Correct siting and sizing of a boiler flue and terminal is essential to avoid poor performance and bother to neighbouring properties. Here, Graham Collins from Baxi Heating takes a look at the different issues and best practice solutions to fit the perfect flue.
When the minimum efficiency requirements for gas fired boilers changed in 2005, as part of the revision to Part L of the Building Regulations, the market switch to condensing boilers was quick to follow. However, as condensing boilers emit a plume of condensate (particularly in the colder months) it places an increased importance on the location of the flue terminal. Installers therefore need to consider all possible issues and solutions available to them.
Sizing things up
Minimum and maximum lengths vary depending on the boiler make, model and output. You can find out this information with the purchase of a unit or by contacting the manufacturer for help and assistance if ever unsure.
A current production Baxi, Main or Potterton system or combi boiler with a 60/100mm concentric flue fitted, for instance, can be up to a maximum length of 10m. For those installations that require the flue run to be longer, the flue size can be increased to 80/125mm, providing a maximum flue length up to 20m.
Calculating the equivalent flue length is a fairly straightforward process – you simply have to remember that for current production Baxi, Main or Potterton boilers the first elbow on a concentric flue is free. From there,you will need to make a one meter reduction for each 90º bend and a half meter reduction for every 45º bend.
It is important only to use those flue components as shown in the installation instructions. Should the decision be made to operate the boiler with a non-specified flue, then the boiler may not function as it should. In cases of improper sizing, you may find that the ignition is delayed and the combustion is affected, which could result in serious damage to the boiler.
Site it right
Alongside the sizing, siting the flue terminal correctly is of the utmost importance. As specified in BS 5440 – 1, there are a number of siting considerations that must be made when installing a flue to prevent the products of combustion from entering a homeowner’s property or that of their neighbour’s. Leaving enough distance between the window of a property and the flue terminal is crucial to do this – 300mm is the minimum distance required when installing a horizontal flue that is adjacent to an opening window.
Importantly, though sometimes overlooked, if a Baxi Multifit plume displacement kit is utilised the flue terminal air intake can be sited a minimum of 150mm from an opening window or door. To avoid causing a nuisance or putting neighbouring homeowners at risk, the distance from a fanned draught appliance terminal installed next to the boundary line must not be less than 300mm. However, the use of a Baxi Multifit plume deflector can reduce this minimum to 25mm.
While manufacturers, such as Baxi, provide detailed guidance on permissible lengths of the flue and BS5440 Part 1 gives minimum clearances for flue terminal siting, every installation is different. Therefore, it is always worth paying close attention to the siting of the flue terminal to avoid putting the homeowner in the unenviable position of having to get the nuisance flue re-sited.
Options are available when re-siting the flue is not a viable option. Flue accessories, such as plume displacement kits and plume deflectors can be used to point the pluming away from the area that it is likely to cause a nuisance.
Giving support
Just as a boiler must be properly hung and supported on a homeowner’s wall, the flue also requires adequate support. BS5440 requires that installed flues are supported throughout their entire length, and at Baxi we specify that they are supported at least once every meter. Without the required support, a horizontal flue-run may dip, leading to an accumulation of condensate water in the flue pipework which could prevent proper boiler function.
Most manufacturers will provide installers with a means of complying with these requirements. For instance, all Baxi Multifit flue components are supplied with support brackets where necessary, meaning there really is no reason not to comply.
Good outcomes
A boiler flue that meets the manufacturer’s sizing and siting requirements and legal regulations shouldn’t be a cause for concern. Once installed correctly the flue can then be largely forgotten about until the next annual service, which is the best outcome for both installers and their customers.
