Flue Dilution

A flue dilution system is normally installed where difficulties are encountered in the provision of a conventional flue, or indeed where savings can be made if the only other alternative is a high rise chimney.

Flue dilution is a handy solution where there is no acceptable route for a conventional chimney. It is only suitable for VLS (very low sulphur, i.e. <0.04%) fuels up to a total combined gross heat input rating of 6MW.

How Does It Work?

The general principle is to mix ambient air from outside the building with the flue gas in order to dilute the CO² content to less than 1%. Traditionally this has meant that only natural gas and LPG are suitable, however the oil industry is making great strides to reduce the sulphur content of gas oil and it will not be long before the VLS benchmark of the Clean Air Act is reached. It is worth pointing out that flue dilution does not remove the smell inherent with a particular fuel, hence it is likely that any VLS oil-based fuel when used on a dilution system may still create significant odours at the point of discharge.


The location of the discharge must be compliant with the Chimney Heights Memorandum of the Clean Air Act (also now shown in BS6644 and IGE UP10). In common with all flue and chimney systems, local authority approval must also be sought for a flue dilution system. Indeed it is not uncommon for certain areas to have their own policy on flue dilution systems, particularly where noise is a concern.


The shift in the commercial boiler market, largely driven by government ERP regulations, towards condensing units, has a significant evolutionary effect on flue dilution. The flue gas temperature from these very efficient appliances is far lower than traditional atmospheric and pressure jet boilers, and as a result condensation is created within the flue. By then mixing this warm, moist gas with cooler air, the mixed gas temperature is reduced further, creating more condensation. There are a few critical considerations in this situation:


  • Material of flue system – we only ever use 316 grade stainless steel on condensing applications and flue dilution systems.
  • Capability of the flue system to handle condensation in huge quantities – for this reason we only use our PS flanged system.
  • Pluming at the discharge – the suspended water molecules in the diluted flue gas will be discharged out through the louvre to atmosphere. In cold ambient temperatures this can often be significant and can cause a genuine nuisance to local residents and building occupants. Therefore careful consideration of the discharge position is of the utmost importance, as is control of system drainage.

Principles Of Flue Dilution

Each system is designed on the principle of diluting the vented products of combustion sufficiently to reduce the CO² content to the required level of below one per cent (BS 6644:2005+A1:2008) and less than 100ppm for carbon monoxide.


The Clean Air Act dictates that this type of system can be fitted to gas-fired appliances rated at up to 6MW, the principle being that fresh air and products of combustion from the boilers are drawn into a duct by a fan and consequently diluted, thus guaranteeing the low CO² and carbon monoxide discharge levels.


With over 30 years experience in this type of installation, we can ensure compliance with the Clean Air Act, D1, BS 6644:2005+A1:2008 and IGE/UP/10 Part 1, Edition 2. Advice is also offered on noise reduction methods in conjunction with our acoustic consultant where necessary.


Our technical design team carefully analyses each system on its merits, taking proper consideration to comply with the Clean Air Act regulations as well as Environmental Health Noise Control requirements. Regulations dictate that only very low sulphur fuelled appliances are used with this type of system design.


In accordance with the Clean Air Act, combustion products and air from the inlet are mixed by the fan in the centre of the system until the required dilution of less than 1% CO² is achieved. The diluted flue gases are then exhausted through the outlet.

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