The importance of chimney design

Chimney Design Article
The HETAS Technical Team talk chimney design

Every solid fuel heating appliance installed in the UK will be connected to a suitably sized chimney system, to allow for products of combustion created during the burning of solid and biomass fuels to escape from the appliance and be safely emitted to outside. For all combustion appliances, this process of combustion product dispersion is governed under UK Building Regulation requirement J2 which stipulates;

J2 – Discharge of Products of Combustion

Combustion appliances shall have adequate provision for the discharge of products of combustion to the outside air.

Chimney Design and ADJ

For solid fuel burning appliances, compliance with the regulation is pretty straightforward. Approved Document J (ADJ) provides the necessary guidance to ensure that a masonry or twin wall metal system chimney is available and is suitable for the appliance to be connected to. Further details are given in ADJ that may need to be considered to ensure combustion products evacuate safely, and that the suitability of materials, minimum flue liner diameter and minimum chimney height are adhered to for the correct and safe operation of the appliance.

Consideration of the termination point of the chimney is also important when installing a new chimney, to ensure other neighbouring properties are not affected by products of combustion.

Functioning of a Chimney System

Chimneys work through the simple process of convection. Warm air rises and cold air sinks. Even without any fire in the appliance a chimney may provide an “initial draw” simply because the air inside the property is warmer than the air outside of the property. If the opposite occurs (i.e. the inside temperature of the property is actually cooler than outside) then the chimney may not be able to provide this initial draw and the chimney itself may require warming to get it started, so that a fire can be lit without any nuisance spillage of smoke etc from the appliance.

Once the fire is going and the fabric of the chimney has warmed up nicely, a good chimney will provide the driving force to enable the products of combustion to be carried safely away to outside.

Why Chimney Design is Important

When designing a new chimney or installing an appliance into an existing chimney, flue height should always be a key consideration, especially in situations whereby an increase in flue draught is required to meet appliance manufacturer prescribed flue draught tolerances.

Clause 2.8 of Approved Document J stipulates that 4.5m height is a satisfactory benchmark on which to design a chimney’s overall height, however it is important to note that a chimney at this height may not necessarily be effective in fully evacuating all products of combustion.

Other design considerations should be taken into account including such things as the number of bends and off-sets, the chimney diameter and the length of chimney that is exposed to outside conditions.

The correct operation of any chimney depends on a balance between the driving force from the hot air rising inside it and the overall resistance caused by the walls of the chimney, any changes in direction and how much cooling occurs due to the external walls being exposed to the elements. However, generally and within reason a taller chimney will provide a better draw or draught.

If as an installer you are experiencing problems with flue draught, then thinking about making the chimney taller where possible should always be considered. This may be necessary especially in single story properties or installations in a single-story extension to stand the best chance of achieving sufficient flue draw.

Thatched Roof

It is important to note that the requirements for chimney height and termination differ slightly for installations in properties with a thatched roof, guidance of which is available with HETAS Technical Note TN_007 – v2:2019 (login required).

Alternative Approaches

There may be situations whereby the minimum 4.5m chimney height cannot be achieved (i.e. bungalow installations). In these circumstances it is important to undertake a calculation in accordance with BS EN 13384-1, to confirm the suitable height of the chimney.

This calculation will provide the installer with a predicted flue draught, dependent on the appliance output, efficiency and chimney height, number of bends, minimum flue diameter and location of the chimney (i.e. internal or external). This result can then be compared to the appliance manufacturers stated tolerances to ensure the predicted flue draught is suitable for the appliance to operate safely and correctly.

Any flue designs below 4.5m in length, it is prudent to check with the appliance manufacturer that the specific appliance will work in accordance with the chimney design specified.

It is also advised to check any stipulations either on the property itself or local or regional requirements before construction or modifying any chimney.

Chimney Design support

For more information, please contact the HETAS Technical Helpline at technical@hetas.co.uk or call 01684 278194.

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