The flue gases fed to a chimney should be hot enough to promote an adequate draught and must be kept as hot as possible until they exit at the top. This is best achieved by constructing the chimney in lightweight insulated materials that heat up rapidly. The chimney should also be contained within the warmth and shelter of the building for as much of its length as possible. Twin wall sectional chimneys use just 25mm of insulation so it is particularly important to minimise their exposure to the elements. Clearly the situation shown at right is undesirable and should be avoided.
Any masonry chimney is by definition ‘stone cold’ and will quickly chill hot flue gases fed to it. The best way to counter this is to fit a low-mass flexible liner which heats up rapidly and helps to keep the flue gases hot throughout its length.