What is External Air?

External air, also known as direct or outside air, is an increasingly important safety requirement for a stove which is being installed in a room with an extractor fan or in a modern, virtually air-tight home, especially one with mechanical ventilation heat recovery (MVHR). The direct external air supply (DEAS) allows the stove to take its combustion air from outside the room the stove is installed in, normally from outside the building.

An external air supply means that hardly any of the warm air inside the room the stove is helping to create is wasted by the stove having to use it. For good quality modern stoves this means that the stove is effectively room-sealed. This is important because extractor fans and MVHR can adversely alter the air pressure in a room which in turn affects the pressure in the stove's flue and potentially its safe performance. Without such a room seal in an extreme case this could lead to a leakage of dangerous CO into the room. All of this is dependent on the quality of the air-tight seal on the door, with the best stoves having their air tightness tested to the tough German DIBt standard.

Prior to 2018 it was prohibited to install a stove in a room (eg kitchen / diner) where there was an extractor fan, never mind a home with whole-house MVHR. Although current Building Regulations Document J and its national equivalents, as well as many stove instructions manuals, continue to state this prohibition the good news is that this rule has now changed. The recent revision to BS8303, which covers the installation of stoves, now allows for stoves with DEAS to be fitted in such situations – but only if the appropriate spillage tests, as stipulated by the standard, have been successfully carried out during commissioning.



Ultimately the success of these spillage tests relies upon the effectiveness of the room-seal and this will very much depend on the design and build quality of the stove and its door seal. Generally speaking good quality stoves with DEAS will provide a functional room seal suitable for most applications. DIBt tested stoves however have a proven room seal and are therefore highly recommended for safe operation in homes with MVHR systems. Many modern stoves would probably pass the DIBt test but because of the prohibitive costs of this test some manufacturers have undertaken their own tests so that they can safely declare that their stoves are suitable for operation with MVHR. The French inset stove maker Fondis would be a really good example of this.  

Even for basic installations where there is no mechanical extraction, a stove with direct external air supply can still offer the homeowner some advantages. Since the stove has it's own outside air supply this means there is no need to install the dedicated air vent required for some installations by Building Regulations Document J and its national equivalents. An external air supply should also help eliminate cold draughts. Again, the appropriate BS8303 spillage tests should be undertaken during commissioning.

The external air inlet grille and the non-combustible ducting required to complete the DEAS system are normally sold separately from the stove. Some stoves come complete and ready to be connected to the ducting but others may also require the purchase of an optional 'air box' manifold which can either be factory fitted or supplied to be fitted by the stove installer. These days there are many stoves with DEAS capability but among The Stove Yard's favourites are Max Blank, Hwam, Wiking and the latest British-built models from Charnwood.