Generator Stack Height

In Massachusetts, the requirement for generator exhaust stacks is commonly overlooked or misunderstood.

The 2015 International Mechanical Code (IMC) Section 501.3, as referenced by 780 CMR 28.00, requires that any air removed by a mechanical exhaust system be discharged outdoors at a point where it will not cause a public nuisance and will not be readily drawn in by a ventilating system in the subject building or any other building. Additionally, the IMC provides requirements for minimum exhaust outlet clearances including at least 10 feet from property lines, 3 feet from exterior walls and roofs, 10 feet from operable openings into buildings, and 10 feet above adjoining grade. IMC Section 915.1 also references the 2015 Edition of NFPA 37, Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines for the installation of liquid-fueled stationary internal combustion engines and gas turbines. NFPA 37 requires generators located on roofs or outdoors to be at least 5 feet from openings in walls or from structures having combustible exterior walls unless a 1-hour rated enclosure is provided (NFPA 37, 4.1.3.1 & 4.1.4)

In Massachusetts, generators installed on and after March 23, 2006 must also comply with 310 CMR, Department of Environment Protection Regulations. 310 CMR 7.26(42) addresses regulations to prevent air pollution from emergency engines or turbines and includes specific requirements relative to exhaust stack heights. These requirements are much more direct and, in many cases, are more stringent than the IMC as follows:

  • General Requirements:
    • Exhaust stacks must be configured to discharge the gases vertically and must not be equipped with any part of device that impedes the vertical exhaust flow of the emitted gases (e.g. shanty caps or egg beaters).
    • Emissions on sensitive receptors including people, windows and doors that open, and buildings with fresh air intakes must be minimized.
  • Generators with a power output ≥ 300 kW: Minimum stack height of 10 feet above the facility rooftop or the emergency engine enclosure, whichever is lower.
  • Generators with a power output ≥ 1 MW: Minimum stack height of 1.5 times the height of the building on which the stack is located. If the stack is lower than 1.5 times the building height or lower than the height of a building within the stack’s area of influence, the owner must submit documentation that the operation of the generator will not cause an exceedance of any National Ambient Air Quality Standard. This would typically involve computer aided dispersion analysis to confirm. The stack’s area of influence includes any buildings within a distance 5 times the building’s height or projected area, whichever is less.