Flue Draught and Minimum Flue Heights

“The flue is the engine of the stove.”

There are three certainties in life; death, taxes and the fact wood burning stoves will not work without an appropriate chimney. Flue pipes do not exist to solely exhaust smoke away from the stove, for within the correct chimney something wondrous occurs and the magical draw is created, essential for the successful operation of your log burner.

What is Flue Draught/Draw?

Let’s begin at the beginning – hot air rises, but why? When air is heated it expands and becomes less dense (or lighter) than the colder air around it and so floats or rises above the heavier cooler air. Therefore, when the air within a flue system is warmed by the fire of the stove, it too becomes lighter than the cold air outside of the system and will start to rise up the chimney, creating an updraught which effectively pulls fresh air in behind it, feeding the fire. This is the all-important draw, the holy grail for a successful stove. And controlling this draw is the key to an efficient, controllable and complete burn which is why we say the flue is the engine of the stove…

How to Create a Draw

First you need heat so get that fire going but bear in mind, you may experience some smoke or dampening of flames until things are up to temperature and the draw is established. A big tip here is to use the ‘upside down’ method of starting a fire which focuses heat on the flue. You can also artificially warm the air within the chimney using a blow torch or such like. You will also need sufficient air supply for this so ensure you have the regulatory ventilation for your size stove.

Being that life is never simple, there are a few things that can get in the way of creating your draw such as weather conditions, atmospheric pressure and nearby obstructions.

  • High winds or a change of wind direction will push everything back down the flue but Anti-Wind Cowls can help with that.
  • Changes in atmospheric pressure or outside temperature (especially in Spring and Autumn) can quickly bring about a reversal of the draw turning your updraught into a dreaded downdraught.
  • Nearby structures, roofs and even trees affect wind patterns blowing across or down your chimney.

Once the draw is created and your fire is established, you need to maintain it and the focus here is to keep the heat within the gases so they continually rise. Two key ingredients for this are 1) good insulation and 2) little to no turns or horizontal runs. Insulation such as that found already in Twin Wall flue pipes or Vermiculite granules which fill a chimney void keeping the heat within the flue for as long as possible and prevent the cold outside air from penetrating the system. Every turn in a chimney is an obstruction to the rising gases and so they will linger, cool and fall back down the chimney, killing the draw.

Flue Heights

The height (or length) of a flue system also plays a major role in the quality of draw created as the longer the flue system, the greater change in pressure between the top and the bottom. Extending this pressure difference will only improve the draw. It will come as no surprise that the building regulations have a say in flue heights, stipulating that no flue be less than 4.5m in total length and enforcing how high a chimney can terminate:

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