Wet stacking can be prevented by periodically applying a proper electrical load to a diesel generator. A load bank is commonly used for this purpose because it forces the engine to operate at a higher load percentage, raising combustion temperature and cylinder pressure so fuel burns completely instead of accumulating in the exhaust system.

In many projects I’ve worked on, wet stacking almost always appears in generators that run for long periods at very light load. This is especially common in standby installations where the generator starts regularly but carries little or no electrical demand.

Load banks solve this problem by artificially applying electrical load, allowing the engine to operate under conditions similar to real-world power demand.

Why Low Load Causes Wet Stacking

A diesel engine is designed to operate under a certain load range. When the load is too low, several things change inside the engine.

First, combustion temperature drops.

Second, cylinder pressure decreases.

Third, fuel injection may not burn completely.

When this happens repeatedly, unburned fuel and carbon deposits begin to accumulate in the exhaust system.

Typical symptoms include:

• Black or oily liquid coming from the exhaust pipe
• Carbon buildup in turbochargers
• Fouled injectors
• Reduced engine efficiency

In the field, I often see this when generators run at less than 30% load for extended periods.

What a Load Bank Actually Does

A load bank is a device that converts electrical power into heat using resistive elements.

Instead of powering real equipment, the generator feeds electricity into the load bank. The load bank then dissipates that energy as heat, allowing the generator to operate under a controlled electrical load.

This creates operating conditions that are much closer to real-world demand.

Once the load is applied, several things improve inside the engine:

• combustion temperature rises
• cylinder pressure increases
• fuel burns more completely
• carbon deposits begin to clear

In many cases, running a generator under a proper load bank test can even help clean up early carbon buildup that started forming during low-load operation.

In practical maintenance programs, engineers often use portable or resistive load banks to apply controlled load during testing. If you're interested in how these devices work in real generator testing scenarios, you can also see our overview of diesel generator load banks.

Typical Load Levels Used in Load Bank Testing

In most generator maintenance programs, load banks are used to bring the generator to a realistic operating level.

A common testing pattern looks like this:

• 25% load for initial warm-up
• 50% load for stabilization
• 75% load for performance testing
• occasionally 100% load for short verification

From my experience, even running a generator at around 60–80% load for a short period can significantly improve combustion conditions.

This is why load bank testing is widely recommended for standby generators that rarely experience real electrical demand.

Where Load Banks Are Commonly Used

Load banks are especially important in systems where generators rarely operate at meaningful load.

Typical examples include:

Hospitals

Backup generators are started regularly for testing, but the building load may not be connected during the test.

Data centers

Generators must be verified under real operating conditions before commissioning.

Telecommunication sites

Remote generators often run for maintenance checks without carrying the full network load.

Industrial backup systems

Facilities sometimes test generators without switching the entire plant to generator power.

In these situations, load banks simulate real electrical demand and keep the engine operating in a healthy range.

Load Banks Also Reveal Hidden Generator Problems

Another reason engineers use load banks is that low-load operation can hide performance problems.

When a generator runs at very light load, issues such as cooling problems, fuel delivery limitations, or voltage instability may not appear.

Once the generator reaches higher load levels, these problems become visible.

That is why load bank testing is often part of commissioning procedures and preventive maintenance programs.

Load Banks and Generator Sizing Problems

In some projects, the real issue behind wet stacking is not only operating behavior but also generator sizing.

I’ve seen many installations where the generator was selected much larger than the actual load requirement. As a result, the unit rarely reaches an efficient operating range.

Oversized generators frequently run below 30% load, which creates ideal conditions for wet stacking.

If you're interested in the sizing aspect of this issue, I explained it in detail here:

Can an Oversized Diesel Generator Cause Wet Stacking?

Understanding this relationship between sizing and load behavior is important when diagnosing persistent wet stacking problems.

Relationship With No-Load Operation

Many people assume that wet stacking happens mainly during long periods of no-load operation.

In reality, the problem is more closely related to sustained low-load operation, not just brief no-load starts.

This is why generators are often started at no load only briefly before load is applied.

I discussed this operating practice in more detail in the main guide:

Is It Good to Start a Diesel Generator at No Load?

That article explains when no-load starting is acceptable and when it becomes harmful.

Conclusion

Load banks prevent wet stacking by forcing diesel generators to operate under a sufficient electrical load. This raises combustion temperature and cylinder pressure, allowing fuel to burn completely and preventing carbon buildup.

From a practical standpoint, load bank testing is one of the most effective ways to maintain healthy combustion conditions in standby generators that rarely carry real electrical demand.