How do I calculate an appropriate chimney height for furnaces/boilers not subject to controls under the Clean Air Act?
Individual gas boilers with less than 366.4 kW thermal input are not regulated under the Clean Air Act, although installations may be subject to building regulations and British and European Standards. The Chimney Heights Memorandum (HMSO, 1981) may be used for boilers with thermal inputs between 150 kW and 366.4 kW and this is generally considered to be good practice. It is unlikely that individual boilers with thermal inputs less than 366.4 kW will lead to exceedances of the air quality objectives over any significant area.
Boiler manufacturers are able to meet energy requirements in some cases by installing more than one small boiler on a modular basis. Individually, these boilers may have heat input ratings less than the Clean Air Act lower limit of 366.4 kW, although the aggregate capacity of several installed boilers is greater than the limit. In such cases, if each small boiler is served by its own separate chimney, the provisions of the Act regarding chimney heights should not apply (see FAQ 101). However, Section 64(6) of the Act (general interpretation) indicates that where a number of furnaces or boilers are served by one chimney then, for the purpose of the determination of chimney height under Section 14 of the Act, these will be treated as one furnace/boiler.