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Dealing with fume discharge at low levels

The current guidance on low-level discharge of commercial room sealed gas condensing appliances deserves serious consideration, according to Jason Lee
Low-level discharge on room sealed gas fired condensing appliances has become commonplace in the residential/domestic market and is adequately covered under BS 5440-1: 2009 for appliances whose input does not exceed 70kW net. In the majority of cases, only one appliance is discharged at low level per house. Over recent years we have seen an increase in the number of modular room sealed condensing appliances being used in commercial buildings, leading to an array of adjacent terminals discharging combustion products at low level. Whether this is acceptable with respect to the Clean Air Act 1993 and the Environmental Act 1995 is a question that we, as chimney manufacturers, are often asked.

Low level discharge of gas-fired commercial appliances is nothing new and is covered by the 3rd Edition of the Chimney Heights 1956 Clean Air Act Memorandum (CAAM) when used with a flue dilution system serving low sulphur fuels. A fan dilution system is specifically designed to dilute the concentration of CO2, CO and NOx down to specific levels. Historically, fan dilution systems were designed around atmospheric appliances. However, today, we are also seeing such systems used on condensing appliances with disastrous effects.

There exists a fundamental difference between the products of combustion being discharged by a fan dilution system at low level and a low level terminal serving a room sealed condensing appliance. The products of combustion from a room-sealed appliance are not diluted. Their constituents are the same as for any type of gas appliance, including those of a similar size regarded as 'conventional' (i.e. for connection to a natural draught chimney), where a low level discharge traditionally would never have been considered unless connected to a fan dilution system.

So the question remains, can multiple room-sealed appliance/s be discharged at low level? The answer is, in essence, addressed under Annex F 'Best practice guides to the dispersion of combustion products' of BS 6644: 2011, Specification for the installation and maintenance of gas-fired hot water boilers of rated inputs between 70kW (net) and 1.8MW (net) (second and third family gases). Section F.4.1 of BS 6644: 2011, which is entitled, 'Determining the overall inputs to be used in chimney calculations', offers a clear answer to our question.

'Where multiple boilers are installed adjacent to one another in the same boiler house and discharge their products of combustion individually through the same wall or adjacent walls into the same external region, the total input of all the boilers should be used to determine the chimney height.'

Following on from Section F.4.1, guidance is offered based on overall input ranges. These have been summarised below:

Inputs below 150kW (gross) - Where the overall input is between 70kW (gross) and 150kW (gross) for second family gases or equivalent third family gases, the CAAM does not apply and the manufacturer's installation instructions should be followed. For those boilers that are specifically certified for horizontal termination, the low level discharge of the combined products of combustion should be acceptable in this case. If the individual rating of the appliance/s is below 70kW net input, then the spacing and positioning of the terminals should conform to BS 5440-1 and associated Building Regulations. Above this value, the spacing and positioning of the terminal should conform to IGEM/UP/10.

Inputs between 150 and 366.4kW (gross) (light-to medium commercial boilers - Where the overall input is between 150kW (gross) and 366.4kW (gross) for second family gases or equivalent third family gases, the CAAM or equivalent should be used to determine the uncorrected chimney height, regardless of appliance type.

The final height is then corrected to take into account building effects and the overriding minimum requirements stipulated under the CAAM.

Inputs above 366.4kW (gross) (industrial and large commercial) - Above this value, compliance with the Clean Air Act is mandatory and the terminations need to meet the requirements of the CAAM or equivalent. In some cases, where there are complicated structures or the topography of the surrounding area is complicated, site-specific dispersion modelling may be required.

Under all input ranges, once appliance inputs and terminal positions are finalised, the appropriate Local Authority should be advised of the proposed design prior to installation taking place.

In cases 2 and 3, there is no provision for low-level discharge unless a fan dilution system is used and meets the requirements of the CAAM.

Questions and answers

What if the circumstances are such that a suitable chimney height cannot be achieved?

In these cases, site-specific dispersion modelling may be required. In such cases it is important that the appropriate Local Authority is consulted at an early stage in the design in order to establish that the proposed discharge positions are acceptable and will not cause significant local exceedance of air quality objectives. Depending on the agreed outcome, the Local Authority may issue a Design Verification Notice or a Dispensation Notice in order to approve both the design and termination position.

Is low level discharge still an option under points 2 and 3?

Low level discharge is still acceptable under the CAAM; however only as a fan dilution system when designed in strict accordance with the recommendation of the document. The dense plume of flue gas associated with low level discharge condensing appliances is already a known nuisance issue and can be distressing when seen. A fan dilution system dramatically increases this effect especially on cold days. Although the plume may be within acceptable concentration limits, it is a huge nuisance factor as well as a potential safety hazard and concern to those who experience it.

How do I meet the guidance under points 2 and 3?

In order to meet the requirements and guidance offered in BS 6644: 2011, the chimney needs to comply with the CAAM or equivalent and terminate at high level. The guidance uses the calculation methods in the CAAM, but the input is based on the total combined appliance input. This data and method is used to determine the un-corrected chimney height.

The chimney height is then corrected to take into account other factors such as building height and other topographical considerations. Today most room-sealed appliances are available with a vertical flue kit as well as a horizontal option. Also in most cases the appliance/s can use a conventional system chimney complying with BS EN 1856-1.

Through careful chimney system design using calculation methods as detailed in BS EN 13384-1/2 it is possible to exhaust a number of appliances into a common manifold with only one chimney rising to termination. Alternatively fan dilution could be considered. However, due to technical design considerations, high plumage, maintenance and potential nuisance factors, it is often avoided in favour of a more conventional chimney system.

The guidance explained

In consideration of the guidance offered in BS6644:2011, it is not acceptable to exhaust six appliances each with a gross input of 50kW at low level on the same wall. Although the individual input falls within the requirements of BS5440-1 and the appliance may well be supplied with a low level discharge kit, this is not the guidance offered under Annex F Section F4.1.

This guidance requires that the terminal location is assessed based on the total combined input for all appliances which in this case would be 300kW. In this case, Section F.4.3 covering inputs between 150kW and 366.4kW gross requires that the chimney height is determined based on the requirements of the CAAM regardless of appliance type.

The final chimney height is then corrected based on the building effects and the overriding minimum requirements of the memorandum. To achieve this would require either a common chimney or individual chimneys running internally or externally to the building and terminating at the corrected chimney height as calculated by one of the aforementioned methods.

// The author is technical sales manager at SFL Flues & Chimneys //
24 June 2013


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