How do I calculate an appropriate chimney height to ensure emissions of NO2 from small gas boilers controlled by the Clean Air Act do not lead to an exceedence of the Air Quality Strategy's objective for nitrogen dioxide (NO2) prescribed in the Air Quality Regulations 2000, SI No 928?


The third edition of the Chimney Heights Memorandum (HMSO,1981), provides guidance to local authorities in exercising their functions under section 14 of the Clean Air Act 1993. The Memorandum is not a statutory document. It provides a relatively simple method of calculating the approximate height of chimneys desirable in normal circumstances. The guidance may also be applied to those furnaces/boilers which are not statutorily bound by the Act. The Memorandum was last revised in 1981, and therefore predates the current prescribed air quality objectives.

In practice, for conventional small gas boilers burning very low sulphur fuels (including natural gas) and discharging above roof level through natural draught or mechanically assisted flues, the chimney height determined by the Memorandum will be sufficient to prevent local exceedences of the air quality objectives for NO2. For advice on chimney height calculations in respect of emissions of sulphur dioxide from small boilers and the potential for exceedance of the SO2 air quality objective. For further information on please see the Supplementary Assistance section of this website.

Since 1981 condensing boilers have been widely installed throughout the UK and can benefit the environment by providing improved energy efficiency. However, the flue gases are discharged at much lower temperatures than conventional boilers and thus the buoyancy and momentum of the discharge is much reduced. The efflux velocity for these boilers is likely to be less than the target velocity of 6 m/s at full load for boilers up to 2.2MW specified in the Memorandum. Nevertheless, the stack height determined by the Memorandum will normally be sufficient for condensing boilers with low NOx burners (<0.02g/MJ) with thermal input less than 1 MW burning very low sulphur fuels and discharging above roof level. Larger boilers will require more detailed assessment, using an appropriate dispersion model.

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