Thank you for your question.
In order to perform properly the Firefox 8 needs a flue draught of minimum 12 Pascals and a maximum of 15 Pascals water gauge.
If your chimney is outside of these measurements you may need a chimney liner to correct the flue draught.
A Hetas engineer will be able to measure the flue draught for you and recommend the best course of action. (http://www.hetas.co.uk/find-installer/ )
We sell a wide range of chimney products at competitive prices, so please contact our stove experts if you need anything.
Hope this helps?