Size a chimney liner from appliance output and flue height.
Appliance flues follow the outlet size rather than being calculated, while open fires need around a tenth of the opening area. Each bend reduces effective height and therefore draught. Oversizing a liner cools the flue gases, which reduces draught and increases condensation, so bigger is not safer with chimneys.
Chimney Liner
Open fires need a flue about a tenth of the opening area; appliances follow their outlet size
Open fires need a flue about a tenth of the opening area; appliances follow their outlet size Appliance flues follow the outlet size rather than being calculated, while open fires need around a tenth of the opening area. Each bend reduces effective height and therefore draught.
Oversizing a liner cools the flue gases, which reduces draught and increases condensation, so bigger is not safer with chimneys.
This calculator takes 5 inputs: Appliance output, Chimney height, Appliance type, Fireplace opening area, Number of bends. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.