The terms flue and chimney are often used interchangeably. The flue is the working part of the chimney, conveying the products of combustion safely to the atmosphere. The chimney includes the shaft within which the flue is housed. A flue works under negative pressure drawing the product of combustion from the boiler. A successful “chimney draw” is dependent on variable principles:

 - The difference in air pressure between the boiler and the top of the chimney (height)

- The difference in temperature between the boiler’s exhaust gas and the outside temperature.

- The quality of insulation along the length of the chimney.

- The route of chimney. (The straighter and more vertical the chimney the better).

- Heat recovery system.

- The sound produced during combustion is emitted as airborne noise at the chimney head (flue gas silencer).

- Boiler Flue regulations (minimum height; as applicable).

- Facility requirement

It is essential to choose a flue diameter that matches the output from the boiler. An adequate air supply is also required for the boiler to operate safely and efficiently. When planning the exhaust pipe, all factors has to be closely considered and implemented in the design.