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FAQ 99 – Calculating Chimney Height Below Roof Level

How do I calculate chimney height for fan-diluted emissions below roof level?

The Chimney Heights Memorandum advises that certain fan-diluted emissions may be discharged at heights below the roof level of the building or directly through the roof without a definite chimney. This might lead to local exceedances of the prescribed annual mean objective for nitrogen dioxide in areas where there is already a significant background concentration of nitrogen dioxide or where there are places where members of the public might be regularly exposed to the discharge. The potential for exceedance of the objective may be assessed using the LAQM Industrial Emissions Screening Tool.

Alternatively, the table below provides thermal input rate screening limits for fan diluted boilers with low NOx burners (<0.02g/MJ). The table is based on the information provided in the Review of the Clean Air Act Provisions for Dispersion from Small Gas Boilers, linked below. Larger boilers will require more detailed assessment using an appropriate dispersion model.

Screening Limits for Boiler Capacity Based on Allowable Boiler Heat Input (MW)
Distance of Nearest Receptors (m) 20 50 100 200
Annual Background NO2
Concentration – 30µg/m3
0.6 2.1 5.7 16.6
Annual Background NO2
Concentration – 32µg/m3
0.5 1.7 4.6 13.3
Annual Background NO2
Concentration – 34µg/m3
0.4 1.3 3.4 10
Annual Background NO2
Concentration – 36µg/m3
0.2 0.8 2.3 6.6
Annual Background NO2
Concentration – 38µg/m3
0.4 1.1 3.3
Annual Background NO2
Concentration – 40µg/m3
0.2 0.6 1.7


Review of the Clean Air Act Provisions for dispersion from Small Gas Boilers

Released: September 2005 (PDF, 426KB, 37 pages)

Hosted on www.uk-air.defra.gov.uk

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