Without a flue or chimney, normally brick built, your boiler would not function, but unless they are designed and installed correctly, chimneys lead to condensation.
All fuels produce gases and water vapour when burnt. A chimney provides a safe means of carrying away these gases. The hot flue gases and vapour rise because they are light than the colder outside air which displaces them from below. The height of the chimney influences the velocity at which these gases rise, together with the difference in temperature.
Designing a flue
Always install your flue to reduce the risk of condensation, bearing in mind the following:
1. If possible have a chimney or flue built inside the house.
2. Insulate it to reduce heat losses.
3. Line it with a non-absorbent and smooth impervious material.
4. It should be the correct size to comply with the boiler output.
5. Keep it as straight as possible. If bends are inevitable, then make them no less than 45° to the horizontal.
When installed inside the house the flue retains its own heat and reduces the risk of condensation. If the flue is outside the house then use a double skinned steel flue, with insulation between the skins. If an asbestos flue is used a drain tap has to be fitted at the lowest part of the flue, or at a suitable bend. Check with local health and building regulations before buying an asbestos flue.
A flexible stainless-steel flue liner provides a convenient method of lining an existing flue if installing a boiler in a fireplace opening. This type should not, however, be used with solid-fuel burning appliances as the waste gases will cause corrosion of the steel.