Guidance on flues and ventilation

Gas Safe Register’s Technical Team review the changes and updates made in 2023 to BS 5440 Parts 1 and 2, covering chimneys/flues and ventilation. This article appears in the July-August 2024 issue of Registered Gas Engineer magazine.

BS 5440 Part 1
Most of the changes made to these standards came in Part 1 of BS 5440, which covers chimney/flues. Many concerned removing outdated information that is no longer relevant: for example, most of the guidance about the construction of SE ducts and U ducts has been stripped out of the document but guidance on their maintenance remains to allow for appliances still connected to these types of chimney/flue system.

While a lot of historical information was removed, there have also been some important additions, particularly in the definitions section. The bulk of them is that all pathways into a property, ie, windows, vents, doorways, etc, are now simply referred to as ‘openings into a building’. 

The Gas Safe Technical Team often receive queries around weep vents and the distances from which boiler chimney/flues should terminate from them. This is now clearly addressed in the definitions – weep vents are not considered as an opening into a property.

There is updated guidance on chimney/flue outlets terminating into a lightwell or courtyard. As before, chimney/flues from a room-sealed or fan-assisted open-flued appliance can terminate a maximum of 1 metre below the uppermost level of an enclosed space such as a lightwell. 

The chimney/flue of any gas appliance in a new development or complete building refurbishment is now required to connect on to a communal flue system. However, the chimney/flue can terminate in the inner courtyard of an existing building provided that the narrowest dimension across the open area of the courtyard is at least that of the height of the tallest structure forming the courtyard. 

This means that, for example, if a building is 12 metres high, the width of the courtyard needs to be at least 12 metres across in order for the appliance to be able to terminate there. 

Where these restrictions cannot be met, the installation is treated as a new development or complete building. Appliances could still be installed in the uppermost floor of the building, as long as the terminal is no more than
1 metre from the top of the enclosed space. Any existing appliances fitted in a courtyard that do not meet this criteria can be left operational, provided that satisfactory combustion analysis can be achieved, and the appliance is
operating safely.

When it comes to chimney/flue routing and termination, much of the existing information has remained unchanged. However, the standard states that the routing of a chimney/flue cannot pass through either a fire compartment or a property other than that which the appliance serves. 

Figure C.8 in Annex C is a diagram of a house, showing the termination distances required from openings, etc. This has been updated to show additional scenarios and distances, including but not limited to vertical terminals from a wall or other vertical terminals, terminals facing an opening, and terminals perpendicular to an opening on a different wall. 

It is important to remember that these distances may differ from those given in the appliance manufacturer’s instructions (MIs), which take precedence for the installation.

Finally, a checklist has been included in Annex D to help you confirm whether a proposed chimney/flue location is suitable.

BS 5440 Part 2
Important changes to Part 2 cover ventilation. These include updates to many of the tables and figures, including a change to how they are numbered. For instance, the table showing minimum permanent ventilation requirements for flueless appliances has been changed from Table 6 to Table 4. The information in this table remains unchanged.

The main updates surround ventilation provision, including that air vents passing through a fire-rated compartment are permitted only where the ventilation opening is direct to outside, and no vent shall penetrate a protected area.

Any new air vent shall not communicate with a roof space. Existing air vents communicating with a roof space may be acceptable as long as the total free area is satisfactory, and any opening is checked to ensure it is free of any obstruction. 

Another important change is on a ventilation outlet connected to a cooker extract or gas tumble drier. Previously this would not be considered an opening into the building, but this provision has been removed provided that the terminal is fitted with non-return flaps. Therefore, the ventilation outlet is now considered to be an opening. 

Industry standards
The industry standards covering chimney/flues and ventilation for domestic gas appliances (BS 5440 Parts 1
and 2) were reviewed and updated in 2023, and published with new titles:
• BS 5440-1:2023 Chimneys, flue pipes and ventilation for gas appliances of rated input not exceeding 70kW net (1st, 2nd and 3rd family gases) Part 1: Design, installation, commissioning and maintenance of chimneys – Specification
• BS 5440-2:2023 Chimneys, flue pipes and ventilation for gas appliances of rated input not exceeding 70kW net (1st, 2nd and 3rd family gases) Part 2: Installation and maintenance of ventilation provision for gas appliances – Specification

Industry Standard Updates 121 and 122 cover the guidance and changes made to BS 5440 Parts 1 and 2. You can read and download all Gas Safe Register Industry Standard Updates by logging into your online account here.

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