How do I calculate chimney height for multiple gas boilers/furnaces which are individually below the Clean Air Act threshold of 366.4 kilowatts and are not served by one single chimney but several separate ones?


Section 64(6) of the Act indicates that where more than one boiler discharges through a single chimney then the chimney height should be determined on the basis of the aggregate capacity of the boilers. However, where each boiler of capacity less than 366.4 kW discharges through an individual chimney the Act may not apply and discharges below roof level may be allowed. However, there is a risk that such installations of modular boilers might lead to local exceedences of the prescribed annual average objective for nitrogen dioxide 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 close to the point of discharge.

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