September 20, 2026
Posted by: Aceget
Category: RECD & Emission Control Devices

A correctly sized and properly installed RECD should not reduce the usable power output of a diesel generator in any way you can measure in daily use. A device that is too small, blocked, badly routed or fitted to an engine already running close to its exhaust limit can cost power, raise fuel consumption and push exhaust temperatures up. The difference between those two outcomes comes down to a single technical quantity, exhaust back pressure, and to whether anyone measured it.

Many owners hear only one half of that sentence and worry. Others hear only the other half and stop asking questions. This guide gives you the whole picture by treating generator output as a ledger, a list of debits and credits that turns the number on the nameplate into the power your building can actually draw. Once you see which lines an RECD touches, and which it cannot touch, the question stops being a worry and becomes a test you can run.

The Kilowatt Ledger

Start with a simple idea: the figure on your generator’s nameplate is a starting balance, and several things draw it down before the power reaches your switchboard. The table lists them in the order they occur.

Ledger line	What it is	Can an RECD change it?
Engine rating basis	The power the engine maker declares for stated conditions and a stated exhaust restriction	No
Site conditions	Ambient temperature, altitude, air quality, room ventilation	No
Engine auxiliaries	Cooling fan, pumps and other loads taken from the engine	No
Exhaust restriction	Total resistance in the exhaust path: silencer, pipework, stack and any emission device	Yes, this is the RECD’s line
Alternator efficiency	Electrical and mechanical losses in the alternator	No
Power factor of the load	The relationship between kVA and kW	No
Condition over time	Wear, fouling, blocked filters and choked passages	Only through maintenance

Of seven lines, one belongs to the RECD. That does not mean the line is unimportant, because a bad entry there can hurt the whole account. It does mean that when someone says “the generator lost power after the RECD,” there are six other places to look before blaming the device.

Line by Line

Engine Rating Basis

Every generator engine is rated by its maker for a given set of conditions, and the rating assumes the exhaust system will not exceed a specified back pressure. That specified figure is the engine’s “allowance.” Any combination of components that stays inside the allowance leaves the declared power intact. Any combination that exceeds it eats into performance.

This is why the phrase “does an RECD reduce power?” cannot be answered without knowing what else is in the exhaust and what the engine’s limit is.

Site Conditions

Hot air is less dense, and so is air at altitude. Both reduce the oxygen available for combustion and raise cooling demand. Turnkey Industries, in its article on generator derating and site power limits, describes derating as the adjustment applied when site conditions differ from the rating basis used by the manufacturer, and lists temperature, altitude, ventilation restrictions and the electrical load profile as the main drivers. It also warns that treating derating as a single percentage shortcut is often misleading.

The relevance for RECD owners is simple: a hot summer afternoon in a poorly ventilated generator room can reduce usable output whether or not a device is fitted. If a set feels weak in May and fine in January, suspect site conditions first. Our guide to common causes of generator overheating covers the ventilation side.

Engine Auxiliaries

The fan, water pump and other engine-driven items take a share of the engine’s output before it reaches the alternator. Worn belts, fouled radiators and a fan running harder than intended can make that share larger. Again, this has nothing to do with the RECD.

Exhaust Restriction: The RECD’s Line

Every item in the exhaust path resists flow: the manifold outlet, bends, the silencer, straight pipe, the stack and any emission device. Together they set the back pressure at the engine outlet.

DieselNet’s page on engine exhaust back pressure explains the mechanism: higher back pressure means the engine must do more work to push exhaust out, or extract less energy through the turbine, which leads to higher fuel consumption, reduced boost pressure and diminished engine performance. It also raises exhaust temperature, which can overheat exhaust valves and the turbine. The same page reports that manufacturer limits for diesel generator sets typically run from about 6.7 to 10.2 kPa.

Aurora Generators makes a similar point in plain language: excessive restriction can reduce power and load acceptance, and it notes that catalysts, particulate filters and other emissions components add model-specific requirements and can change restrictions as operating conditions change. It also observes that the right limit, measurement location and test condition vary by engine model and rating.

Our own explainer, how exhaust back pressure affects a DG set, lays out the same effects in an installer’s terms.

Alternator Efficiency and Power Factor

Even a perfectly healthy engine cannot deliver more than the alternator converts. Losses in the alternator, and the relationship between kVA and kW, decide what your equipment sees. If you are unsure of the difference, our guide on reading generator kVA and kW ratings explains it. An RECD sits in the exhaust and has no electrical link to the alternator.

Condition Over Time

New equipment starts with a clean sheet. Filters load with soot, catalysts foul, and passages narrow. For an emission device, this is the line where neglect creates a power problem. Our article on what a DPF is and how it helps notes that a heavily ash-loaded filter can measurably affect fuel economy and power output. Regular servicing under the RECD maintenance guide keeps the line clean.

The Budget Idea: How the Exhaust Allowance Is Spent

Think of the engine maker’s maximum back pressure as a budget. Every component spends part of it.

Total back pressure = silencer + pipework and bends + stack + RECD

A few consequences follow.

Sizing matters more than the device’s brand. An RECD sized for the engine’s exhaust flow should sit comfortably inside the budget. One that is too small spends too much of it.
Everything else in the line competes. If the site already has a long pipe run with several bends and a restrictive silencer, the RECD may be the item that pushes the total over the limit. The problem then lies partly in the site design, and the fix may include rerouting or upgrading pipework as well as choosing the right device.
The clean-day reading is not the final reading. Restriction rises as the device collects soot and ash. A sensible installation leaves headroom at commissioning so the reading stays inside the limit until the next service.
The measurement point counts. The engine maker states where back pressure should be measured. A reading from a convenient port elsewhere may not represent what the engine feels.

For a sense of how much a full exhaust system can spend, Turnkey Industries’ article on exhaust systems and backpressure specifications quotes ranges for US generator components: industrial silencers in the region of 6 to 12 inches of water column, with lower-noise options higher, against typical engine limits of 15 to 25 inches. Those figures are for a different market, so do not treat them as your numbers. What they show is that a silencer alone can take a large slice of the budget, which is why the whole path must be added up.

Kirloskar’s RECD product page says its device is designed to maintain low back pressure within permissible limits, and that phrasing captures the right expectation: the goal is to stay inside the engine’s allowance, not to have zero effect on the exhaust.

A Worked Example (Illustrative Numbers Only)

The numbers below are round figures chosen to show the arithmetic. They are not measurements from any specific generator or device.

A generator is rated at 250 kVA with a power factor of 0.8. Its electrical output is 250 multiplied by 0.8, which gives 200 kW.

Case A: total back pressure stays inside the engine maker’s limit. The engine delivers its declared power, and you draw the full 200 kW. There is no measurable loss to record.
Case B: back pressure exceeds the limit and the engine loses, say, 1 percent of its power. The usable output falls by about 2 kW, from 200 kW to 198 kW. Nobody on site would notice at half load, but a set that already runs near its limit might struggle with a large motor start.
Case C: the same 1 percent shows up as extra fuel instead. If the set burns 50 litres an hour at that load, 1 percent is 0.5 litre an hour. Over 200 running hours in a year, that adds up to about 100 litres. Small, but avoidable.

Again, these are only for illustration. The lesson is that the effect scales with how far past the limit the engine sits, and that the fix is to measure and stay inside the limit.

For more on running costs, see our guides on diesel consumption in generators and tips to reduce diesel generator fuel consumption.

What “Too Much Restriction” Looks Like

If a device or an exhaust system is asking too much of the engine, you may see some of these signs. None is conclusive alone, so use them as prompts to measure.

Black smoke at loads where it was not seen before
Higher exhaust temperature readings than the baseline
Slower load pickup, with voltage or frequency dipping more than usual when a big load is applied
Higher fuel consumption for the same electrical load
The set struggling to reach rated kW on a load bank
Turbocharger behaviour that changes, such as a different sound or a temperature alarm, reported by your technician
Exhaust leaks or sooting at joints, which suggest pressure building up upstream

Our article on black smoke from generators helps you separate restriction from other causes such as injectors or air supply.

The Acceptance Test: Prove Nothing Was Lost

The best answer to any worry about power is a before-and-after measurement. Agree on the test with your vendor before installation, write it into the order, and complete it on handover.

What you need: A resistive load bank or a site load large enough to reach the test points, a calibrated pressure gauge or manometer at the point the engine maker specifies, a means to record fuel used (a flow meter or a tank dip method), and an exhaust temperature reading.

Paste-ready acceptance sheet

Test point (% of rated kW)	Back pressure (kPa)	Exhaust temperature	Electrical output (kW)	Fuel used per hour	Voltage and frequency stable?	Smoke observed
25% before RECD
50% before RECD
75% before RECD
100% before RECD
25% after RECD
50% after RECD
75% after RECD
100% after RECD

How to run it:

Warm the set to normal operating temperature before recording anything.
Hold each load step long enough for readings to settle, and record the same items at each step.
Perform the “before” set of readings on the day before installation, or as close as possible, under similar ambient conditions.
Repeat every step after installation, again at stable temperature.
Compare the tables. Back pressure at 100 percent load is the critical figure: it must be inside the engine maker’s limit for your model.
Add a step-load test if your site has large motors: apply a known block of load and note the voltage and frequency dip and recovery time.
Photograph the instruments and store the sheet with your compliance file.

Agree the pass criteria in advance. Sensible criteria include back pressure at rated load within the engine maker’s limit, rated kW reached on the load bank, no new smoke, and fuel consumption per kWh not materially different from the baseline. Because we cannot know your engine’s limit, ask the maker or dealer for the figure and the measurement point, and put both in writing. If you cannot obtain a “before” test, a “before” reading of back pressure from the old exhaust is still valuable.

Six Situations Where Power Concerns Are Real

Some setups deserve extra care.

A set already near its exhaust limit. Long pipe runs, many bends, a low-noise silencer or a narrow stack can leave little room for a new device.
A generator working near full load for long hours. Prime or continuous duty sets have less margin than a standby set that seldom exceeds moderate load. See our note on load factor and duty cycle.
Hot, dusty or high-altitude sites. The set is already derated, so any extra restriction hurts more.
A small or undersized device. A vendor keen to cut cost may propose a smaller unit than the engine needs.
A set with existing after-treatment. Stacking devices adds restriction and can upset control systems. See whether an RECD can be added to a generator with an existing DPF or SCR.
Heavy motor starting or step loads. Load acceptance depends on boost pressure, which back pressure can reduce.

In each case, the response is the same: measure and check the margin, do not assume.

When You Think Power Dropped After Installation: A Five-Step Triage

Suppose your set seems weaker after the RECD. Work through these in order before deciding the device is to blame.

Rule out site and season. Has ambient temperature risen? Is the generator room hotter than before? Compare with the same time last year.
Rule out the load. Has the connected load changed, or has the power factor worsened? A change in load profile can look like a loss of power.
Check the basics of the engine. Air filter, fuel filter, fuel quality, injectors and cooling system. Our monthly maintenance checklist and generator air filter guide cover these.
Measure back pressure at rated load at the maker’s specified point. Compare with the engine limit and with the commissioning reading.
Inspect the installation. Look for crushed pipes, misaligned flanges, an internal obstruction or a device fitted the wrong way round. Our article on common RECD installation mistakes lists frequent faults, and the step-by-step installation guide shows what a proper job includes.

If step 4 shows a reading above the limit, take it to your vendor with the numbers. A specific reading gets a quicker, more useful reply than a general complaint.

Selecting a Device So That Power Stays Intact

Protect your output before you buy.

Provide accurate engine details. Nameplate data gives the vendor what they need to size the device. Our guide to DG nameplate photos for an RECD enquiry shows what to capture.
Match the kVA range. Devices are offered for defined size ranges. On Aceget’s RECD page, models run from 25 kVA to 2500 kVA, and there are individual pages for sizes such as 250 kVA. Choose the page that matches your set, then confirm the details with the team.
Read the certificate. The test report lists the approved engine range. See how to read an RECD test certificate.
Ask for back pressure data. Request the expected value at rated load for your engine, and the maximum the vendor considers acceptable.
Review the whole exhaust path. A good installer looks at the silencer, pipework and stack as one system.
Plan servicing. A device that stays clean keeps its back pressure low. See the maintenance guide.
Check the warranty position. Read how an RECD can interact with your generator’s OEM warranty before installation and get the maker’s view of the back pressure limit in writing.

If your set is sized for peak demand that it rarely reaches, a load review may reveal plenty of headroom. Our article on load calculation basics can help you check.

The Trade-Off in Perspective

A small, measurable cost in exhaust restriction is the price of catching particulate matter, and regulators have judged that price worth paying for in-use diesel sets. A well-designed installation makes that cost so small that it disappears inside the engine maker’s tolerance. The wider gains are described in our overview of the benefits of installing an RECD on diesel generators, and if smell is part of your reason for asking, see whether an RECD can reduce diesel exhaust odour. If your generator rarely runs, our guide on whether an RECD suits a DG set that runs only occasionally explains how to keep the device healthy.

Frequently Asked Questions

Will an RECD reduce my generator’s kVA rating?

No. The kVA rating is the electrical capacity set by the alternator and the set’s design, and an RECD has no link to the alternator. What can change, if back pressure is too high, is the engine’s ability to deliver its full power, which in turn limits the usable load.

How much power does an RECD take?

There is no single figure that fits every generator. The effect depends on the size of the device, the engine’s exhaust limit and the rest of the exhaust path. A correctly sized device on an engine with spare margin should leave rated output unchanged. Only a measurement on your set can give the real answer.

Can an RECD increase fuel consumption?

If it pushes back pressure above the engine’s limit, yes. Higher restriction increases pumping losses and can raise fuel use. If the device keeps the total inside the limit, any change is usually too small to notice. Track fuel per kWh before and after.

Should I derate my generator after installing an RECD?

Not by default. Ask the engine maker whether any derating applies for your model with the specific device, and confirm that back pressure at rated load stays inside the limit. If both answers are favourable, no additional derating should be needed. Site derating for heat and altitude is a separate matter.

What if my vendor cannot give a back pressure figure?

Treat it as a warning sign. A supplier who sizes devices for engines should know what the device adds and should be willing to measure it at commissioning. If they cannot, find one who will.

The Bottom Line

An RECD affects only one line of your generator’s power ledger: the exhaust restriction. Keep the total inside the engine maker’s limit, and rated output should stay intact. Push past it, and power, fuel economy and exhaust temperature all suffer, with the effect growing as the device fills with soot and ash.

That makes the practical steps clear: give the vendor accurate engine data, insist on a back pressure figure, run the before-and-after acceptance test, record a baseline, and service the device on schedule. If your set shows any of the warning signs, use the five-step triage before assuming the device is at fault.

To discuss a specific set, contact Aceget with your nameplate details and exhaust layout, or explore the RECD range.

Does an RECD Change a Generator’s Usable Power Output? The Kilowatt Ledger