What is a downdraught?
A flue draught is the movement of air through a chimney or flue. This is caused by the “buoyancy” of hot gases rising up the chimney or the pressure differential between the chimney base opening and the flue terminal. Therefore the hotter the flue and gases, the more buoyancy or draw in the chimney. Adequate chimney draught is necessary to induce sufficient air for combustion through the flue and to clear the products of combustion from the appliance up the flue and disperse them into the atmosphere. An uninsulated/single wall chimney cools the flue gases rapidly and draught is never as good as it is in a well-insulated flue.
Poor draught causes
The most common cause of poor draught is a lack of air supply to the fire or appliance. This in many cases is due to a lack of free air “ventilation” to the room where the appliance is installed. The size of the room has no bearing on this; large rooms still need correct ventilation. With that said, appliance manufacturers generally indicate the required free air requirements in their installation instructions. As per HETAS regulations permanent room air vents should be installed in the room where the fire or appliance is positioned.
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