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The higher insulation standards and reduced air permeability of new build and low energy homes pose particular problems regarding the adequate supply of combustion air that the stove will need to perform safely and efficiently and therefore such homes need special consideration. Unlike old housing stock, which is incredibly leaky and which will have an air permeability greater than 5 m3/hm2, air supply to a stove rarely presents an issue, in a post-2008 home there simply isn't enough advantageous 'free' air and therefore additional ventilation is required for the stove – whatever its heat output.
Document J (or its country equivalent) stipulates that a dedicated open air vent' of the appropriate size based on 550mm2 for each kilowatt of the nominal heat output of the proposed stove and the amount of combustion air it needs should be installed – but of course that would defeat the air tight standard that you're trying hard to achieve. Fortunately stoves that feature direct external air supply (DEAS) capability now provide you with an alternative approach to the dedicated air vent'. Instead of using the air in the room, which is then resupplied by the air vent' and which consequently causes cold draughts, the DEAS stove is effectively 'room sealed' with combustion air supplied directly to the stove from outside and the combustion gases leaving through the flue. How good this room seal is depends on the quality of the stove, with the best of them being classed as '100% room sealed' and some also being tested to the prestigious German DIBt standard specially developed to allow stoves to be safely installed in passiv homes.
In addition, the revision of BS8303 Installation of Domestic Heating Burning Solid Fuels in 2018 now provides installation, risk assessment and commissioning procedures related to appliances with DEAS, including considerations for duct diameter, total duct length and the air inlet terminal's external position. Most importantly the BS8303 standard outlines a series of critical spillage tests, including under worst case depressurisation, for low energy homes, particularly where mechanical extraction is present. These tests have been designed to determine that the DEAS stove will operate safely and efficiently to ensure that carbon monoxide (CO) cannot leak into the home – provided it is regularly serviced and the flue is also swept.
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