Monday, December 9, 2019

Diesel Exhaust Requirements

The exhaust from a diesel engine driving a fire pump is an often-overlooked item.  The high heat put out by the exhaust is a hazard to occupants and can greatly contribute to overheating the room itself. NFPA 20 (2013 edition), section 11.5 covers "Engine Exhaust" and should be carefully reviewed. However as a starting point, the following items should be considered.

Exhaust Discharge Location

The first step is to determine where you are going to discharge the exhaust gases. NFPA 20 section 11.5.3 for "exhaust discharge location" has the generic common-sense requirements of do not discharge where you might hurt people, damage the building, or directed on combustible materials. But very little specific requirements are provided. There is a loose reference in NFPA 20 back to NFPA 37 (Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines). In addition, the International Mechanical Code (IMC) section 915.1 directs us back to NFPA 37 for guidance.

NFPA 37-2018 edition section has the same generic recommendation that "Exhaust systems shall terminate outside the structure at a point where hot gases, sparks, or products of combustion will discharge to a safe location."

Not well referenced by the codes around exhaust, are the requirements for intake louvers.  These requirements provide more hard numbers that the exhaust sections.  The IMC section 401.4 requires intake louvers to be a minimum of 10-feet away from any hazardous or noxious sources (diesel exhaust would be considered noxious).
Example Exterior Exhaust Discharge - Not near any doors

Exhaust Sizing

Don't assume that just because the engine and muffler have a 4-inch connection, that you should run 4-inch all the way to the exterior.  Back-pressure on the engine due to excessive distances to the end of the exhaust pipe can cause poor engine performance. NFPA 20 states that the exhaust shall not be smaller than the engine outlet size and be as short as possible. NFPA 20 section A.11.5.2 further provides the generic guidance that if you have more than 15 linear feet (4.5 meter) of exhaust pipe, you should increase your exhaust one pipe size for each additional 5-feet (1.5 meters) of length.

However, rather than using this generic rule-of-thumb, we recommend consulting with the manufacturer.  Both Clarke and Cummins have some very easy to use calculators so that there is no question about exhaust sizing. Links to their web pages are provided below:

Exhaust Through Walls/Roofs

Obviously you don't want to put your 1200 F degree metal exhaust against wood studs and siding. NFPA 20-2013 section 11.5 provides the following guidance:
  • COMBUSTIBLE ROOF - Guarded at the point of passage through the combustible roof by ventilated metal thimbles that extend not less than 9-inches on each side (above and below) of roof construction and are at least 6-inch in diameter larger than the exhaust pipe or duct.
  • COMBUSTIBLE WALL - Metal ventilated thimble not less than 12-inches larger in diameter than the exhaust.
NFPA 37-2018 section 8.3 is also applicable for exhaust gas temperatures less than 1,400F (760C). Based upon a quick review of the data sheets for Clarke and Cummins, the typical exhaust temperature does not exceed 1,100 F. The requirements of NFPA 37 are basically the same as NFPA 20.
Diesel Exhaust Wall Thimble - Exterior

Diesel Exhaust Wall Thimble - Interior


The exhaust pipe shall be covered with high temperature insulation or otherwise guarded to protect personnel from injury. This should also include the required minimum 12-inch flexible piece shall be installed between the engine and the exhaust piping (this piece is provided standard with all packages provided by Anvil Fire).

Diesel Engine Exhaust - Without Insulation

Diesel Engine Exhaust - With Insulation
Contact Anvil Fire ( for support and sales of any product referenced in this article.