What is a boiler flue?
A boiler flue is a pipe or duct attached to a gas central heating boiler to safely transport harmful waste gases, also referred to as products of combustion (POCs)produced by the boiler out of the property and into the atmosphere. Water vapour, carbon dioxide, and, if the boiler is not working properly, carbon monoxide are among the waste gases produced by a gas central heating boiler.
A modern boiler flue contains two ducts of tubes, the inner tube is used to disperse the products of combustion while the exterior tube serves as the air intake duct for the boiler.
The importance of boiler flue installation, regulations
To ensure the safety of the property and the occupants, a boiler flue must be installed by a Gas Safe registered engineer who must follow the regulations regarding boiler flue separation distances. These regulations ensure:
• The products of combustion are safely removed to open air.
• The products of combustion do not re-enter the property.
• The products of combustion are not directed towards neighbouring properties.
What does a boiler flue look like?
Majority of homes in the UK and many businesses have gas central heating boilers and, as a result, a flue. Boiler flues are either horizontal pipes that protrude from an exterior wall or vertical and protrude out of the roof. A flue’s size and shape prevent waste gases from accumulating around the terminal and ensures that they are expelled far enough into the atmosphere to prevent them from entering the home again.
What are horizontal flues?
Horizontal flues are typically short pipes that protrude from an exterior wall and are positioned to keep waste gases from accumulating around the terminal and to expel the gases far enough into the atmosphere to keep them from re-entering the home. Horizontal flues are the most common type of flue system and are appropriate for the majority of properties. A vertical flue may be used instead if the terminal is within close proximity into a neighbouring property or if there isn’t an accessible exterior wall.
What is a vertical flue?
A vertical flue is a type of flue system that is installed vertically from the top of the boiler and unlike a horizontal flue does not exist through the wall. Vertical flues are commonly used when the boiler or heating appliance is located where there isn’t adequate access to an external wall or too close to a neighbouring property. Vertical flues have the advantage of being a more practical option for homes or buildings with limited outdoor space.
Vertical flues, on the other hand, can be more difficult to install than horizontal flues because they require a secure and weatherproof installation and they may not be appropriate for buildings with flat roofs or other distinctive architectural features. They may also require expertise of other trades and services such as scaffolding and roofers to complete the task, therefore being more costly.
What are twin pipe flues?
Twin pipe flues are a type of flue system used in heating appliances such as boilers that use two separate pipes . One pipe serves as exhaust for products of combustion, while the other serves as an air intake for the appliance to ensure proper combustion.
This type of flue system is intended to increase appliance efficiency and provide better ventilation. Twin pipe flues are especially common in condensing boilers because they are designed to recover as much energy as possible from waste gases, making them more efficient and environmentally friendly. The use of twin pipe flues also helps to reduce the risk of carbon monoxide poisoning by ensuring that the appliance is correctly burning fuel and producing a sufficient supply of air for proper combustion.
Twin pipe flues are not as common as standard flues but they have been used by some manufacturers such as Keston to good effect. They can be popular in modern development of flats where the boiler is located centrally within the property, as the twin pipe system allows flexibility and longer flue runs.
What is the difference between condensing and non condensing boiler flues?
The main difference between condensing and non-condensing boiler flues is how they handle the boiler’s flue gases.
Flue gases from non-condensing boilers are typically vented directly to the outside, with little or no attempt to recover any of the heat contained in the exhaust. As a result, non-condensing boiler flues may be less energy efficient than condensing boiler flues. The flue gases from non condensing boilers tend to be a higher temperature, resulting in the flue getting hotter. This emphasises the importance of locating the flue in the correct place to avoid risk of burning or damage to property.
Condensing boiler flues are intended to recover heat from the boiler’s flue gases. This is accomplished by passing the flue gases through a heat exchanger, which extracts heat from the gases and uses it to preheat the cold water supply that comes in.
Condensing boilers are more energy efficient than non-condensing boilers due to the preheating of the incoming cold water supply. As a result, fuel bills are lower and carbon emissions are lower.
Another significant distinction between condensing and non-condensing boiler flues is that condensing boiler flues produce more acidic condensate. This is due to the heat exchanger’s lower exhaust temperatures, which can cause the condensate to be more corrosive than condensate produced by non-condensing boiler flues. This means that condensing boiler flues must be made of corrosive resistant materials such as PVC.
Horizontal condensing boiler flue Installations
Horizontal condensing flues should have a slight upward slope. This ensures that any condensate generated by the flue drains back into the boiler or another suitable drainage point.
If the flue is installed with a downward slope or level, condensate can pool inside the flue, causing corrosion, blockages, and other problems.
It is important to note that the exact angle required for a horizontal condensing flue will be determined by the boiler and flue system installation requirements. This can vary depending on the length of the flue, the size and type of boiler, and the type of building served.
As a result, when installing a horizontal condensing flue, it’s critical to follow the manufacturer’s instructions or seek the advice of a qualified gas engineer. A properly installed and maintained flue system is critical for the safe and efficient operation of the boiler as well as the safety of the building’s occupants.
Types of flues on gas appliances
Gas appliances have 3 types of flues
- Flueless
- Open flue
- Room Sealed
Flueless Gas Appliances
A flueless appliance is one that does not require a chimney or flue to vent exhaust gases outside. A flueless appliance uses the natural air and ventilation present in a property (adventitious air) to operate. Flueless appliances generally below a maximum output of 7Kw and examples of flueless appliances are water heaters and cookers
Open Flue Gas Appliances
An open flue is a type of flue that draws combustion air from the room in which the appliance is located and vents exhaust gases to the outside via a flue or chimney. These were typically available on older boilers and are not very common.
Room Sealed Gas Appliances
A room-sealed flue is a type of flue that draws combustion air from outside the building rather than from the room in which the appliance is located. Room sealed appliances are further subdivided into two categories
- Fan assisted flues:
These appliances draw combustion air from outside the building and use a fan to assist in exhaust gas removal. Modern gas fired boilers commonly use fan-assisted room-sealed flues
- Balanced flues:
These appliances draw combustion air from outside the building and vent exhaust gases to the outside via a balanced flue. The balanced flue ensures that the pressure inside the appliance is equal to the pressure outside the building.
In this article we will specifically be looking at boiler flue regulations that apply to fan assisted flues
What gases are expelled from a boiler flue when the boiler is in operation?
When natural gas is burned in a boiler, several by-products are produced. Water vapour and carbon dioxide for example, are typically harmless and are simply vented to the outside atmosphere via the boiler’s flue or venting system.
But if a boiler isn’t working properly or isn’t well maintained and serviced regularly, it can emit a more dangerous by-product, carbon monoxide. Carbon monoxide is a colourless, odourless gas that in high concentrations can be fatal.
Carbon monoxide poisoning can result in a variety of symptoms such as headaches, dizziness, nausea, and confusion. It can cause unconsciousness and even death in severe cases. If you have any gas powered appliances in your property including boilers, you must have a working carbon monoxide detector installed.
A qualified and Gas Safe registered engineer can help to ensure that your boiler is working safely and efficiently, and that the risk of carbon monoxide production is minimised. It’s also critical to make sure your home has enough ventilation and that any vents or flues are kept clear and unobstructed to allow for proper air flow.
What are the regulations for boiler flues and where should they be located?
As mentioned above, in this article we are exploring the boiler flue regulations for modern condensing boilers that are most commonly installed and the distances away. Balanced flues and open flue appliances have different requirements.
| Terminal Flue Position | Fanned Room Sealed flueDistance |
| Directly below an opening, air brick,Openable window | 300mm |
| Above an opening, air brick,Openable window | 300mm |
| Horizontally to an opening, air brick, Openable window | 300mm |
| Below temp sensitive components Eg plastic gutters, soil pipes etc | 75mm |
| Below eaves | 200mm |
| Below balconies orCarport roof | 200mm |
| From a vertical drain pipeOr soil pipe | 150mm |
| From an internal or External corner | 300mm |
| Above ground roof or balcony | 300mm |
| From a surface facing the terminal | 600mm |
| From a terminal facing the Terminal | 1200mm |
| From an opening in the car port Into the dwelling | 1200mm |
| Vertically from a terminal On the same wall | 1500mm |
| Horizontally from a terminal On the same wall | 300mm |
Please also note while these are the regulations for boiler flues, the manufacturers instructions supersede these regulations and should always be consulted for accurate guidance.
Other Boiler Flue Installation Standards
The appliance and chimney configuration must be installed in accordance with the manufacturer’s instructions, which must be left at the property where the boiler is installed for future reference and guidance.
- All flues should be installed and maintained according to manufacturer’s standards
- The terminal must be placed in such a way that combustion products can safely disperse at all times, and
- It must not be placed near any opening that could potentially allow flue gases back into the home, or near heat sensitive materials.
- Flues should not be installed near heat sensitive material
- The flue must be at least 2.1m above the ground if it faces a public space.
- When facing the boundary line, the flue terminal must be at least 600mm away from it,
- and at least 300mm away when running parallel to it.
- Any gas flue terminal within 2m of the finished ground level must have terminal guards. Guards should be installed on all flue terminals that are less than 2m above finished ground level and can come into contact with people.
- Horizontal flues should not be over extended and protrude excessively from a building. They should be cut down to the correct size
- Gas Boiler Flues should only be installed and checked by Gas Safe registered engineers
- Flues with any visible damage or dents should not be installed
- The boiler flue must be installed in a safe and secure manner and must not be obstructed in any way.
- The boiler flue must be installed so that it does not come into contact with any flammable materials
What happens when boiler flue terminals are correctly sealed?
When boiler flue terminals are properly sealed, harmful gases such as carbon monoxide are prevented from returning into the property. The purpose of boiler flues is to expel gases from the boiler to the outside of the building. If there are gaps or leaks in the flue system, dangerous gases can enter the property. Properly sealing the flue terminal can help to reduce health and safety risks.
In addition, unsealed gaps around boiler flues can allow moisture to enter the fabric of the building and create issues with the building or the boiler flue itself. It is vitally important that the opening around the boiler flue is adequately sealed with the appropriate materials
Fire cement or other appropriate materials, such as silicone or high temperature sealant can be used to seal boiler flue terminals. Fire cement is a type of high temperature resistant cement that is frequently used to seal gaps or joints in flue systems. When using fire cement, it is important to follow the manufacturer’s usage and application instructions.
To seal a boiler flue terminal with fire cement make sure the area is clean and dry first. Using a putty knife or similar tool apply the fire cement around the flue joint or gap. Firmly press the cement into the gas making sure to completely fill any gaps. Smooth the fire cement in place with a tool being careful not to leave any gaps or air pockets.
It is critical to note that fire cement should only be used on joints or gaps that are not subjected to stress or movement, as it may crack or break if subjected to movement or vibration. If the joint or gap is subjected to stress or movement, a flexible high temperature sealant may be a better option.
In any case the boiler flue would always be sealed by the qualified gas engineer during the installation of the flue
What happens when boiler flues are blocked or obstructed?
Blockages or obstructions in front of room sealed gas boiler flues can affect boiler operation and efficiency. A room sealed boiler draws air from outside the room to support combustion and discharges waste gases through a flue.
Obstructions in front of the flue can limit air intake resulting in incomplete combustion and poor boiler operation. This can lead to decreased efficiency and increased wear and tear on the boiler over time resulting in costly repairs.
Obstructions in front of the flue can also cause an imbalance in air pressure resulting in the boiler shutting down and not restarting until the obstruction is removed. It is important to keep the flue clear of any obstructions to ensure proper ventilation and efficient boiler operation.
If you suspect that your flue is blocked, stop using the appliance immediately and do not use it again until it has been inspected for safety by a Gas Safe Registered engineer. Regular boiler servicing and inspection of the flue system can also aid in the detection and resolution of issues before they become more serious.
Is it necessary to relocate my flue?
If after reading this article you believe your boiler flue does not meet the standards outlined you may need to relocate the flue.
There are several options available to you if your current boiler flue termination does not meet the regulation for boiler flues:
- Changing the boiler flue from horizontal to vertical, or vice versa
- Install a boiler flue plume kit
- Rerouting your existing flue run
If you are unsure, contact a Gas Safe registered engineer.
Boiler flue checks and maintenance regulations
In addition to boiler flues being installed in accordance to boiler flue regulations, they need to be inspected and maintained to ensure they are operating safely and efficiently. Whilst there are no specific requirements for flues to be checked on their own, safety and maintenance checks are implemented within a standard boiler service or during a Gas Safety Certificate inspection. The below guideline and checklist should be considered when inspecting flues and the checks should be carried out by a Gas Safe Registered engineer
- Regular maintenance of the flue must be carried out in accordance with the manufacturer’s instructions.
- If the flue becomes damaged or worn, it must be replaced immediately to ensure the safe operation of the heating system and the occupants
- Boiler flues should be checked externally for blockages, obstruction or signs of corrosion
- Perform combustion performance analysis to ensure boiler and flue are operating correctly
- If the flue is not working correctly, it must be repaired or replaced immediately to prevent the risk of fumes building up inside the building.
- All homes and buildings in the UK that contain a fuel burning appliance must be fitted with a working CO alarm.
- The CO alarm must be placed in a location where it can be easily heard, such as near the bedroom
- Regular testing of the CO alarm must be carried out to ensure it is working correctly.
Boiler flues in voids or concealed spaces.
If you have a gas appliance installed in a void space such as a wall or ceiling cavity the flue may become blocked or damaged. This can result in toxic flue gases, like carbon monoxide accumulating inside your home. To reduce this risk get your flues examined on a regular basis by a competent Gas Safe engineer.
There are a few things you may do to check for potential hazards if you suspect you have a gas appliance put in an empty space. For example, staining or discoloration on your walls or ceilings could indicate that exhaust gases are escaping from the flue. You should also be on the lookout for any unexpected odours, such as the odour of gas, which could indicate a problem with your appliance.
To meet regulations there two options available to consumers:
- Installation of inspection hatches so the flues can be inspected for damage and tested for CO Leakage
- Installation of continuous CO monitoring detection device that shuts off the boiler if it detects CO
Final Thoughts
In conclusion, it is important to ensure that boiler flues are installed and maintained in accordance with the regulations set by the UK government. Regular maintenance of the flue, along with the installation of a working CO alarm, can help to prevent the risk of fumes building up inside the building and ensure the safe operation of your heating system. If you are unsure about the regulations for boiler flues, or if you need any further information, please speak to a qualified heating engineer.