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Flue dilution systems - a lot of hot air?

The inner workings of flue dilution systems needn't be viewed as a 'dark art'. Bob Sharples explains why flue dilution is still a viable option to discharge CO2 safely at low level
Let's go back to commercial boiler basics with the components of combustion: fuel, oxygen and a source of ignition; different types of commercial boiler fuel - natural gas, liquid petroleum gas, coal, oil, and wood (in varying types) all of which as we know are carbon based fuels. All boilers during their operation also produce other products of combustion (POCs).

An efficient boiler will produce 11 to 12 per cent CO2 in its flue gases. As a result of combining the carbon in the fuel with the oxygen in the air these POCs are gases that are potentially dangerous in volumetric amounts. It is therefore necessary to ensure that these products of combustion are discharged safely into the atmosphere.

A flue is a bespoke duct to carry out this process by normally discharging the POCs at a high and safe level away from any potential inhalation or ingestion by the public. They can vary in construction from cast iron, steel and stainless steel (stainless steel is mainly used in instances of high condensate saturation levels due to the high efficiency and low exhaust gas temperatures of modern commercial condensing boilers.) These flues have to be designed to be a certain height under regulations, have to comply with local planning requirements, and are therefore expensive to manufacture and subsequently expensive to the end client.

So let's look at some of the challenges facing modern commercial building designers and boiler system installers?. We've all seen the tall, metallic chimneys jutting at high level from the sides of commercial premises, and in some cases these are unavoidable within the specific design of not only the building but also the overall boiler system.

However, unsightly and expensive chimneys continue to be a bugbear of commercial building and plant designers. Imagine the problems with this design. You have to place an expensive stainless steel chimney up the outside of a building. As you would expect, neither party are too happy with this approach, as it is more difficult to incorporate, there may also be planning issues arising from this type of design that those involved need to be aware of.

A flue dilution system is a cost-effective, tried and tested solution to these problems.

Discharged from low level

This arrangement of ducting incorporates a fan unit which mixes fresh air with the products of combustion, normally at a ratio of 10 parts air to 1 part product of combustion, which then allows safe lower level disposal directly to the outside: in lay-mans terms, take fresh air from low level, mix it with the products of combustion and discharge it all out at low level.

It may seem complicated, but it's not rocket science, in fact the principle works very well in practice. Flue dilution is sometimes perceived in industry as a 'dark art' with the regulations covering its use complicated and fraught with hurdles of compliance, but once you understand them Flue Dilution is actually a very simple concept.

The Regulations covering this type of system are:

  • The 3rd Edition of the 1956 Clean Air Act, Chimney Heights Memorandum, the only stipulation being the need to advise the local environmental health officer (EHO) of the proposed installation.

  • British Standard (BS) 6644: 2005 + A1: 2008: Specification for the installation of gas fired hot water boilers of rated inputs between 70kW (net) and 1.8MW (net).

  • Institution of Gas Engineers and Managers (IGEM)IGE/UP/10(Edition 3) Installation of flued gas appliances in industrial and commercial premises.

  • The dilution system concept, as explained above, is to take dilution air from a duct connected to the outside the boiler room, combine it with the products of combustion and then discharge the diluted combined gas mixture through an outlet grille at low level.

    You can also provide the dilution air from the boiler room itself, however this requires a large amount of additional ventilation ducted directly from outside to replace the air drawn into the flue system and the Institution of Gas Engineers and Managers (IGEM)IGE/UP/10(Edition 3) needs to be consulted or a specialist calculation provided by the dilution fan manufacturer (as given with the Airflow flue dilution range) needs to employed to correctly asses this level of ventilation, and is normally supplied for both high and low level discharge positions.

    For low level (intake): 1,080 cu cm plus 9 cu cm for each kW input in excess of 60kW rated input.

    For high level (extract): 280 cu cm plus 2.25cu cm for each kilowatt input in excess of 60kW rated input.

    Also included as a safety feature in all dilution systems is a regulatory pressure device or pressure safety switch, normally on the dilution fan itself, so in a rare occasion that a problem with the diluting flue fan or flue duct system arises, it will fail safe and prevent operation of the gas burner under the following conditions:

  • Loss of fan air supply due to blocked intake, discharge flue or fan motor inlet (diluting air)

  • Stalled fan motor

  • Interrupted power supply

  • For normal operation this switch allows the boiler to fire when the fan is operating correctly and dilution air is entering the fan. Most flue dilution fans, depending on their manufacturer, can handle the following fuel types:

    Natural gas, or liquefied petroleum gas (LPG) and are compatible with atmospheric, condensing, modular and pressure jet boilers (with draught diverter).

    Flue dilution systems are limited to the to the fuel types given above because there is no way to shut off an oil fired or solid fuel boiler immediately in the event of a break in the flue gas flow through a dilution fan, or fan failure.

    The volume flow rate for correct dilution is calculated as follows (based on clause IGE/UP/10 - Edition 3)

    Q = Fn x rated net heat input (kW)

    Where: Q = volume flow rate (cu m/s) and Fn = Fan dilution factor (10.8 for natural gas or 12.8 for LPG)

    Another misconception surrounding flue dilution systems is that you can only dilute the POCs of a single boiler. While designers and installers of flue systems will adhere to the calculations required by the regulations for single boiler dilution it is possible to create multiple boiler dilution systems as well. By ensuring you use the correct size of flue dilution fan - based on the total kW input - plus a common header (and the use of lockable dampers and draught diverters to stop the build it of POCs re-entering the system) multiple boiler dilution can be easily achieved.

    As boilers strive to achieve the highest efficiencies, and new technology enters the marketplace, flue dilution still remains a viable and cost effective alternative to expensive and unsightly chimneys for commercial boilers using gas based fossil fuels.

    // The author is industrial product manager at Airflow Developments //
    7 March 2013


    Paul Martin
    14 June 2017 07:15:26
    Hi are you able to tell me the minimum distance for a flue terminating next to flue dilution intake vent. Kind regards
    Alan Evans
    29 July 2014 08:25:31


    Just one question!

    What are your thoughts on fan-dilution systems serving condensing applications?



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