The exhaust system of an emergency power supply (EPS) provides a means for exhaust gases from the engine to travel to the outside atmosphere. The components of the exhaust system include the exhaust manifold, turbocharger (if so equipped), flex sections, muffler hangers, supports, end pipe, thimble, insulation, and rain cap. While there is not much maintenance involved other than visual checks, compliance can be an issue with incorrect testing procedures.


NFPA 110 (2019 Edition) 8.4.2* Generator sets in service shall be exercised at least once monthly, for a minimum of 30 minutes, using one of the following methods:

  1. Loading that maintains the minimum exhaust gas temperatures as recommended by the manufacturer
  2. Under operating temperature conditions and at not less than 30 percent of the EPS standby nameplate kW rating

What does the above paragraph have to do with exhaust systems? Currently, NFPA 110 requires generators to be tested monthly under a load that is at least 30% of the nameplate rating. As an alternative to the 30% of nameplate requirement, it is allowable to test the generator with a load “that maintains the minimum exhaust gas temperatures as recommended by the manufacturer.” This is of interest to those facilities whose generators are unable to be tested consistently at the minimum 30% of the nameplate rating.


There are a couple of ways to perform monthly testing and take accurate exhaust temperature readings.

Option A: Thermocouple

On the high-end side of the expense scale is a temperature probe unit, or thermocouple, which is embedded in some portion of the exhaust system or cylinder head and sends data automatically. Once installed, tracking exhaust temperatures would be a snap. The drawback to this system is the price: they’re rather expensive to retrofit – upwards of several thousand dollars per engine. Another disadvantage is that their use is limited to recording temperatures for the specific location in which they are embedded.

Option B: Infrared Instrument

A less costly, and more practical option involves the use of infrared instruments. With this approach, however, the question of where to obtain the readings becomes the issue. The best place to get the highest readings is at the exhaust manifold-to-cylinder head connections which are closest to the combustion chamber where the manufacturer takes their readings.

Unfortunately, these connections are not easily accessible on some engines, and attempting to obtain readings here may present a real threat to personal safety and should be avoided. We recommend the safer option of specifying a location on the exhaust manifold where all exhaust ports empty their gases and obtaining temperatures here each month. Since your specified location may be some distance from the combustion chambers, the temperatures you obtain will be cooler, and the data must be adjusted for this difference.

Option C: Annual Supplemental Load

If a facility is unable to make the 30% of nameplate rating and does not have the exhaust temperatures that demonstrate the generator is sufficiently loaded, an annual supplemental “load run” will need to be performed. As stated in NFPA 110,, installations that do not meet the requirements of 8.4.2 shall be exercised monthly with the available EPSS load and shall be exercised annually with supplemental loads at not less than 50 percent of the EPS nameplate kW rating for 30 continuous minutes and at not less than 75 percent of the EPS nameplate kW rating for 1 continuous hour for a total test duration of not less than 1.5 continuous hours.”

While the monthly test loading of a generator above the 30% level is not required by NFPA 110 beyond the above circumstances, we recommend loading the generator to 75-100% of the nameplate “on occasion” to benchmark exhaust temperature readings starting at a no-load level. The graph below is a record of such Exhaust temperatures.

Left = Left Turbocharger
Right = Right Turbocharger
MFG = Manufacturer Specifications

Manifold temperature data can provide a very accurate picture of the condition under which an engine is running. Readings that are too low – after adjusting for the temperature differential between the combustion chamber location the manufacturer used for their recommendations and the location the facility chooses to obtain their data – can be a clear indication that wet stacking conditions are present, and appropriate action can be taken to alleviate the problem.