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Chimney Flue Size Chart

Learn How to Determine Chimney Flue Size

When you’re trying to install a new wood or gas-burning appliance that will require a chimney flue, it can be complex to understand how to determine chimney flue size. 

Below, you can look at our chimney flue size chart to discover what flue size your chimney requires.

Flue pipes should have the same diameter or equivalent cross-sectional area as that of the appliance flue outlet. They should not be smaller than the size recommended by the appliance manufacturer.

Flues should be at least the size shown in the table relevant to the particular appliance and not less than the size of the appliance flue outlet or that recommended by the appliance manufacturer:


Minimum Flue Size

Fireplace with an opening of up to 500 x 550 mm

Ø200 mm 

Fireplace with an opening excess of 500 x 550 mm or a fireplace exposed on two or more sides

15% of the total face area of the fireplace opening/s

Pellet burner or pellet burner meeting the requirements of the clean air act

Ø125 mm 

This may be reduced to no less than Ø100 mm when permitted by the appliance manufacturer and supported by calculation according to BS EN 13384- 1:2002

Other closed appliance of up to 30 kW rated output burning any fuel

Ø150 mm ; Ø125 mm when DEfra approved

Closed appliance of above 30 kW and up to 50 kW rated output burning any fuel

Ø175 mm 

Common Chimney Flue Sizing Mistakes

Many people make a few common mistakes when trying to find the appropriate chimney flue size. Let’s take a closer look at a few of these and the potential results:

Oversized flues - People often think that bigger is better. However, with chimney flue sizes, you want it to fit well within a size range. An oversized flue can result in unwanted consequences, including the following:

  1. Insufficient Heat Retention
  2. Lack of Velocity
  3. Inefficient Airflow
  4. Difficulty in Maintaining Consistent Temperature
  5. Reduced Stack Effect

Undersized flues - A flue that is too small hampers the ability of the system to function appropriately. Some of the potentially disastrous consequences of an undersized flue include the following:

  1. Limited Airflow
  2. Inadequate Draft
  3. Increased Back Pressure
  4. Smoke Spillage
  5. Creosote Accumulation


Our chimney flue size chart can help you make the appropriate choice when sizing your new chimney flue. Ensuring that you pick the right size is critical and can mean the difference between an efficient system and one that barely works.

Another useful installation that you may wish to consider is to install a chimney flue liner. This useful tool helps to protect your brick-and-mortar chimney from the erosive effects of combustion gases.

If you’re not sure what your best size option is for your new application, or you wish to know more about which products will be the perfect fit, feel free to contact us. We’d love to help you design your new chimney flue system to be as effective and efficient as it possibly can by using the right tools and chimney flue sizes.