Plume management

4th Jul 2017


By Neil Macdonald, Technical Manager, HHIC

In April 2005, changes to Part L all but created a mandate, requiring condensing boilers to be installed in all normal circumstances at the time of boiler replacement. The subsequent surge in condensing boiler sales brought new considerations for heating engineers to tackle, and alongside condensate discharge pipework, in particular these related to installation of the boiler flue/chimney, and its point of termination. No longer was siting of the flue/chimney (in accordance with the manufacturers’ instructions) only influenced by heat-related factors (e.g. potential damage to eaves), or ensuring products of combustion could not recirculate, or enter the building and others nearby, but there was then, as there is now, a need to consider the potentially complex and subjective nature of “nuisance” pluming from condensing boiler flue-terminals.

We all know that condensing boilers emit a plume of condensate (particularly in colder weather), and whilst manufacturers’ installation instructions will generally echo the guidance of Part L and the Domestic Building Services Compliance Guide (which mentions the need to consider nuisance), they frequently focus on the minimum flue terminal clearances needed to ensure the flue operates safely, and to design specification. Individual site conditions will vary and should pluming from a condensing boiler flue-terminal be deemed a statutory nuisance, there is a very real chance that the local authority may insist on it being re-sited, as well as the potential for civil action (e.g. from the client or neighbour). Therefore, the need for installers to remain vigilant when specifying/installing condensing boiler flue terminations (over and above compliance with the appliance manufacturers’ minimum clearances), is more important than ever.

Since 2005, the rise of approved boiler plume-management-kits (PMK’s) has provided the installer with an invaluable installation option, helping them navigate around these potential pitfalls ….which brings me onto the main point of this article……

In recent times, HHIC members have advised that they are encountering an increasing number of unapproved flue systems and components being installed to gas boilers, most often in the form of unapproved plume-management-kits (PMK’s). Unless of a type/specification approved by the appliance manufacturer, such kits will provide little, if any, reassurance that the appliance and flue system will operate as intended, with potential repercussions for safety and consumer warranties.

PD/CEN/TR 1749:2014 is a European technical report which classifies appliances based on the method of evacuation of the flue gases. In the UK, most commonly C1 (horizontal flue), C3 (vertical flue) and C5 (non-balanced flue/air – such as PMK’s) appliances are supplied. In these cases, the flue system is specified by the appliance manufacturer and forms part of the testing and certification requirements at the approvals stage (e.g. CE marking under the Gas Appliances Directive). It is therefore of paramount importance that the manufacturers’ specified flue system is utilised, in order to ensure validated levels of safety and performance. The appliance, flue components, or both, will come with detailed technical instruction for correct installation, including guidance on jointing, support and maximum flue lengths. Category C6 appliances are the only exception, where a proprietary flue-system is not specified by the appliance manufacturer, although they are still likely to offer advice on appropriate products and materials (e.g. MUPVC pipe suitable for C6 twin-flue products).

Whilst the manufacturers’ instructions can be seen as the essential reference document, this advice is supported by BS 5440-1, stating as it does that plume-management-kits can only be used when provided or specified by the appliance manufacturer. The Gas Safety (Installation & Use) Regulations also place an all-encompassing legal duty upon engineers to ensure that they do not modify gas appliances in such a way that results in them ceasing to comply with the standards to which they purport to comply (e.g. original approvals - CE marking). The appliance test standards used to certificate the appliance-type under the Gas Appliances Directive include a range of appliance/flue combination tests, the results of which may be impacted by a change in flue specification. These include acceptable leakage rates from flue components, maximum acceptable flue-gas temperatures for the materials used, and also “blocked-flue” tests, which ensure the appliance does not operate unsafely in this scenario, where the specified flue-system is utilised.

Also of concern to HHIC is the potential impact on consumer warranties where unapproved flue components are installed. The repercussions range from condensate leaking externally (a potential nuisance and freezing hazard above paths etc.) to recirculation of products of combustion, risking nuisance lockout, internal appliance corrosion and elevated levels of Carbon Monoxide, all likely to invalidate the consumer’s commercial warranty where due to inappropriate materials/components.

To ensure safety, reliability and to protect consumer warranties, always specify and install the appliance manufacturers’ approved flue system components to “type-C”1 gas boilers

1 C6 appliances are specified, approved and sold without a proprietary flue-system and so this guidance does not apply here. Ref: PD/CEN/TR 1749:2014