Boiler installations 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. Installations can be become more complex if you are restricted to just one type of flue system.
Charlie Mowbray, Product Manager at Keston Boilers looks into how twin flue boiler systems can offer a solution to installations that require a longer flue run. For example, Keston Boilers are able to run their flues up to 27 metres allowing flexibility on the boiler position. The flexibility this length provides allows for boilers to be positioned in locations that cannot be flued concentrically directly through an external wall, such as those in a basement room.
Fluing requirements for boilers not exceeding 70kW are detailed in British Standard BS5440-1: 2008 and provides guidance on what is essentially the main function of flue termination, namely that the products of combustion are effectively discharged to the outside air in a safe location. Any termination needs to be positioned appropriately with respect to surrounding features on the building and designed to prevent any debris getting into the system that could have an effect to the performance. Relevant details are outlined within the boiler manufacturers’ installation guides which ensure the safe operation of the appliance and prevent the position of the flue causing detriment to others.
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. Installations that use a concentric flue system can be installed vertically when leaving through the roof, or horizontally leaving through a wall.
Twin flue boiler systems will perform the same task as concentric systems; however, they do 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 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 27 metres in total. So, for example the air flue can be terminated three metres away from the boiler allowing the remaining 24 metres left for the exhaust flue run.
Twin flue systems will offer a solution for complicated boiler installations, for instance if the boiler placement is in the basement of the property. Historically, boilers were installed in kitchens across the UK. This stems from to the days when kitchens were situated in the basement of houses. Heating homes in the UK has developed over time, originating from the use of wood, then coal and eventually to natural gas. During these changes to fuel type the position of the boiler may have remained the same, within the basement. Basement boiler installations require a longer boiler flue to meet the requirements for flue termination. 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.
There are also certain properties where twin flue could be used to benefit the installation, for instance “landlocked” flats. These types of properties lack 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. This means a longer flue will be needed to extend towards the roof for the combustion gases.
There are also certain properties that have additional legislation to consider regarding flue runs, such as listed buildings. There are over 500,000 such properties in the UK which are protected so that they are preserved for future generations. Amongst many other regulations, there are some that would affect the flue run. Flues cannot be positioned in front of the property or any part that would be seen from a highway, meaning that a more flexible flue solution will be needed for to meet the requirements.
There are a number of benefits to using twin flue. Ultimately, the preferred outcome for the customer is cost savings, across both gas bills and installation. One of the benefits for twin flue installations would be the cost difference. Twin flue boiler installations require 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 sold at a fraction of the price when compared to proprietary concentric flue systems.
It is also advantageous to retrofit existing twin flue installations rather than moving a boiler to fit a concentric flue. 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 not need to be taken into account. As a retrofit replacement a twin flue solution would be far simpler.
When using a twin flue system there are some additional measures that need to be considered, such as the Corgi/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 have required to be installed at appropriate points along the flue runs to ensure safety checks can be completed. The inspection hatches are intended to allow visual inspection of the concealed system rather than having full physical access. To ensure flues can be examined thoroughly, inspection hatches should be 300mm 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 hatches has been completed.
There are other elements that need to be considered outside of BS 5440-1: 2008. 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.
British built since 2013 Keston Boilers offer heating solutions for both domestic and commercial installations, with unique twin flue capabilities that offer heating solutions without compromise. Free training is available at the Training Centre of Excellence sites in Hull, Leeds and Reading.
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 solutions.
Versions of this article first appeared in Professional Heating & Plumbing Installer, October 2019.