipes or by inflating a tube within the masonry chimney and backfilling it with a stone mixture that sets. The chimney liner seals the chimney against the risk of leaking back into the house and also insulates the chimney, a warm chimney works better.

For further information on flues please read our articles on the Chimneys and Flues section.

Article written by Ryan Brocklehurst

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