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Flueing and Ventilation requirements
Chimneys systems for open flued appliances come in various types and materials ranging from masonry, brick or concrete, rigid metallic, flexible metallic liners, vitreous enamel and plastic.
Original rigid non-metallic Asbestos chimneys systems can also be used if it is good condition and does not require any modification to the structure and they meet the requirements of BS 567 and BS 835 and the up-to-date requirements for the correct routing and termination etc. Reusing an asbestos chimney would require a full risk assessment and special safety precautions when carrying out this work.
A Chimney systems design and construction varies on the type and size of the appliance its serves. It also should comply with relevant local building regulations, in England and Wales Approved Document J and BS 5440-1:2008 “Flueing and ventilation for gas appliances of the rate in put not exceeding 70kW net” and in consultation with the appliance manufacturer’s installation instructions.
Chimneys are also required to comply with the British Standard appropriate to their construction. A chimney is defined in BS EN 1443 as a structure that is made up of a wall or walls enclosing a flue or flues to remove the products of combustion from a type B or C (open flued or room sealed) gas burning appliance to the outside air. It can also be designed either to remove the products of combustion by natural convection (thermally) or with the aid of a fan to force or draw the PoC’s into the chimney system.
Masonry and brick chimneys can be used for certain gas appliances without a flue liner dependant on the length of run and guidance is given in Table 13.9 of the gas regulations. All Back Boiler Units are required to be flued using a flue liner and most gas boiler manufacturers require flue liners to be used when using a masonry or brick constructed chimney.
Poured concrete liners cannot be used for new brick or masonry chimneys but are an alternative to flexible stainless steel in the correct environment and compliant with gas type and appliance. This type of liner is required to be installed by a competent installer approved by the British Board of Agrement (BBA).
Brick or Masonry chimneys were commonly lined with clay, salt-glazed clay or high alumina cement pipes if built after 1965 and as such will generally meet gas regulations for most open-flued gas appliances subject to manufacturer’s instructions. Brick or Masonry chimneys built before 1965 may still be appropriate depending on condition and the appliance type but must be lined using suitable liner. It is essential that this type of construction that the route of the entire chimney is checked to ensure that the lining is continuous and that there is no obstructions or restrictors. The chimney may require sweeping if not previously been used for a gas appliance. The appliance shall not be connected through any fuel burning redundant equipment but shall be installed directly into the chimney system. Where a solid fuel back boiler is left in position, it should be treated to prevent any build up in pressure. Technical bulletin 101 highlights the HSE’s safety alert risks from redundant solid fuel back boilers. A chimney system should also only be able to serve one appliance.
A flue flow test should be carried out to ensure that the flue/chimney is not damaged by cracks or missing brick works, to brick or masonry constructed chimneys or that there is no corrosion to the flue pipes, bends or liners that would prevent the correct operation of the flue. The flue flow will also check the adequate ventilation for combustion. A flue flow test is carried out using a smoke pellet designed for the purpose. The pellet is placed at the base of the chimney/flue and all doors and windows to the room should be closed. Once the pellet is ignited it will produce approximately 5m³ of smoke. The flue should draw the smoke and a visual inspection taken along the route to guarantee that there is no escaping smoke through any part of the chimney/flue except at the proper terminal to the outside environment.
The flue may be preheated by supplying heat from a blow torch for a minimum of 10 minutes if the flue is unwilling to draw the smoke, also if there is an existing appliance this can be used to heat the chimney/flu prior to the flow test.
Flues operate with a basic natural method. Where the products of combustion are warm, rise to be replaced with fresh air. Where the PoC’s are extracted to the outside environment by means of an open flued terminal, it is important to prevent a location which may affect the correct operation. Natural wind movement, creates positive and negative pressure zones which may effected the flues performance in removing completely the products of combustion or where the PoC’s may affect neighbouring properties.
Fig 1 below illiterates where the building is located 20 meters near an embankment which is the same height of the building of 10 meters. Although a natural draught flue is fitted in partial compliance of table 13.11. I.e. “ no part of the flue outlet should be closer than 1.5 meters measured horizontally to the roof surface or 600mm above the ridge. The natural movement of the wind may cause a negative pressure on the flue terminal resulting in the PoC’s not being fully removed.
Fig 2 below illiterates where the same building located 20 meters near an embankment which is the same height of the building of 10 meters. Now fitted with a flue terminal at a height of 2 meters and fitted with an extractive terminal allowing for the full extraction of PoC’s.
Fig 3 below illiterates a building that has just had a loft conversion. The open flue terminal shown serviced the original boiler. This may lead to thermal inversion and complaints of periodic fumes due to the fact that heavier cooler air can enter the building through the flue terminal and the lighter warm air is dispersed to the atmosphere through the dorma window in the loft conversion.
Fig 4 below illiterates the same building. Although there are several solutions to prevent this such as “sealing all unnecessary high level openings” I believe would prove unsatisfactory to usage of the loft conversion. This may reduce the complaints of periodic fumes but will not eliminate it. A solution would be to replace the flue in line with table 13.17 and 13.19 where the flue terminal is not less than 1.5 meters from the roof apex and not less than 600mm above the roof line and not less that 600mm to the dormer opening.
Fig 5 below illiterates a building in which the open flue terminal is located in accordance with table 13.11 but it is located on the windward side of the building. The wind blowing across the terminal will result in may result in an increase or decrease flow upwards within the flue and intermittent down draught. The down draught can result in the products of combustion being forced back into the room in which the appliance is located and out through windows/opening on the leeward side.
Fig 6 To alleviate the risk of the wind blowing across the terminal and resulting in increased or decrease flow upwards within the flue and intermittent down draught. Ventilation of adequate fresh air supply should be provided on the windward side within the room in which the appliance is located and not on the leeward side. The flue terminal should also be raised out of the positive pressure zone.
A vertex flue is classed asa Vertical flue to outside air with air supply ducts in loft. Draught diverter in loft above air inlet servicing a room-sealed appliance.
The Vertex flue shall be installed in accordance with the instructions provided by the chimney manufacturer and BS 5440-1. It takes the supply of combustion air from the roof void through a Draught Diverter which must be located at least 300mm above any loft thermal installation. The flue should be fitted vertically however if a change in direction is unavoidable, the flue above the Draught Diverter must run for a least 600mm before any bend. The offset section of flue shall not exceed 20% of the total length from the appliance to and terminated through the roof with a normal terminal.
The Vertex flue operates by taken adequate fresh air for combustion from the loft space. Provision shall be required to be made for an adequate unobstructed air supply to the roof space in which the draught break is located. The draught divert allows fresh combustion air in to be used by the appliance while preventing any drown draught from the terminal. It also prevents excess flue pull on the products of combustion inhibiting the combustion process.
The draught diverter allows the POC to be diluted with fresh air allowing for the cooler fresh air to aid travel of the products of combustion. As the flue is require to be manufactured using stainless steel and runs through the roof void it may be susceptible to condensation therefore there is a requirement to remove this condensate.
Vertex flue and natural draught open flue systems are somewhat similar in that they both use the natural thermal physics of hot air rising being replaced by an equal quantity of clean cooler air both have different processes. The natural draught open flue system operates using a single piped flue where the appliance which it serves takes the fresh air for combustion is taken from the room in which the appliance is located. While the Vertex flue operates using two sections of flue pipe. The first which is usually apart of boiler is made up of two pipe system the inner one to remove the products of combustion and an outer flue pipe to bring fresh clean air to the appliance. The first section is connected to the second by a draught diverter, which allows separated fresh air to be supplied to the appliance from the roof void, while allowing some fresh air to be mixed with the products of combustion to aid extraction. Due to the way the vertex operates is will not be prone to down draughts as the natural draught system as the fresh air intake and the outside air will be at the same pressure and relatively the same temperature. One issue with the vertex draught diverter is that the area around the diverter needs to remain free and the area of the roof void is usually used as a store for unwanted items by the owners which may introduce a risk.
Natural draught balanced flue and fanned draught balanced flue appliances are room sealed. Both incorporate a ducting system that allows fresh clean air for combustion to be taken directly from the outside atmosphere into the appliance. Both these types of appliance are fundamentally safer than open flued appliances as they require no air or cause air movement in the room in which they are located and products of combustion cannot generally leak back into the room.
Natural draught balanced flue appliances are driven by the hot POC gases creating a negative draw on the fresh air to aid combustion and as such have relatively short flue. Generally the length of the wall on which it is mounted. These appliances are only suitable to be mounted on an external wall within the building. The flue terminal has the outlet situated centred surrounded by the fresh air intake, with both ducts having the same cross sectional area. Due to the closeness of the intake and extract duct within the terminal any gusts of wind will effect both ducts similarly and the burner should not be effected due to the pressures cancelling each other out.
Fanned draught balanced flue appliances utilise a fan to pressurise the movement of fresh air into the appliance and the products of combustion to the external atmosphere. The addition of a fan allows for the flue system to be of a concentric nature with significantly smaller cross sectional area of that of the natural draught balanced system. It also allows for longer flue runs either vertical or horizontal or a mixture and therefore more versatile locations of appliances. The provision of adequate ventilation of fresh clean air for combustion to both of these appliances is vital, but use of fanned draught balanced flue systems in most circumstances require less minimum clearances for structural and temperature considerations for the external terminals, but should still be installed in compliance to manufacturers installation instructions.
Fanned draught balanced flue appliances are manufactured in two style either positive or negative pressure case systems. Appliances in which the fan is designed to draw the fresh air in to the appliance is referred to being an appliance under positive pressure. Where the casing is under negative pressure is where the fan is designed to drive the products of combustion from the appliance. The positive pressure case system is more at risk of contaminating the room with products of combustion if the combustion chamber casing is not securely. The levels of carbon monoxide will also sharply rise due to the poor combustion that will take place.
Balance flue terminals should be situated away from any openings to the building to allow for the complete dispersal of the products of combustion to the atmosphere and to ensure that they cannot accumulated in motionless pockets around the building, be carried back into the property or transmitted into neighbouring properties.
Guidance to the minimum distance from the flue rim to any building opening such as doors, opening windows, air bricks etc. is given in BS 5440-1 and Approved document J. approved document J also defines the boundary as “land or buildings belonging to and under the control of the building owner.” The boundary is also considered to extend to the “centreline of any adjacent waterways or routes such as paths, streets, rights of way, rivers, canals or railways.” Terminal should be positioned not less than 600mm facing and 300mm parallel to the boundary line. Fanned balanced flues can be angled to 45° to give a clearance of 600mm on a 300mm boundary. Terminals should also not be located within 2 meters of an opening in an adjacent building. The boundary lines allow for any neighbouring property to be extended without have an effect on the performance of the flue to discharge the products of combustion in a safe manner.
The dimensions noted previously, detailed in BS 5440-1 and within manufacturers installation instructions are set-out in a detached building. Room sealed appliances and therefore balanced flue terminals can fitted within extensions within semi-detached or terraced buildings, in which the extension is built right up to the boundary line. Appliances which are fitted at these locations and breach the neighbouring boundary maybe subject of trespass compliant from the neighbour, concerned of products of combustion entering the neighbouring building. In the event that a condensing boiler is fitted in cold weather the products of combustion tend to condense in the cold air and form a plume. Pluming kits are readable available from the boiler manufactures to minimise the effect of pluming. The neighbour may in the future build an extension ultimately blocking or restricting the flue. The neighbour may also consult with the local Planning Office, or the Environmental Health Officers, who may under the powers of the Environmental Protection Act 1990, switch off and cap off the offending appliance to prevent further use.
Carports or building extensions have additional requirements in that they should be completely open on at least two sides and that the terminal should be at least 600mm from the roof for a natural draught terminal of 200 from a fan flued terminal allowing for the products of combustion to freely disperse. As with basement, light wells, etc. terminal should not be located in areas in which the products of combustion will not easily disperse.
Areas surrounding terminals should be protected to prevent burning or scorching. Where the terminal is a low level it may be necessary to fit a cage over it to prevent anyone or animal coming into contact with the hot surface. Similarly where the flue is fitted at high level near the roof eaves. The eaves require protection of a heatshield.
The heat input rating also affects the location of the flue terminal as the higher the rating of the appliance the higher level density of product of combustion around the terminal. Resulting in the need for a wide disperse area.
“Se-duct” and “U-duct” are categorised as shared chimney systems into which service multi, room sealed appliances. These types of chimney systems can be found in multi storey single buildings but predominantly can be found in blocks of flats. Both Se and U ducts require specialist design as the correct dimensions of the duct or ducts are critical to ensure the appropriate operation. Both duct types use the same chimney system to supply air for combustion while serving to remove the products of combustion from the same appliance.
Se-ducts are fabricated from standard sized pre-cast concrete segments stacked to form one vertical chimney. The fresh air intake for combustion is supplied from a low level duct at the base of the Se-duct. The combustion air duct should run from one side of the building to the other to prevent negative pressure or from a single duct from a neutral pressure zone such as a court. The intake ducts shall be protected from debris by removable grills. The grills should be sized as to allow a 10mm diameter ball but no that of a 16mm diameter ball. The products of combustion shall exit the duct beyond the roof level with the terminal position at least 6m above the upper most installed appliance and in line with table 13.11 of the domestic essential gas safety guidance. This may mean that the appliance on the top floor has to be ducted separately from the other appliances in the building. Where the roof is pitched the bottom of the terminal outlet shall be above the ridge level of the roof.
U-duct systems are used when it is not feasible or achievable to have the air for combustion supplied from low level. U-ducts are similar in construction to the Se-duct but consist of two vertical stacks with a connection chamber at the bottom. Allowing for air for combustion to be supplied from above roof level down to the appliances using one duct while the products of combustion are discharged using the other duct.
It is important that the only appliances suitable for Se and U duct chimney systems are used on these installations. Only Natural Gas appliances that the manufacturer confirms can be used on these flue systems should be allowed. In the majority of cases the originally design of these types of chimney took into account only the relevant calculations that would have been based on standard efficiency appliances.
The flue gases differ in temperature between high efficiency and standard efficiency boilers and could potentially affect the performance of the chimney.
Due to the materials used in the production of these chimneys they are not normally suitable and may react to condensate, the by-product of condensing boilers.
BS 5440-1 stipulates that only appliances that are classified and approved as type C2 and C4 are suitable to be used with Se-duct and U-duct systems. Although there is no issue whether the replacement boiler is that of a system boiler or combination.
It is also recommend that appliances installed within the top third of the chimney are commission to ensure that the combustion air used by the top-most appliance contains a level of no more that 1.5% per volume of carbon dioxide.
Technical Bulletin 123 and the Institution of Gas Engineers and Managers provide guidance regarding appliances suitable for Se-duct and U-duct system.
Each appliance fitted to these types of ducts should be fitted with a label explaining that they are fitted to a Se-duct/U-duct flue system. It should also state that the when these systems were originally designed, the relevant calculations would have been made based on standard efficiency appliances being utilised. Should not be remove without consulting the person responsible for the building and should also provide the contact details.
Fanned draught flue appliances utilise a fan to pressurise the movement of fresh air into the appliance and the products of combustion to the external atmosphere. The fan can be an integral part of the appliance (positive pressure) or located in the outlet to a chimney or flue system (negative pressure) being supplied as part of the appliance and in compliance with manufacturer’s instructions.
The addition of a fan allows for the flue system to be of a concentric nature with significantly smaller cross sectional area of that of the natural draught balanced system. It also allows for longer flue runs to be either vertical or horizontal or a mixture and therefore more versatile locations of appliances. Where balanced draught flue appliances require it be fixed to and external wall allowing to be vented through a small length of duct. Fanned draught flue appliances can be mounted on an internal wall some distance from the external wall. The fanned draught appliances still have a limitation as to how far the fan duty can push the products of combustion and pull the air for combustion from and to the appliance but some fanned draught flue appliances can be fitted up to 30m from the terminal. This is always subject to manufacturer’s instructions. To combat drown draughts or high wind effect in high-rise blocks and to allow for longer duct runs appliances are fitted with centrifugal fans instead of axial fans as centrifugal fans are designed to have a higher duty allowing to remove damp heavy air as is the products of combustion. In the case of condensing appliances the flue must be angled to allow any condensate to run back into the appliance and not rest within the flue. Bends within the flue cause resistance and affect the air flow with in the flue reducing the length. Therefore bends should be sweeping and kept to a minimum in line with BS 5440-1. The flue duct terminals shall be sited so that the wet combustion products are unlikely to cause damage or nuisance.
With Fanned draught flue appliances becoming increasingly popular around 1995. Allowing builders of new builds to have more scope in the design of a building as to where the appliance was installed and making the most of the available space. This also applied when replacing a balance draught flued appliance, as it gave the heating engineer more scope to fit the boiler in an unobtrusive place such as a cupboard in the middle of the building.
Flues which could look unsightly could easily be routed within the ceiling void or boxed in. in some cases flues from one property could pass through another property on its way to the terminal. This type of installation became common from the turn of the century within new build flats and apartment blocks. In addition to the ceiling voids and purpose made enclosures any other enclosure such as floor voids, service risers and roof voids which prevent a visual inspection of the length of the flue are common in class.
This practice could lead to the increased risk of dangerous levels of carbon monoxide leaking within the building if there was a combination of an appliance not operating correctly and a fault on the flue.
The heating installations may have been installed and commissioned before any purpose made enclosure where installed reliant on the next service date before this would have been identified. Although it has always been a legal requirement of the Gas safety (installation and use) Regulations, (GSIUR) that chimneys and flue should be safe and that engineers should check the flues. Other trades and professionals would only look at their own functions and not be aware of this requirement.
The gas industry has worked zealously to ensure other relevant British standards, Industry guidance, building standards and manufacturer’s instructions have now all adopted the same information that flues within voids require to be periodically inspected and require a means to do so. Building Regulations Approved document J is seen as best practice by all Health and Safety organisations has been amended to include the requirements to install inspection hatches and gives guidance on the size and the locations of each hatch.
Corgi introduced a technical bulletins in 2007, TB 200 when it became aware of the trend by building designer to place flues from fanned draught flued appliances within an intermediate ceiling void in new build multi storey flats and apartment without the facility for inspection at commissioning and through the life of the installation during routine maintenance and servicing. TB 200 reiterates the requirements of the Gas Safety (installation and use) Regulations checks that the flue is continuous throughout its length, all joints are correctly assembled and appropriately sealed. Also that the flue is adequately supported throughout its length. TB200 emphasise is on adequate risk assessment and once taken into account information provided by, the user, checks on the flue system to check the system complies with manufacturer’s instructions and walking the length of the flue run checking for signs of staining on the ceiling, Operational checks of the appliance, The guidance suggests that if theses checks are satisfactory the system can be deemed to be “Not to Current Standards”. If there are concerns the installation should either be deemed “At Risk” or “Immediately Dangerous”.
In October 2008 the Health and Safety Executive released a safety alert titled “Gas boilers – flues in voids” as a result of a death earlier in 2008. The advice given was owners to contact their builder or a registered gas engineer. If a property owner, ensure they have their gas boiler serviced and flue checked at least every 12 months by a Gas Safe registered engineer. If a tenant living in rented accommodation then bring the alert to the attention of your landlord or managing agent and to fit an audible CO alarm.
The HSE alert led to the publication of TB017 and TB008 edition 2 in 2010. TB008 Stated that inspection hatches were now incorporated by the Building Regulations Approved Document J and landlords would be required to retro fix inspection hatches. Landlords were given a grace period of two years to implement these works by 31st December 2012. The bulletin still had emphasise on adequate risk assessment even providing an example colour coded risk assessment. The guidance suggests that if theses checks are satisfactory the system can be left operational and deemed to be “Not to Current Standards”. If there are concerns the installation should either be deemed “At Risk” or “Immediately Dangerous” and the appliance should be switched off Carbon monoxide detectors were now recommended for each room in which the flue ran.
TB 008 edition 2.1 was published in April 2012. This bulletin confirms that the only way that an engineer can comply with the GSIUR is to retrospectively fit Inspection hatches if none are present. The technical bulletin also suggest that endoscopes or carbon monoxide monitoring maybe an alternative in confirming that the integrity of the concealed flue is sound. Although it does state that these methods must meet the requirements of GSIUR 1998. In existing systems where there is no provision to access and inspect the flue system throughout its length a risk assessment is required as detailed in the “Chimney systems in voids risk assessment process”. This includes if the flue runs through a neighbouring property. Although the system within the property in which the appliance is located is in full compliance with GSIUR. If the flue which runs through a neighbouring property cannot be inspected up to 31 December 2012 the appliance may be left operational but after this date it should be classed as at risk and with the permission of the owner the appliance should be turned off and relevant labels attached. The owners will be able to refuse to have the appliance turned off in this scenario the engineer should ask the consumer to sign a declaration to state that they “accept responsibility for the defects which could result in a serious incident”. The bulletin also informs of the need to report any Immediately Dangerous installations to the HSE under the Reporting of Injuries, Diseases and Dangerous Occurrences Regulations (RIDDOR).
TB 008 edition 3 highlights the fact that in the case of exiting voids they may not be suitable for an inspection hatch due to the size or if a hatch was fitted may not provide suitable access to allow for an inspection of the whole flue. In these situations to allow the system to comply with GSIUR a carbon monoxide void monitoring safety shut-off system may be fitted. A (COSSVM) is a system which monitors the air within the void for signs of carbon monoxide and If detected the system will shut off the appliance. Another example of exception to hatches is where a vertical flue passes through a flat or pitched roof. Although there are certain criteria that have to be meet such as there is no changes in direction, there is no signs of distress to indicate there may be an issue, the length of the flue does not excessed a single component length and that documentation confirms that there is no joint within the concealment. Where the flue runs through a neighbouring property there shall be all reasonable steps taking to get access for inspection if all steps fail and the rest of the appliance checks are correct then the appliance can remain functional. The engineer in addition must leave documentation with the neighbouring property explaining the requirements and the need for access and register the information regarding the installation with the Flues in Voids database hosted by Gas Safe. The technical bulletin also recommends that where there are inspection hatches and the whole installation complies with manufactures instructions and GSIUR 26(9) that room monitoring carbon monoxide detectors are installed in each
neighbouring properties and although room in which the flue passes.
The technical bulletins produced since 2007 have progressively informed of ways to ensure the safety of the consumer and the occupier of the practice of running a flue through another property is now not acceptable, there will still be systems which will not be fully compliant until these existing appliances are refurbished.
The concealed room-sealed fan-draught boiler assessment process flowchart is an activity diagram which gives a graphical representation of the actions to plan the correct procedures to flow when dealing with a boiler which has its flue enclosed within a void. This flowchart is made up of rectangle “process boxes” with diamond shape “decision boxes” which checks an input or condition before carrying on. The boxes are linked with arrows to show direction of the flow.
Following the chart will allow the gas engineer to determine and distinguish the correct procedure and providing the correct information in dealing with flues in voids.
The first decision box asks whether the flue is concealed or not, and if not directs the engineer to a process box. Which gives advice that as long as the boiler is in accordance with manufacturer’s instructions and passes the checks required by GSIUR 26(9) the boiler can be deemed safe, and left operational. If the flue is concealed the engineer is directed to a decision box asking if the flue is routed through an adjacent property. If so they are directed to another direction box which asks if there is adequate hatches and carbon monoxide detectors present. With hatches and CO detectors present the engineers directed to the process box which explains that if the boiler is installed in accordance with manufacturer’s instructions and passes the checks required by GSIUR 26(9) the boiler can be deemed safe, and left operational.
The same decision box will direct the engineer to another direction box which asks if entry to the adjacent property is available. If yes the engineer is directed back through the process that if there are inspection hatches and CO alarms and as long as the boiler is in accordance with manufacturer’s instructions and passes the checks required by GSIUR 26(9) the boiler can be deemed safe, and left operational. Where no access can be gained the engineer is directed to a process box to take all reasonable steps to verify that the flue is reliable throughout its full length by attempting to gain access by engaging with the property owner, building representative, recorded delivery letter. If all access fails the property details shall be recorded on the Gas Safe, Flue in Voids database. The engineer is then further directed to a decision box which asks if there is any recorded evidence of any historical issues or repairs concerning the flue section, which runs through the neighbouring property. If no evidence is apparent the engineer is directed to a discussion box which explains that, if it can be concluded from examining the rest of the installation and that there is no apparent evidence that the flue is unsafe and that the boiler is installed in accordance with manufacturer’s instructions and passes the checks required by GSIUR 26(9) the boiler can be deemed safe, and left operational.
Where evidence does exist the engineer is directed to a decision box which asks does the installation represent an immediate danger. If yes then the installation should be classed as Immediately Dangerous, warning labels should be attached and with permission the boiler should be decommissioned. A RIDDOR report should also be filed on the HSE website. If no permission is granted to decommission the boiler then the engineer is required to contact the Gas Emergency Contact Centre.
Where at the decision box that the flue is not routed through an adjacent property the engineer is directed to another decision box which asked whether there is an satisfactory method to examine the entire flue system along with CO alarms are fitted with the answer being no the engineer is directed to a decision box that enquires if a carbon monoxide safety shut-off void monitoring system is installed. If yes is it operating correctly if yes then the engineer is directed to a process box which declares that as long as the boiler is in accordance with manufacturer’s instructions and passes the checks required by GSIUR 26(9) the boiler can be deemed safe, and left operational.
The boiler installation should be classed as At Risk is there is not an operating COSSVM the engineer is then directed to process boxes in which they are instructed to perform all appropriate safety and combustion checks to ensure that the boiler is operating safely and correctly. At risk warning notices are to be attached, warning notice issued and the boiler should be turn off with permission from the responsible person/landlord. The engineer should also recommend to the responsible person that suitable CO protection equipment should be installed and existing CO detectors tested.
When no carbon monoxide safety shut-off void monitoring system is installed at the decision box. The engineer is directed to whether there is any evidence of staining or leaks along the route of the flue. Where the is no evidence the engineer is directed to class the system as At Risk and perform all appropriate safety and combustion checks to ensure that the boiler is operating safely and correctly. At risk warning notices are to be attached, warning notice issued and the boiler should be turn off with permission from the responsible person/landlord. The engineer should also recommend to the responsible person that suitable CO protection equipment should be installed and existing CO detectors tested.
A sign of staining leaking or any sign of distress on the route leads the engineer to class the system at Immediately Dangerous and warning labels should be attached and with permission the boiler should be decommissioned. A RIDDOR report should also be filed on the HSE website. If no permission is granted to decommission the boiler then the engineer is required to contact the Gas Emergency Contact Centre.
Permanent ventilation for complete combustion is required for most gas appliances. Balance draught and fanned draught room sealed appliances draw air in directly from outside using ducts and expelling the exhaust gases, products of combustion back to the atmosphere.
Open flue appliances, use the air for complete combustion from the room in which they are located, and expel the products of combustion through a flue/chimney system.
A flueless appliance requires the air from the room in which it is located for combustion and discharges the products of combustion back into the same room. A gas cooker being an example.
It is important the correct level of ventilation is provided when required for each appliance. Adventitious ventilation is where natural ventilation arises when the outside air flows through gaps in and around doors, windows, wooden flooring etc. it is estimated equivalent to 35cm². Adventitious ventilation is normally sufficient ventilation for appliance that has a heat input up to 7kW.
Manufacturer’s instructions are required to state the minimum ventilation required for their appliance and must be used in the first instance. Guidance is also provided for the requirement for ventilation and flueing of all gas appliances of rated heat input not exceeding 70kW (net) can be found in British Standard BS 5440-1 and BS 5440-2
Ventilation within compartments is a requirement for providing air to achieve complete combustion for certain gas appliances, but also to maintain airflow to prevent some appliances overheating.
Room sealed boilers located with in a compartment do not require ventilation from the compartment for complete combustion but may require ventilation to prevent the appliance overheating.
Older room sealed appliances operate by having a single chamber in which the hot gases pass the heat exchanger these gases (POC’s) are expelled through the flue which is often at the top of the boiler before exiting through the wall. These gases can reach a temperature of approximately 180°C. Often this type of boiler may also contain a cast iron heat exchanger. The combination of heat losses from these appliances can sufficiently increase the temperature within a compartment if not ventilated. If ventilation is provided into the room in which this type of appliance is located it can be calculated into the heat loss of that room. The compartment should be ventilated at the top and bottom, allowing cool air to vent through the bottom and warmed air to escape through the top vent.
Modern high efficient condensing boilers are now designed that once the products of combustion gases pass through the first chamber containing a stainless steel or aluminium light weight heat exchanger, they pass through a second chamber containing a secondary heat exchanger significantly reduces the gases (POC’s) temperature and are expelled through the flue at approximately 55°C. Therefore there is no great heat emitted from this type of appliance and no ventilation is required.
The minimum air vent requirements should be in line with manufactures instructions. Guidance is provided by BS 5440-2 giving a minimum size of 10cm²/kW net. If the compartment is on an outside wall then the ventilation can be taken direct to the outside. As this air is cooler the vent may be smaller calculated as a minimum size of 5cm²/kW. The vents should be located top and bottom of the compartment, but located so not to have an adverse effect on the performance of the boiler
Open flued appliances require ventilation direct from the location in which there in for complete combustion and as such the ventilation vents need to ensure an adequate air supply. The ventilation can be provided directly through the wall to the atmosphere at high and low level. Although the ventilation requirements for combustion is 5cm²/kW net over 7kW. Vents provided for combustion and ventilation should be a minimum of 10cm²/kW net at low level and high level at a minimum 5cm²/kW net. Where ventilation is provided through an adjacent room the minimum sizes should be minimum of 50% greater than the ventilation to the atmosphere. BS 5440-2 states that the minimum vent to the compartment must be a minimum of 20cm²/kW net low level vent and 10cm² for the high level vent. Ventilation cannot be taken from a bathroom or shower room, if the appliance exceeds 12.7kW net the ventilation cannot be taken through a bedroom or bedsit area.
Air vents used to supply fresh air can be manufactured in different styles and materials but must be sized in compliance with BS 5440-2 to supply the correct amount of free air required by the manufacturer’s instructions. BS 5440 gives a calculation taken into account air supplied by adventitious air so that an appliance with an input rating of less than 7kW does not require additional ventilation. If the appliance is above this rate and additional ventilation of 5cm²/kW is required. example being a 28kW net boiler
of free air is necessary.
The vent openings are required to be smaller than 10mm² but larger than 5mm² if the vent holes are tapered then the size of the tapered end is what’s calculated.
Vents should not be stepped but continuous, and within cavity walls the vent should be sleeved using a duct with a cross sectional area not less that the free air required, the vent or duct should not incorporate any closing device or insect screen.
The vents should not be located be sited in areas in which they can be easily blocked with leaves, snow, flooded or where they can become contaminated by car exhausts or from the flue terminals of any appliance.
Internal air vents are required to be installed at no more that 450mm AFFL to prevent the spread of smoke in the event of a fire. Intumescent air vents can also be use but cannot incorporate a closing device. Internal air vents that are installed in series from room to room are required to be at least 50% larger that the vent to the atmosphere.
Air vents should not interconnect with stairwells or lift shafts, bathrooms, or vented roof or under floor areas that are shared with other dwellings.
Radon gas should be considered when installing a vent supplying the air from under a suspended floor space.
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