A balanced flue gas fire requires a concentric flue system. A concentric flue is essentially, one steel pipe within another. There are renovation kits available to convert into an existing chimney, to create the concentric system using multi-fuel flue liners. The inner pipe of the concentric system, expels the flue gases from the fire, up to the roof termination. The outer pipe supplies the combustion air (oxygen) to the fire, to burn.
Most balanced flue fires on the market require a very efficient flue route, involving an initial 1m vertical rise of flue pipe, before a 90° bend through to a wall termination (otherwise known as the ‘up and out’ system). The flue route can be taken further horizontally, up to 7m away on most models, or alternatively vertically though to roof termination. When routing the flue vertically to the roof, our manufacturer Kalfire now allows a direct 45° elbow off the top of the unit, allowing more flexibility with a chimney breast design*. There are however restrictions with the maximum distances and/or bends which can be included within a balanced flue system, ensuring sufficient extraction of flue gases, and supply of combustion air.
*minimum 3m vertical route.
We then can compare this with the requirements of an open fireplace, which requires around 4.5-5m of straight vertical flue route to achieve a high likelihood of natural draught. However, natural draught within an open flue is unfortunately not an exact science, as there are a number of factors which will contribute to the draw, which cannot be pre-determined.
Open gas fireplaces require either a twin wall stainless steel flue system, or can be connected into an existing chimney. A smooth surface on a flue, e.g. twin wall flue or a liner, will create less friction in the draw, so will hopefully allow the flue to pull better. However, variables such as temperature, wind speed, location of the property, will all effect the extraction of the flue gases, making each individual installation unique.
Also, incorporating a number of bends into a flue route will restrict the draught, as the flue gases will slow down once they hit the obstruction. Therefore, a straight vertical flue route will have much higher chances of natural draught, compared with route including a number of 15/30/45° bends. There are a number of regulations to consider when introducing 90° bends into a flue route, specifically on wood burning appliances.
If natural draw cannot be achieved in an open fireplaces flue system, an extraction fan, or reducing the volume of air in the fireplace design can be discussed.