What is external air?

External air, or direct air, as it is also known is a fairly new development in stove technology in the UK and Ireland and is becoming increasingly important, especially when you decide to fit a stove in a relatively new house, where the builders have gone to great lengths to ensure that the house is virtually air tight and that very little heat (and therefore air) is allowed to escape. 

The external air feature will allow the stove's main combustion air to be taken from outside of the building. This makes the stove more efficient and safer because it is not using the warm air it has just created (well, at least not much) or the air that the room's occupants will also need to breath. A typical stove can consume between 15 to 25 cubic metres of combustion air every hour and an average UK and Irish living room has a volume of air around 40 to 50 cubic metres, so you can see that a stove operating in a modern draught-proof room could quickly present problems for the occupants. In a new dwelling an external air supply to the stove ensures that there's an unlimited flow of combustion air which will help with the effectiveness of the flue up-draught thus also helping to avoid the potential escape of dangerous carbon monoxide from the stove as the stove's fire burns down. So, external air should be a prerequisite for any stove planned for a new home but, in our opinion, it really isn't that critical in older houses which can be fairly draughty anyway and the supply of air is not in question. 

It is worth noting here that the latest version of UK Building Regulations Document J still insists on a dedicated air supply (via a small non-closable air brick) for a stove even if the main combustion air is also supplied externally and even if it is less that 5kW output when it is to be operated in a dwelling built after 2008. This is contrary to most other European countries and is something that the SIA (Stove Industry Alliance) in conjunction with Hetas and Gastech are keen to change through a programme of laboratory tests, the findings of which will then be used to argue the case for parity with Europe. 

There are different types of external air supply too. Usually it is pre-heated (but not always) which means that the chill is taken off the colder external air as it enters the stove's combustion chamber and thus helps to keep the fire-chamber hotter, which makes the stove more efficient and cleaner burning. Some external air systems only supply the primary air (not really needed when you burn wood) with the secondary air (the main air for the combustion of wood) still taken from the room where the stove is located, hence our earlier comment. The best external air systems supply both pre-heated primary and secondary air.
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