Commercial heating flues

Gas Safe Register’s Technical Team provide an overview of commercial flue systems and the aspects that you must consider. This article appears in the July-August 2024 edition of Registered Gas Engineer magazine.

What are the regulations and standards?
When planning and installing a commercial gas heating appliance, you must ensure that the flue systems are suitable and meet the requirements of BS 6644:2011(1) and IGEM/UP/10 Edition 4(2). 

You must get prior approval from the local authority for the height and termination of flues serving appliances in excess of 333kW net total installed load, unless the installation is a like-for-like appliance exchange. In all cases, flue heights must meet the requirements of IGEM/ UP/10 and BS 6644:2011 unless the local authority issues a waiver.

Consult the flue system manufacturer’s instructions to confirm that the system is suitable for use with the appliances. Where there is any doubt, both the flue system designer/manufacturer and appliance manufacturer should be consulted before installation. 

You should carry out a fire-rating test to ensure that the complete flue system is installed, or shielded, to prevent it being the source of ignition of any part of the surrounding structure and to prevent the products of combustion from entering any part of the building.

Flue construction
All flue materials must be suitable for the safe operation of any connected appliance. In particular, the material shall be fit for purpose in mechanical strength, temperature and corrosion resistance.

Where an appliance can burn more than one fuel, the materials chosen and the flue construction shall consider the constituents of all fuels. A gas or oil-fired appliance shall not be incorporated into a biomass flue system, for example.

Specifically designed sealable points shall be provided in a suitable location for the analysis of combustion gases from individual appliances and from fan-dilution systems.

Flue routes
The route of any flue system shall be as direct and straight as practicable while ensuring the effectiveness of the discharge of combustion products. The use of horizontal runs and 90° bends or elbows should be avoided on natural-draught systems, except those parts forming headers to common chimneys.

Chimneys shall be terminated in a position where the products of combustion will disperse readily into the environment without causing a danger or nuisance to people or property.

They must be supported properly throughout their length. Joints in chimneys inside buildings shall be either located where they can be inspected, where the leakage of combustion products may enter normally occupied spaces, or be able to be tested for integrity. Unlike domestic applications, flue joints are not always accessible and there are various methods to test a commercial flue system to ensure its integrity. 

Where necessary for servicing and maintenance, means shall be provided for the disconnection of the flue from any appliance. All flue systems shall be fitted with adequate test points in the form of a suitable fitting to permit smoke, combustion or leakage testing, as appropriate.

The surface temperature of single-wall flues shall not exceed 70°C; for double-wall flues, the inner flue surface shall not exceed 250°C. Combustible materials adjacent to the chimney must not be subjected to temperatures
over 65°C. 

Where the flue pipe enters a common flue system, the design shall ensure that no interaction, such as combustion reverberation between appliances, can occur.

Where the chimney/flue pipe enters a masonry chimney, the entry shall be inclined upwards at least 45° and not protrude into the chimney beyond its inner face or, if not practicable, where entry into the base of the chimney is less than this, the entry shall not impede the ability of the chimney to exhaust combustion gases safely and correctly.

Connection shall not be made within, or terminate below, 250mm from the base of the chimney and there must be a means to inspect the base of the chimney and flue.

Condensation in flues
Where condensation might occur other than during first start-up of the burner, the flue should be constructed to dispose of water condense to a drain.

Non-condensing boilers
If condensation within the flue is conceivable at times other than during first start-up, an insulated flue, eg, double-walled flue, should be used. Double-walled or otherwise insulated flues should be used for exposed flues.

Condensing boilers
Flue components, including liners, shall be impervious to condensates and resistant to corrosion. The flue shall be designed to operate under positive pressure and be sealed. The condensate drain shall have an internal diameter of not less than 22mm.

A condensate trap should be installed as close as practicable to the condensate outlet at the base of the flue (see below).

Condensate pipework shall be protected against frost and be corrosion-resistant. It shall not permit the leakage of combustion products or condensate into any building or plant room. Where condensation could occur, the flue and chimney shall be designed so that the condensate can flow readily to a drain. The slope shall be a minimum of 2.5°. The system shall prevent the emission of combustion products from any condensate drain, particularly those relying on water seals that may dry out.

Any horizontal condensate pipe shall be supported adequately to prevent sagging and installed so that condensate always flows towards the drain.

Open-flue boiler flues: Type B
If an open-flue boiler is to be fixed to an existing flue system, you must first ensure that the boiler is suitable. Unless stated otherwise, the cross-sectional area of the flue natural-draught system shall not be less than the flue outlet of the boiler.

Room-sealed boiler flues: Type C
The flue system shall be assembled and installed in accordance with the manufacturer’s installation instructions, with the flue terminal positioned externally so as to allow dispersal of the products of combustion and the intake of air.

Flue termination
A terminal shall be fitted if any dimension across the flue outlet is less than 170mm. Flue terminal positions for fanned or natural-draught flues shall be clear of obstructions and clear of any opening into a building: for example, a window, a door, or air inlet. The minimum clearance from horizontal and vertical flue terminals to openings into a building is provided in Figure 7 of IGEM/UP/10 and is based on the heat input of the appliance(s). 

Mechanically assisted flues
Manufacturers should be consulted before using mechanically assisted flues, to determine the appliance suction conditions and limitations to ensure safety. The resistance of the flue shall be calculated by the flue system designer and the fan should be sized to ensure the full volume of combustion products are removed from the connected appliance(s) under all weather conditions. If this is not achieved, it will result in the appliance(s) locking out.

Consideration should also be given to changing suction conditions that may occur due to the modulation of the air and gas flow to burners. 

Fan diluted flues
It is recommended not to install fan dilution systems above 2MW net because the separation distances between inlet and outlet louvres may become excessive. Fan dilution systems shall be designed to ensure the products of combustion are diluted to give maximum levels as follows:
• CO2: 1%
• CO: 50ppm
• NOx: 5ppm.

Duct diameters must ensure that duct velocity is 6m/s to 8m/s. The preferred design is for the air intake to be taken directly from outside. Only in exceptional circumstances, where taking air directly from outside is impractical, may air for dilution be taken from within the plant room, provided that:
• The additional air is accounted for within the appliance installation space ventilation
• The natural high and low ventilation openings shall be at least 0.5 times the extract fan rating (m3/s)
• Taking air from within the appliance installation space does not affect the safe operation of the appliances(s) or combustion performance
• Louvres providing ventilation to the plant room are connected directly to outside air, and combustion products cannot re-enter the plant room, with discharge grilles sited on a different wall from the air inlet grilles.

Manual flue dampers
Any damper or stabiliser shall be constructed of suitable grade stainless steel. A manual flue damper shall not be fitted except in cases of common and fan-diluted flues, when it shall be locked in position. 

Any control or balancing damper shall not block the flue by more than 75 per cent of the cross-sectional area. Manual flue dampers shall not be fitted into a structural masonry flue, such as a brick-built chimney. A position indicator should be fitted to each flue damper.

Automatic flue dampers
Automatic flue dampers shall not be installed unless they are specifically allowed by the boiler manufacturer’s installation instructions. Where one is fitted, it must incorporate the necessary safety devices and interlocks.

A flue damper shall only be fitted in the secondary flue unless the appliance uses a forced draught burner and does not have a flue break. A flue damper shall not be fitted into the common flue of multiple or combined appliance installations and should be located so it can be removed easily for inspection, repair or maintenance.

Failure of a flue damper to be proven in the correct open or closed position will lead to a lockout condition, preventing the appliance from operating.

(1) BS 6644: 2011 – Specification for installation & maintenance of gas-fired forced convection air heaters for commercial & industrial space heating (2nd & 3rd family gases)
(2) IGEM/UP/10 Edition 4 with amendments March 2016 & February 2017 – Installation of flued gas appliances in industrial and commercial premises