The flexibility of twin flue

Some light commercial boiler applications may require a longer flue run, but a twin flue system can provide the installer with a more flexible solution. Charlie Mowbray of Keston Boilers highlights some of the key benefits. 

Boiler installations for large domestic or light commercial projects can present a number of challenges for installers, as not all are the same. There are many elements that need to be considered such as the boiler position, flue termination, and regulations regarding the property, to name a few, and installations can become more complex if you are restricted to just one type of flue system. 

Twin flue boiler systems can offer a good solution to installations that require a longer flue run, allowing for boilers to be positioned in locations that cannot be flued concentrically directly through an external wall.

 The vast majority of domestic boilers are designed with a concentric flue system where the air supply and flue gases are managed within one flue. They are designed as a duct within a duct, with the exhaust gases taken away through the inner duct and the combustion air drawn in through the outer duct. 

Twin flue boiler systems will perform the same task as concentric systems, but allow further flexibility in distance. Both systems have an air intake and an exhaust flue, but in a twin flue installation the flues do not run within one another – they run separately. This type of system offers a different way to install boilers as it allows further options where the flue runs can go. 

For example, the twin flue runs are not restricted to run parallel, meaning that the air position will not be limited by the same restrictions placed on the termination of the combustion flue. This added benefit means the combustion gas pipe can run further to the desired termination point while the air pipe can run through the nearest external wall next to the boiler. The maximum combined length for both flues on a Keston boiler is 27m in total, so the air flue can be terminated 3m away from the boiler, allowing the remaining 24m left for the exhaust flue run. 

Cost saving
Twin flue systems will offer a cost-effective solution if the boiler placement is in the basement of the property. Rather than dealing with the additional work and cost of moving the boiler position, twin flue solutions can be used to give the extra length that is needed without any hassle.

If the original boiler position was within an area that did not have straight access to an external wall, the consideration of moving the boiler, gas and water pipework and flue runs to suit a concentric system would need to be taken into account. However, rather than moving a boiler to fit a concentric flue, a retrofit replacement with a twin flue solution would present a far simpler and more cost-effective solution. 

As well as avoiding the cost of having to re-site the boiler, twin flue systems can be significantly cheaper than proprietary concentric flue systems.  The Keston twin flue system requires manufacturer approved small diameter (50mm) muPVC (PVC-C) solvent weld pipes, one for the air duct and one for the combustion gases. This solvent weld pipe is a fraction of the price of many other flue systems.

There are also certain properties where twin flue could be used to benefit the installation, such as ‘landlocked’ flats where there probably won’t be sufficient external wall space for the flue termination to be placed. For example, there may be a balcony or windows near where the flue termination would normally be.  

There are also certain properties that have additional legislation to consider regarding flue runs, such as listed buildings, where regulations may prevent flues from being positioned in front of the property or any area that would be seen from a highway. 

Flues in voids
When using a twin flue system there are some additional measures that need to be considered, such as the Gas Safe technical bulletin TB200/TB008 regarding flues in voids. Where boilers are located away from an external wall the flues are likely to run through the ceiling or wall voids. In these cases, inspection hatches will need to be installed at appropriate points along the flue runs to ensure safety checks can be completed. 

To ensure flues can be examined thoroughly, inspection hatches should be 300 by 300mm in size, and no flue joint within the void should be more than 1.5m away from the edge of the nearest inspection hatch. Making space for these hatches could mean additional remedial work would need to be done, such as added plastering after the hatch has been completed.

There are other elements that need to be considered outside of BS 5440-1: 2008, the British Standard that sets out the fluing requirements for boilers not exceeding 70kW. All flues need to be adequately supported and all joints, other than manufacturer approved push-on or plastic compression connectors, are made and sealed with solvent cement suitable for the manufacturer approved muPVC (PVC-C) flues conforming to BS 6209.

In conclusion
There are many types of installations that would reap the benefits of twin flue systems, such as those in pubs, care homes and landlocked flats. Many of these types of building require a longer than average flue length, which could be determined by the original location of the boiler. Twin flue systems are more cost and time effective for these boiler positions than concentric systems, while also complying with flue termination regulations. They offer a unique resolution for installers with tricky heating challenges.

At Keston Boilers we offer heating solutions for both domestic and commercial installations and free training is available at our Training Centre of Excellence sites in Hull, Leeds and Reading. 

keston.co.uk

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