Is Your Exhaust System Trying to Send You a Message?

The next time you inspect an emergency generator, have a close look at the exhaust system. Follow the exhaust piping, from the engine exhaust manifold, all the way to the pipe termination. Pay close attention and you may spot early signs of issues that should be addressed before they grow to larger, more expensive problems. Here are a few items to look for during monthly or quarterly inspections:


This is the most common and obvious problem. Excessive corrosion deteriorates the structural integrity of the silencer and exhaust piping elements. Eventually, these components will fail and need replacement. Whenever possible, exhaust silencers should be installed indoor, with only the exhaust pipe termination routed outdoor. Although not ideal, an outdoor installation is sometimes unavoidable. If this is the case, exhaust system elements should be specified to have stainless steel construction, especially when the installation is near corrosive environments (coastal areas, chemical processing plants, water treatment plants, etc).

Wet Stacking

Wet stacking is a common issue for standby/emergency generators. When a generator runs with insufficient building loads, oil can get past the piston rings to the combustion chamber. This oil, combined with unburnt fuel (due to the low operating temperature), will make its way downstream to the exhaust piping. Wet stacking can be identified by black exhaust smoke and oily stains seeping from exhaust piping terminations. This problem can be resolved by proper loading of the engine. If building loads are not sufficient, a load bank may be required. This is covered in more detail here.


Backfires are more common on older engines with mechanical fuel systems, but this problem can still occur on modern engines with issues related to their fuel management system. Backfires (in reality “afterfires”) are caused by unburned fuel in the exhaust stream which can ignite when the exhaust pipe is at operating temperature. Backfires generate substantial high pressure in the exhaust piping, and this can lead to catastrophic failure of the silencer. Any sign of stress (bulging, cracks, etc) to the body of the silencer should be investigated right away. As a precautionary measure, pressure relief valves are recommended in exhaust piping systems to relieve potential excess pressures in the event of a backfire. True “backfires” are even worse because pressure builds up and reverts “back” through the engine’s intake system. Although very rare, this can happen if the engine timing is off, and a cylinder intake valve opens when the combustion event is still ongoing.

Exhaust Gas Leaks

Exhaust gas leaks may develop at exhaust pipe flange connections. The flexible connector on the engine exhaust outlet is a common place for gas leaks. If not aligned correctly, the flexible connector may develop cracks at the flanges. Leaks can also develop at stressed joints that have no allowance for thermal expansion (learn more about flexible connectors and bellows). Exhaust leaks can be identified by dark, burnt areas on the insulation material covering these flanged joints. Gas leaks are a serious problem due to their high temperatures (>800F) and carbon monoxide content, both a safety and health concern to maintenance personnel and building occupants. Proper exhaust piping installation is key to avoid exhaust gas leaks.

One last note…

Inspecting an existing exhaust system is sometimes impossible due to the cast-in-place insulation that prevents visual inspection of piping elements. Calcium silicate insulation with an aluminum jacket is a common example of a “fixed” insulation method that I see in the field. To allow easy future inspection of indoor exhaust elements, I recommend removable thermal blankets for the major components of an exhaust system, such as the flexible connector, silencer and any expansion joints/bellows.

Need help?

If you are struggling with issues related to generator exhaust systems, go ahead and reach out for help. With 25+ years experience with engine exhaust systems, I can help with installation guidelines and product recommendations for your specific project. You may contact me here.

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