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Custom Flue Design & Installation

CGBS offer a variety of bespoke flue solutions paired with decades of experience

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CGBS Offer Comprehensive Commercial Flue Services

Flues are a specialty of ours at CGBS. We are extremely well versed in the history, rules and regulations surrounding chimneys and flues. We also have decades of experience designing and installing our own tailor made flue systems to provide innovative solutions that are durable, aesthetic and operate better than anything else available on the heating sector market. We offer a broad range of flues all using the highest quality materials such as stainless steel and plastic but also masonry chimneys such as clay, pumice and ceramic; allowing us to be prepared to tackle any situation you may have for us in regards to your flue requirements.

To briefly detail what flues are used for, before taking a deep dive into the history of them and the common flue options used within the heating and hot water industry: flues are the apparatus that is used to transport the hazardous fumes that are a by-product of the combustion process to the space outside a property, usually above. This prevents them being a danger to anyone in or around the property as it directs the noxious gasses outside to an area or height away from others, in line with Building Regulations requirements.

a flue poking out the top of a chimney

The History Of Flue Design

The primary regulations covering the installations of commercial flues are the Clean Air Act 1956/9, the Institute of Gas Engineers UP10 Edition 4 and the British Standards BS6644:2011, 6230:2011 and 6896:2011. These regulations must also consider the building regulations and any listed or historic house regulations. Local Building Control can advise and ensure changes to existing buildings comply with all these regulations, as well as issuing waivers under certain conditions.

From coal to oil

Flues in a heating plant have changed over the years to meet the requirements of the boilers and fuel used to fire them. Modern cast iron hot water boilers started to appear from the 1860’s firing coke or coal, these boilers were connected to chimneys discharging at the top of the building. This design stayed right through to the late 1950s when coal gave way to oil, as this was the fuel of choice due to the Clean Air Act of 1956. Coal stores were fitted with steel oil tanks and oil burners were fitted to the front of the existing coal firing boilers as a result of this act. When buildings first had heating boilers, a brick chimney was the normal solution as the boilers were originally firing on coal or coke ,in the 1960s these were slowly changed to firing oil in the late 1960s saw the introduction of the purpose-built modular gas boilers, usually cast iron atmospheric. All these fuels had high sulphur content and some were deposited on the walls of the chimney.

From oil to gas

From the 1970s and 1980s, these boilers were either converted to fire on gas or replaced with gas-fired boilers. As 10% of the flue gases from gas boilers is H2O, water droplets to drop out of the exhaust gas when the exhaust gas temperature drops below the dew point of 150°C. This water will then mix with the sulphur deposits to make sulphuric acid which will damage the chimney lining and mortar which could allow flue gases to enter the building.

The 1956 Clean Air Act and The Institute of Gas Engineers

The regulations of commercial boiler flues since 1956 have come under The Clean Air Act which covers all heating plants with over a 150kW input. Since then, the Institute of Gas Engineers have issued guidance on flues and ventilation in their document IGE UP10, now in its fourth edition. In the late 1980s, the British Standards brought out the BS6644 to guide commercial installation and BS5544 for domestic installations, these cover flues, ventilation, gas pipework and safety equipment to ensure the safe operation of the boiler plant. The Institute of Gas Engineers documents allowed variations to the Clean Air Act with flue dilution systems being allowed for systems up to 6mW and more recently, to cover low-level discharges for systems up to 333kW input.

A major change was that all gas boilers’ chimneys had to be lined and made of a suitable material for the boilers connected to them. With power flame or atmospheric gas boilers, this would usually be a stainless steel liner of a 430 or 304 grade and sometimes 316 grade. 430 grade was allowed but as it is a coated steel it was prone to rust if the stainless steel coating was damaged. These flues on non-condensing boilers were designed to work under negative pressure as the flue gas temperature was very high (150-240°C), causing a significant draft in the flue. This meant the flue did not need to be sealed, often being fitted with stabilisers and installed in sections solely pop-riveted together because a correctly sized flue is unlikely to leak or spill.

The new condensing gas boilers work on a different principle, as the flue gas temperature is very low it has little lift and relies on a positive pressure supplied by the boiler fan to remove the flue gasses. This means that the flue must be sealed and to ensure the flue can deal with the acidic condensate, it was made of 316-grade stainless steel. For some applications plastic flues are suitable but they are unlikely to be suitable for outside use as they are not UV protected.

Flue dilution systems and ventilation

From the late 1970s, a discharge at a low level became popular using flue dilution systems. These systems mixed fresh air with the flue gasses to dilute the concentration of CO² in the discharge to under 1% by volume. There are strict regulations on the height from the ground of the discharge, depending on the boiler output. The advantage is that the system does not need a chimney to the roof level, although the flue diameter is big due to the volume of fresh air that needs to be moved to create this dilution. The systems often have a single large fan, which if it fails, the heating must be shut down. Another issue is that noise from the fan can exit the building and be a nuisance to the area near the boiler room.

A new development was the Monodraught system which brought fresh air and combustion air from airspace around the chimney down into the plant room. This meant that the boiler room did not need to be on an outside wall or required fanned air. To comply with the regulations, these systems had to have interlocks on plant room doors into the building and no other openings into other parts of the building to ensure airflow. Often these systems resulted in hot plant rooms and though they are still allowed, the original company that gave the system its name no longer makes them.

The 21st century

After 2000, the regulations started to push for higher boiler efficiencies where  more and more boilers supplied were condensing. This meant the flue gas temperature was much lower and the natural draught was poor, so they needed a fan to push the gasses out. As the flue gasses condense in the boiler to ensure that the efficiency is maintained, a significant amount of slightly acidic water formed in the flue gasses. This is the reason why all flues on condensing boilers must work under pressure and be designed to remove the condensate to conditioners or be allowed to drain away. As all commercial boilers under 400kW must be condensing due to the ErP directive, nearly all systems will have to have a flue system designed to match the boilers and comply with all the regulations. One of the major changes is that some boilers can have flues at a low-level discharge. The Clean Air Act sets this to plant rooms with a total input of under 150kW, but the latest IGE UP10 Edition 4 has raised this to plants with a total input of 333kW. There is a very strict check sheet which must be complied with for low-level discharge to be acceptable.

We can advise on which type of flue will meet your requirements. For larger flue dilution, systems can still be used up to 6,000kW but again with condensing boilers even the best dilution fans will have a limited life and pluming can be a major problem. If this system is the only option, we can select plume reduction boxes on the discharge.

large modern flue attached to the side of a warehouse

Detailed Analysis Of Flue Options

There are many different flue options and these are often dictated by the boiler manufacturers. The boiler manufacturer will stipulate the types of flues, they usually have designations such as B23, C13, C33, C53 etc. Boilers must not be connected to flue systems that they are not CE rated.

Most common flues (C13, C33 and B23)

The most common flues for wall/frame-mounted boilers are C13 and C33 which are room sealed concentric flues with either horizontal or vertical discharge. For both wall/frame and floor mounted boilers, the B23 flue which connects all the boilers to a single flue system and discharges at a high level are also very common. In this system, the boilers take combustion air from the plant room instead of from directly outside as is the case with the C13 and C33 systems. The B23 flues are usually made of high grade 316 Stainless Steel with silicon push-fit joints. The flue must be vertical or sloping, usually about 3°, so any condensate can be drained correctly and if any puddles of condensate sit over the silicon joints, they can fail in time as they will become brittle. Replacement flues for multiple boilers have to be carefully designed to ensure they work correctly for both full and part loads. This is so the flue gasses are correctly removed from the boilers. The running boilers must not put too much back pressure on the non-running boilers either, otherwise flue gases may enter the plant room. Most boilers are fitted with flue gas non-return valves but there is a limit to the toll these valves can take.

As a 100kW boiler can produce 10-11 litres of condensate per hour, there will be condensate in the flue that will need to be drained. You can use plastic pipes inside the plant room for this and these drains must have traps fitted that are designed to deal with the maximum flue pressures. The local water company must be consulted on how the condensate can be disposed of, as this water is acidic they may request conditioners to be conditioned. This particularly the case if the building uses little water that would help to dilute the condensate or that the drains are unsuitable, i.e. cast iron or copper that would fail over time due to the acidity of the condensate.

Flue dilution system

When an existing installation has been installed with a fan dilution system, great care must be taken to replace the system. If a vertical flue can be fitted then that could be a solution or if the total input is less than 333kW, a discharge at a low level may be possible. A flue dilution system can be used with condensing boilers if constructed with suitable fans and high grade 316 stainless steel ducting from the boilers to the system discharge. If pluming is an issue then we can fit the plume removal box before the discharge point. The flue discharge position will be dependent on the total kW rating of the plant and the height of the flue must be taken into consideration with other structures such as the nearest building, openable window or fan intake. For horizontal flues, the height from the ground and distance from the adjoining structure will affect the position or if it will be allowed. Flues may require a fire rating and if so, they will generally require a twin wall flue system.

Other common flue types

Other flues in common use are plastic flues rated for use on boilers. These are generally only used in plant rooms connecting to stainless steel flues which rise in chimneys or internal risers. The riser’s twin-wall is usually used for both fire protection and to minimise heat transfer into the building. For flues running outside, twin-wall flues are again usually used to minimise the chance of condensate freezing in the flue and to allow some heat updraft when the boilers are running. Plastic flues are generally not UV protected so must not be used outside as they will become brittle and fail in time.

Other things to consider

If the boiler manufacturer supplies the flue, their guidance must be followed for size and run lengths as well as meeting all the relevant regulations. All flues must also be pressure tested in position to ensure there are no leaks. This is to meet the latest regulation and the test results must be retained with the plant records.

Sometimes it is easier to keep the existing flue liner which is not suitable for a condensing boiler and fit a laminated Furniflex liner down the inside. This can be a quick solution when time is important. This material can be used to line an existing brick or concrete chimney as it can be made to measure and still retains the maximum cross-section area of the flue or chimney as it is only a few millimetres thick. The flue must be correctly designed by a specialist or boiler manufacturer and the calculations, method of assessment and conclusions of the design and sizing must be retained with the plant records.

We are here to guide you through the options available to ensure the system works and that it will comply with all current regulations.

a curved flue installed to the outside of a building

Boiler Flue FAQs

Your questions about boiler flues answered

Flues are essentially chimneys for your boiler or for other gas fired appliances, almost acting as an exhaust. A flue is a piece of piping that carries the harmful exhaust fumes created from burning a fuel (e.g. natural gas, LPG or oil) outside of the building and into the atmosphere. The harmful fumes in question are Carbon Monoxide, Carbon Dioxide and Nitrogen Oxide, all of which are toxic to humans and needs to be disposed of safely. 

The rules and regulations for flues are expansive, covering topics from location to temperature and are dependant on what type of flue is being used (e.g. natural draught, fanned draught, balanced flues or open flues), as well as the fuel being used (e.g. natural gas, oil or an alternative). There are too many regulations to detail here but for a more detailed look at flue regulations click here to view the Approved Document J on the government website, detailing all the flue regulations that should be followed.

A blocked or clogged boiler is extremely dangerous as it means the toxic flue gasses can’t be disposed of properly and may enter back into your property. Signs of a blocked flue are: staining or soot around flue pipes and their seals, water leaking around the flue pipe as it enters your boiler, an odd smell due to the gasses coming into your property and Carbon Monoxide alarms going off. If you have noticed any of these signs get in touch straight away by calling 0207 9989 005.

Flues normally don’t require planning permission as long as they don’t exceed more than 1m above the highest point of the property’s roof and they aren’t on the principle or side elevation of a highway or designated land such as a national park or conservation area. If the property in question is a listed building you will need to check with your local planning authority to ensure you are adhering to the regulations designated for your property. If you need help finding out who your local planning authority is, please click here.

The length a flue can be run is dependant on the manufacturer’s instructions and the size of the flue being used. Most flues with a 60/100mm concentric size can reach lengths up to 10m and flues with a 80/125mm size can normally reach lengths up to 20m. Again this can vary depending on the manufacturer’s installation instructions and the amount of bends / elbows used, so make sure to check with them before installing a flue.

Boilers flues are likely to drip a small amount of water from the outlet due to the condensation of water vapour, if a large amount of water is dripping from the flue outlet it may be due to the flue not being set at the correct angle. Flues should be slightly angled at around 3° – 5° down towards the boiler so that the condensate drips back down into the condensate trap. If the flue is dripping water elsewhere it could be because a joint is loose and is leaking or the material used for the flue isn’t correct and is corroding. Another reason may be because the flue has become clogged or blocked and the gasses are not being expelled properly. In any of these circumstances, you are best having it checked by a qualified professional as not doing so could have dangerous consequences. Get in touch by calling 0207 9989 005 to book an engineer appointment or for further information.

The gasses a flue disperses are created from combusting a fuel so they can be very hot and make the flue itself hot. This is why boiler flues should be installed in a location where they won’t normally come into contact with people and why they should be located away from any material that can be adversely affected by the heat.

A flue installation needs to be carried out by a gas engineer with the correct ACS qualifications and is registered with the Gas Safe Register. This is to ensure the correct procedures are followed, the regulations surrounding boiler flues are met and the installation is done so safely. If you attempt to install a boiler flue without meeting these requirements, you may risk legal repercussions, also possibly being a danger to yourself and others.

*Optional image or document upload. Details of current boiler such as photos of boiler/plant room and boiler plate with make and model no.