TB035 - flue-and-chimney-design-installation-maintenance

TECHNICAL BULLETINFLUE AND CHIMNEY DESIGN, INSTALLATION &MAINTENANCENote: All figures and tables have been taken from the Building Regulations Approved Document B1Flues and chimneys, carrying hot combustion products out of buildings, can present significant health andsafety risks and the risers and voids they are located in, if not carefully designed, can provide routes for firespread. This technical bulletin highlights some issues surrounding design, installation and inspection forlarge commercial and high rise residential buildings.1. INTRODUCTIONFrom the British Chimney & Flue Manufacturers Association2:The terms flue and chimney are often used interchangeably. The flue is the working part of the chimney,conveying the products of combustion safely to the atmosphere. A chimney includes the outer wall and theflue. A chimney works under negative pressure drawing the product of combustion from the appliance. Asuccessful “chimney draw” is dependent on the following principles:- The difference in air pressure between the appliance and the top of the chimney (created by the height ofthe chimney)- The difference in temperature between the appliance’s flue gases and the outside ambient air.- The quality, density and thickness of insulation along the length of the chimney directly impacts thechimneys performance – therefore internal chimneys are best.- The route of chimney. The straighter and more vertical the chimney the better as there is less resistanceand more reliable draught.It is essential to choose a flue diameter and chimney route suitable for the output from the appliance. Anadequate combustion air supply is also required for the appliance & chimney to operate safely andefficiently – this is determined by the air tightness of the house and the power output rating of theappliance.1 https://www.gov.uk/government/publications/fire-safety-approved-document-b2 http://www.hetas.co.uk/wp-content/mediauploads/BFCMA-General-Guidance-10-12-12.pdf TB/035 1 of 4

FLUE/CHIMNEY INSTALLATION & MAINTENANCETB/0352. FIRE COMPARTMENTALISAION A key part of a passive fire strategy is compartmentalisation of a high rise or large commercial building – it is therefore critical to ensure the integrity of compartments and fire walls is maintained when various building services are distributed. Any chimney pipe passing through a compartment wall or floor which is fire rated should be contained within a riser shaft that has a minimum fire rating equal to half that of the compartment wall or floor itself to prevent the spread of fire. Such a protected shaft may require a fire rating anywhere between 60 minutes and 240 minutes.3. BEST PRACTICE THROUGH FRONT-END DESIGN BOILER/PLANT ROOM LOCATION All too often, insufficient consideration is given to plant room/boiler/flue location during building design. The safest route for a flue system is directly vertically through the building – this also offers the best efficiency for the appliances the system is serving. A common chimney shaft going straight up through the entirety of the building promotes the use of a series of cascade boilers set-up in a district energy system for the building. This enables the overall power output to be reduced as the peak load is spread across the building instead of each dwelling having a boiler designed for peak load for that dwelling. HORIZONTAL FLUE PIPE Horizontal lengths of flue pipe should be used as a last resort and kept to a minimum if necessary due to the increased risk of blockage and soot build up. For solid fuel appliances, 150mm maximum horizontal length is allowable from the back of the appliance to the chimney. Where horizontal flue pipe is utilised for gas appliances, the minimum fall is 3° - therefore there should be a 52mm vertical rise for every 1m length of horizontal flue. Where flues are incorporated in ceiling voids, the void height needs to be sufficient to allow for this minimum fall. JOINTS Joints should be made in strict accordance with the manufacturer’s instructions to prevent the release of toxic flue gases and condensate. Once installed, the system shall be subject to a smoke test to confirm the integrity of all joints as below. Silicone sealant should not be used to ensure a leak tight seal on a joint as the sealant can be quickly degraded by acidic condensate leading to leaks.TB/035 2 of 4

FLUE/CHIMNEY INSTALLATION & MAINTENANCETB/035 SUPPORT It is critical that any flue pipe/chimney is supported adequately over its entire length. Guidance on support centres and type can be sought from the manufacturer’s instructions. Inadequate supporting can allow for the pipe to sag. This increases stressed in the joints and can lead to joint failure and the release of toxic gases and condensate. On horizontal sections, sagging can prevent the condensate from draining as designed and can lead to exhaust blockage affecting the efficiency and safe function of the appliance.4. SMOKE TEST Should the integrity of a chimney or flue be called into question via visual inspection, or is to be re-commissioned after an extended period of non-use, it is advisable to carry out a smoke test in accordance with BS EN 15287-1 for open fires and BS EN 15287-2 for room sealed appliances. It is best practice to conduct a smoke test on all new installations.5. FIRE RATING OF BOILER AND ASSOCIATED DUCT For a gas boiler, the outlet flue duct from the appliance to the common chimney shaft is considered a part of the boiler appliance and the common chimney shaft it connects to is considered a part of the building. If the duct cannot exit the appliance directly through the adjoining wall, leaving exposed flue pipe, this can present issues in determining the appropriate fire stopping and compartmentalisation for the boiler and duct as the boiler appliance itself is unable to be fire tested. One potential solution is to box in the boiler in its own fire rated compartment. Compartment venting, if required by the appliance manufacturer’s instructions, should be provided via intumescent vents which will expand and seal in the event of a fire to maintain the fire compartmentalisation.6. INSPECTION It is a mandatory requirement to have gas appliances inspected and serviced a minimum of every 12 months. Part of the service includes the flue inspection where the full length of the flue, including every joint, must be inspected to confirm integrity. Therefore, it is mandatory for inspection hatches (minimum 300mm x 300mm within 1.5m of each flue joint) to be present to allow the flue to be inspected along its full length in order to confirm, in accordance with Approved Document J, that: • The flue is continuous throughout its length; • All joints are correctly assembled and are appropriately sealed; andTB/035 3 of 4

FLUE/CHIMNEY INSTALLATION & MAINTENANCETB/035 • The flue is adequately supported throughout its length • Any required gradient fall and drain points are included. Alternatively, inspection can be made via other means such as camera.7. COMBUSTIBLES MATERIAL IN FLUE RISER Flue risers should be designed to form a fire compartment, preventing the spread of fire via the riser itself. Therefore, there shall be no flammable material contained within the flue that could directly transmit fire throughout the building. When flues are in concealed voids/risers – this is sometimes viewed as an ideal place to run cables as there are no floor/wall breaks throughout the length. It is important to ensure the flue compartment is free from any combustible materials.8. TO CONCLUDE… …all flue and chimney installation work should be carried out by a registered HETAS installer. All metal flue and chimney products must carry CE certification in accordance with the European Standard and the installation must be in accordance with the manufacturer’s instructions. A carbon monoxide alarm should be included on every installation. Thank you to Poujoulat Chimneys for their assistance in authoring this bulletinFor any technical queries please call 020 7313 4927 or email [email protected]: this document is based on knowledge available at the time of publication and is meant for general purposes, not for reliance on in relation to specifictechnical or legal issues, in which case you should always seek independent advice. No responsibility of any kind for any injury, death, loss, damage or delayhowever caused, resulting from the use of the advice and recommendations contained herein, is accepted by the authors or others involved in its publication(including the Building & Engineering Services Association). 15/08/2017TB/035 4 of 4