Ventilation Requirements for Gas Fires over 7kw

Where there is any confusion over whether a gas fire requires ventilation or not the first port of call for accurate information should be the manufacturers installation manual for the appliance.

The ventilation requirements for open flued gas fires (eg with a chimney) over 7kw should be in accordance with BS5440-2. However this only applies to gas fires installed in the GB region. For other countries the regulations on ventilation can vary and are country specific therefore reference should be made to the local national regulations.

The size of air vent and ventilation required for an open gas fire depends on the net Kw power rating (or heat input) of the appliance. Check the gas fire data plate to see if the heat input is quoted as Gross or Net. You need the net figure and to convert from Gross to Net on a natural gas fire you need to divide the gross figure by 1.11. You then need to subtract 7kw from the net figure.

Gas Fire Air Vent
Modern Air Vents do not cause drafts. Buy a vent to supply the correct amount of air as required by the fireplace manufacturer (shown in the install manual)

Example Calculation

Gross Heat input = 12kw
Net Heat input = 12/1.11 = 10.81kw
Ventilation Required = (10.81 – 7) * 5 = 19.1cm2

Air Vent

The above example states that a minimum of 19.1cm2 of ventilation is required. You can get a vent that is bigger and its most likely you will have to. This must be permanent ventilation and is not the size of the vent grille (very common mistake) it is the volume of free air provided by the vent into the room – from outside. A good quality vent manufacturer will quote this figure to you. The vent must not be blocked up or positioned where it can be blocked accidentally.

Wrong Ventilation

If you do not install the correct amount of ventilation your gas fire will become unreliable and possibly dangerous. When the fire is lit it burns oxygen from the room and this must be replaced by air from outside. If there is not enough ventilation the chimney will not function correctly and the fire will be unsafe and unreliable.

Air Vents Cause Drafts?

Modern air vents do not cause drafts. As with all things in life, you get what you pay for. If you do not want any drafts then spend money on a high quality air vent.


The above information does not apply to flueless gas fires or balanced flue gas fires.

Where an open flame chimney gas fire has a kw rating below 7kw it can be assumed that the room will provide enough ventilation without the use of an air vent, however check the manufacturers instructions first.

Always use a registered gas safe engineer for all servicing and installation work and ensure you have a copy of the manufacturers installation instructions for your gas fire available at all times.

To view our full range of flueless gas fires visit

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