This article was first published in HETAS Technical Bulletin #23.
It is well known that a flue works as a result of two natural principles; pressure difference and temperature difference.
In general, the size of flue is chosen to ensure adequate evacuation of combustion products. The major operating force providing the flue draught to evacuate the products of combustion is the buoyancy of the flue gases, which is mainly dependent upon the temperature of the flue gas itself, but also on the height of the chimney or flue system. An open fire generates a considerably larger volume of flue gas than a closed appliance and therefore needs a larger flue area to suit the size of fireplace opening. In addition, the flue gas temperature from an open fire is considerably less than that for a closed appliance, so the buoyancy (or flue draught) is also less.
Advice with British Standards BS 8303 and BS EN 15287 is also aligned with the advice within Building Regulations (Approved Document J).
This guidance for the relative height of a flue (vertical distance from top of appliance to terminal outlet) is based on a masonry chimney containing up to 4 bends (no more than 45o) being able to produce sufficient draught.
However, there are many more considerations and factors that need to be taken into account to ensure adequate discharge of combustion products. HETAS Training Units discuss the variation of heat input to flues with different appliances – this may be one consideration.
Approved Document J also suggests that an alternative approach, using BS13384 can be used to determine whether a particular chimney design will provide sufficient draught.
Paragraph 2.8 in ADJ states:
2.8 Flues should be high enough to ensure sufficient draught to clear the products of combustion. The height necessary for this will depend upon the type of the appliance, the height of the building, the type of flue and the number of bends in it, and an assessment of local wind patterns. However, a minimum flue height of 4.5m could be satisfactory if the guidance in Paragraphs 2.10 to 2.12 is adopted. As an alternative approach, the calculation procedure within BS EN 13384-1:2005 can be used as the basis for deciding whether a chimney design will provide sufficient draught.
With the increase in Factory Made Twin Wall Flue Systems being installed and the increased efficiency and range of appliance types, it is sometimes necessary to utilise theoretical modelling at design stage, to ensure that the proposed flue will (at least in theory) provide a required draught.
Foremost, appliance manufacturers installation instructions and guidance should be reviewed and followed – these will contain information necessary for flue design and may stipulate requirements (such as minimum height / size) to be considered in conjunction with other guidance.
It should also be noted that utilising a long horizontal flue (in line with BSEN15287 – See TB#1, Nov 2013) stipulates that a calculation to BSEN13384 is required and the report kept with commissioning documentation.
Many flue manufacturers are happy to produce calculations for the design of a flue when purchasing the flue through them and will be able to give good advice on their products.
So what is BS EN13384-1;2005?
EN 13384 is the European norm for chimney calculation which describes the calculating method for chimneys. It is essentially a calculation which takes into account (at least) two conditions: the pressure and temperature requirement, to determine whether a flue will work for the intended appliance. The outcome is either the flue design will meet the standard or not.
The general principles of the standard is based on 4 criteria:
1. the minimum draught at the flue gas inlet into the chimney shall be equal to or greater than the minimum draught required.
2. the minimum draught at the flue gas inlet to the chimney shall be equal to or greater than the effective pressure resistance
3. the maximum draught at the flue gas inlet into the chimney shall be equal to or less than the maximum allowed draught
4. the temperature of the inner wall at the outlet of the chimney shall be equal to or greater than the temperature limit.
The standard has been implemented into computer software packages, with a large database of different flue and appliance details available.
It is important to consider obtaining a report (checking compliance with the standard) where the flue may deviate from the minimum height and maximum number of bends given ADJ or where there are concerns that the appliance and chimney may not be compatible (such as a small, high efficiency appliance connected to a tall exposed factory flue system, or a flue height less than 4.5m.) Using the standard when designing a flue can allow for different routes/models to be theorised and avoid potential costly issues post installation.
Remember however, to ensure compliance with the Requirement J2, full commissioning must always be undertaken on completion, as it is impossible for any theoretical model to cover all factors.