Choosing an RECD for a Generator That Usually Runs at Low Load

A generator that never works hard sounds like a generator that should last forever. In practice, the opposite is often true. Diesel engines are built to burn fuel at a healthy load, and when they spend most of their life well below it, they start to misbehave in quiet ways: oily residue at the exhaust joints, a black streak down the canopy, oil consumption that creeps up year after year.

Add a retrofit emission control device to that engine and a new question appears. The particulate filter inside the RECD needs heat to clean itself. A lightly loaded engine does not produce much heat. So will the device clog?

It can, if the problem is ignored. It need not, if the device and the site are planned together. This guide starts by asking why the load is low, because the answer shapes everything else. It then looks at what low load does to exhaust temperature, how to judge whether an RECD is still the right route, what to look for in the device, and which fixes on the load side give the best return.

First, Find Out Why the Load Is Low

Low load has different origins, and each points to a different remedy. Before you talk to a vendor, decide which of these describes your set.

The set was bought too big

A planner sized the generator for an expansion that never happened, added a large safety margin or simply chose a round number. The connected load was never close to the rating. This is common in new buildings, where consultants size for the full design load and actual demand turns out to be a fraction of it. Our guides on load calculation basics before buying a generator and how to size a generator for a commercial building explain how that happens.

The load shrank after purchase

The set was right when bought, but the building changed. LED lighting replaced older fittings, inverter air conditioners replaced fixed-speed units, a production line moved, staff moved to hybrid working or a tenant left. The DG now carries a fraction of what it used to.

The load is low only at certain times

The set carries a healthy load during working hours but spends long night-time or weekend outages carrying security lighting, a few pumps and a lift. Its average looks reasonable, but many of its running hours are spent at very light load.

The first two are structural and tend to persist. The third is a pattern problem that can often be managed. Most sets that come to us with a low-load concern are a mix of all three.

What Low Load Does to the Exhaust

The temperature of diesel exhaust depends mainly on how much fuel is burned relative to the air drawn in. At light load, the engine takes in almost the same air as at full load but burns much less fuel, so the exhaust is diluted and cool.

The table below gives a rough picture for a turbocharged generator engine, measured at the turbine outlet. Real figures vary by engine and should be taken from the maker’s data sheet.

Load (% of prime rating)	Indicative exhaust temperature at turbine outlet	What it means for passive regeneration
10%	Roughly 150 to 220°C	Generally too cool for useful soot oxidation
20%	Roughly 200 to 280°C	Marginal for many catalysed filters
30%	Roughly 250 to 330°C	Borderline to adequate depending on catalyst
50%	Roughly 330 to 420°C	Usually adequate
75%	Roughly 400 to 480°C	Good
100%	Roughly 450 to 550°C	Good

Two cautions apply. First, the RECD may be several metres downstream, and exhaust cools along the way. Second, catalyst formulations differ, so the temperature at which regeneration becomes effective depends on the device. Ask your vendor for the figure that applies to their product, then compare it with where your set actually runs.

The Two Casualties: Engine and Filter

Light loading harms the engine and the RECD in linked ways.

On the engine side

• Wet stacking. Unburned fuel and lubricating oil pass into the exhaust and collect as a dark, oily residue. The condition is widely documented; see the Wikipedia article on wet stacking.
• Cylinder glazing. Low combustion pressure fails to push piston rings firmly against the cylinder wall, and the wall can become polished. Oil control suffers.
• Carbon build-up. Injector tips, valves and ring grooves gather carbon.
• Fuel in the oil. Unburned fuel washes past the rings and dilutes the lubricating oil.
• Turbocharger oil seepage. With little boost pressure, some turbochargers let small amounts of oil past their seals.

Our article on why a generator emits white smoke describes how unburned fuel appears in the exhaust, and why a generator emits blue smoke covers oil burning.

On the RECD side

• Slow regeneration. Cool exhaust means trapped soot burns off slowly or not at all during normal running.
• Wet deposits. Oily, fuel-rich soot sticks to the inlet face and is harder to oxidise than dry soot.
• Ash build-up. Oil that is burned leaves incombustible ash, which slowly fills the filter channels and can only be removed by cleaning.
• Rising backpressure. As soot and ash accumulate, resistance to exhaust flow increases, which the engine feels as a restriction. Our explainer on how exhaust backpressure affects a DG set covers the consequences.

The engine problems feed the filter problems. That is why the remedy has to address both.

Is an RECD Still the Right Route?

For most lightly loaded sets in regions where emission control is required, the answer is yes, with conditions. A few questions help decide how much work is needed.

What is your engine power band?

Compliance routes in some regions depend on engine size. In NCR, for example, CAQM Direction No. 76 set out different routes for different power ranges. Check how current rules apply to your set; our comparison of RECD vs dual fuel kit may help if both are open to you.

How low, and for how long?

A set that averages 35% with some light-load hours is in a very different position from one that sits at 12% for every hour it runs. Pull a month of load data and work out the share of running hours below 30% of rating.

Can the load be raised?

If load can be added, consolidated or scheduled onto the DG during outages, many problems shrink. If it cannot, the device and maintenance plan must carry more of the burden.

Is the set the right size for the next ten years?

If a building will always need far less than its generator’s rating, replacing the set with a smaller one, or adding a smaller set for light-load periods, may be better value in the long run than retrofitting a large one. Our article on retrofitting an old DG set with an RECD and the comparison of RECD vs a new CPCB IV+ genset help frame that choice.

Three Ways Forward

Most sites end up choosing one of three paths, or a combination.

Path	What it involves	Best suited to	Trade-offs
Fix the load	Bring useful load onto the DG during outages, schedule heavy loads, add periodic loaded runs or a load bank	Sets with pattern-related low load, or modest oversizing	Needs operational discipline; a load bank costs money to buy or hire
Fix the set	Add a smaller set for light periods, or replace an oversized set	Sets that are structurally far too large for the long-term load	Capital cost; space for a second set
Fit for the duty	Choose a device and maintenance plan designed for light-load operation	Sets where load cannot easily change	Higher service attention; possibly a larger or more specialised device

In practice, fitting for the duty is almost always part of the answer, and fixing the load is the most cost-effective supplement.

What to Look For in the Device

When the set will spend a lot of time at light load, these features of the RECD and its installation deserve extra attention.

A catalyst that works at lower temperatures

Ask the vendor what inlet temperature their catalyst needs for effective soot oxidation, and how that compares with your set’s exhaust temperature at its usual load. Some formulations are designed to be active at lower temperatures. The difference between a catalyst that starts working at 250°C and one that needs 350°C can decide whether a lightly loaded set regenerates at all.

Enough soot and ash capacity

A lightly loaded engine sends more soot and ash per hour of useful work. The filter needs enough storage to cope between regeneration opportunities and between cleanings. Discuss the expected cleaning interval at your load pattern.

A placement that preserves heat

Exhaust cools as it travels. Mounting the device close to the engine outlet, using short pipe runs and insulating the pipework all help keep inlet temperature up. On canopy sets, the silencer position is often the best location; whether an RECD can replace a generator’s exhaust silencer discusses that option.

Monitoring that shows the trend

On a lightly loaded set, you need early warning when soot builds faster than it burns. Ask for continuous recording of backpressure and inlet temperature, with alarms at clear thresholds. A device that only shows a reading when someone looks at it is less useful here.

A sensible backpressure allowance

Size the device so that, even with a normal soot load, total system backpressure stays inside the engine maker’s limit with margin.

A clear service plan

Ask the vendor to put in writing how often they expect the filter to need inspection, regeneration support or cleaning at your load profile, and what each visit costs. A plan built around your real data is worth more than a generic interval.

Valid approval

Confirm that the device model is covered by CPCB approval for your engine’s power, under the CPCB procedure for RECD emission compliance testing. Our guide on how to verify a CPCB tested RECD shows how.

Fixing the Load: Options That Work

Bring useful load onto the DG

During long outages, deliberately transfer loads that do useful work: water pumping to overhead tanks, chilled water storage, battery and UPS charging, laundry, kitchen equipment or production that can be shifted. This raises load when the DG is already running, so it costs little extra fuel per useful kWh.

Schedule a weekly loaded run

Instead of a short test at no load, run the set for 30 to 60 minutes at 50% or more of its rating. If building load cannot provide that, a load bank can.

Use a load bank

A resistive load bank converts electrical output into heat in a controlled way. It can be permanently installed and switched in automatically when the building load falls below a threshold, or brought in periodically for exercise runs. It burns fuel without producing useful work, so it is best used in measured doses: enough to keep the engine and filter healthy, not more.

Consolidate onto fewer sets

Sites with several sets running together at light load can often run fewer sets at higher load. A synchronising panel with load-demand control can do this automatically, provided it is set up to avoid frequent start-stop cycling of the second set.

Add a smaller set

A smaller set dedicated to light-load periods, such as nights and weekends, lets the large set rest until a heavy load appears. This is a larger investment but solves the problem at its root.

Replace the oversized set

If the set is old, far too large and expensive to retrofit, a new correctly sized set may be the best long-term answer. Our guide to diesel generator buying for small businesses and the guide for large industries cover sizing for each case.

A Worked Example: The Gurugram Office Tower

Here is an illustrative case built to show the method. A 625 kVA set (about 500 kWe) backs up a 12-storey office building in Gurugram. It was sized when the building was designed for full occupancy with older chillers and lighting.

What the data showed

• Average running load over 90 days: 19% of rating, about 95 kW
• Share of running hours below 30% load: 84%
• Highest sustained load in the period: 41%, during a weekday afternoon outage
• Weekly test: 15 minutes at no load
• Visible signs: oily residue at the exhaust joints, a light black streak on the canopy roof

Why the load was low

All three origins were present. The design load was generous, the building had since moved to LED lighting and inverter chillers, and two floors were vacant. Night outages left the set carrying only common areas.

What was agreed

1. Engine first. The service provider replaced two injectors, cleaned the turbocharger compressor and changed oil and filters. Our article on whether to complete an engine overhaul before fitting an RECD explains when this step is needed.
2. Weekly loaded run. A hired load bank was replaced after three months by a permanently installed 200 kW unit, switched in during the weekly test to bring total load to about 55% for 45 minutes.
3. Outage load management. During daytime outages longer than 30 minutes, the building management system brings chilled water storage and water pumping onto the DG.
4. Device selection. The vendor proposed a device with a lower-temperature catalyst, mounted in place of the existing silencer on the canopy roof with insulated pipework, and continuous monitoring of backpressure and inlet temperature.
5. Service plan. Inspection every three months for the first year, with the interval reviewed against the monitoring data.

What changed, illustrative

• Share of running hours below 30% load fell to about 55%
• Each week now included at least one 45-minute run above 50% load
• The backpressure trend stayed stable over the first six months, with a visible drop after each loaded run

The set is still oversized. The building’s facility team is budgeting for a smaller second set in the next capital cycle. In the meantime, the RECD works.

How to Present Low-Load Data to a Vendor

A clear summary speeds up the design. Include:

Item	Example entry
Set rating and engine model	625 kVA, engine model and kWm from the nameplate
Period covered by data	90 days, June to August
Average load while running	19%
Share of hours below 30% load	84%
Share of hours above 50% load	Under 3%
Typical run length	Median 55 minutes
Test routine	15 minutes weekly, no load
Signs of wet stacking	Oily residue at joints, black streak on canopy
Planned changes	Load bank, outage load management
Exhaust route	Photos and measurements

Our article on DG nameplate photos for an RECD enquiry lists the pictures to include.

Counting the Cost

Low-load operation adds cost in several places, and it helps to see them together.

• Device choice. A device designed for light-load duty may cost more than a standard unit.
• Service. More frequent inspection, regeneration support or cleaning.
• Load bank. Purchase or hire, plus the fuel it burns during exercise runs.
• Engine maintenance. Light-load damage, if left unchecked, leads to expensive repairs.
• Fuel waste. An oversized engine at light load burns more fuel per useful kWh than a correctly sized one.

Against these, a correctly sized set costs capital. Our guide to RECD cost in India covers device pricing, and tips to reduce diesel generator fuel consumption helps with the running side. For many sites, the sensible plan is to fit an RECD suited to light load now, improve the load pattern immediately, and plan the right-sizing decision for the next replacement cycle.

Sector Notes

Low load is especially common in a few kinds of building:

• Schools and colleges, which often have large sets sized for peak summer loads that run during holidays and evenings at a fraction of rating. See RECDs for schools and RECDs for university campuses.
• Office buildings, where hybrid working and efficiency upgrades have cut loads.
• Residential societies, where night-time outages leave large sets carrying common areas only.
• Hospitals, where sets are sized for full critical load plus redundancy, so each set often runs light. See RECDs for hospitals.

Common Misunderstandings

“Running at low load saves fuel, so it is good for the engine.”

It does burn less fuel per hour than running at high load. But fuel per useful kWh is usually worse, and the engine suffers from incomplete combustion.

“The RECD will clean itself eventually.”

Only if the exhaust gets hot enough for long enough. On a set that never leaves light load, that may not happen without help.

“A load bank is wasteful.”

It burns fuel without useful output, so it should be used in measured amounts. Used that way, it protects the engine and the filter and can avoid far larger repair and cleaning costs.

“We will deal with low load after the RECD is fitted.”

It is better to deal with it before. A device chosen without knowing the load profile may be the wrong one, and an engine with existing wet-stacking damage will load a new filter quickly.

Frequently Asked Questions

Can an RECD be fitted to a generator that runs at very low load?

Yes, but low load slows filter regeneration. Choose a device with a catalyst suited to lower temperatures, mount it to preserve heat, monitor backpressure and add periodic loaded runs.

What load is too low for an RECD?

There is no single figure. Many engine makers advise against prolonged running below about 30% of rating, and regeneration often becomes difficult at sustained loads below 20 to 30%, depending on the catalyst and layout. Ask the vendor what applies to their device.

Should I fix the engine before fitting an RECD on a lightly loaded set?

If there are signs of wet stacking, oil burning or injector problems, yes. Fixing them first protects the new filter.

Does a load bank help an RECD?

Yes. A load bank used for regular exercise runs raises exhaust temperature, helping the filter regenerate and the engine burn off deposits.

Is it better to replace an oversized generator than to retrofit it?

Sometimes. If the set is old, far larger than the load will ever be and expensive to keep healthy, replacement may be better value. Compare costs over several years.

How often will an RECD on a lightly loaded set need cleaning?

It depends on the engine, device and load pattern. Ask the vendor for an interval based on your data and review it against monitoring readings.

Give the Engine Some Work

A generator that idles through most of its life is not a gentle case for an RECD. The filter needs heat, and a lightly loaded engine is short of it. Find out why the load is low, fix the engine’s existing light-load damage, give the set regular loaded runs, choose a device with a catalyst and placement suited to cooler exhaust, and watch the backpressure trend. Then decide, with real numbers, whether the set is the right size for the years ahead.

Aceget reviews load data for every RECD proposal and supplies CPCB type-approved devices for DG sets from 25 kVA to 2500 kVA. For the size discussed in this article’s example, see the 625 kVA DG set RECD. Send your load records to Aceget for an assessment, or browse the retrofit emission control device range by rating.