October 27, 2026
Posted by: editor
Category: RECD & Emission Control Devices

Sooner or later, every RECD owner reaches the same fork. The back-pressure has crept up and will not come down after cleaning, or a service visit has found a crack, and someone has to decide: is an RECD filter core replacement enough, or does the whole unit need to come off the chimney and be replaced?

Getting this wrong is expensive in both directions. Replace only the core inside a housing that is quietly rusting through, and you may be paying for a second shutdown, crane and re-commissioning within a couple of years. Replace the complete assembly when the housing had a decade of life left, and you have spent perhaps twice what you needed to.

This guide defines the terms, compares the two options, lists the checks to run first, gives a numbered decision tree and closes with an illustrative five-year cost comparison for a 625 kVA set.

What “core” and “complete assembly” actually mean

Suppliers use different words for the same parts, so agree on vocabulary first.

The core

The core is the replaceable active part of the device: the component that actually captures or converts particulate matter. Depending on the design, it may be called a filter element, substrate, cartridge, monolith, catalyst brick or filter module. Some larger devices use several smaller cores in parallel.

If you are unsure how your own device is built, our explainer on how an RECD works and what is inside it is a good primer, and DieselNet’s overview of diesel particulate filter types goes deeper into substrate technologies.

The complete assembly

The complete assembly is everything that makes up the installed device:

The outer housing or can, including its welds and any access doors
Inlet and outlet cones or transition pieces
Flanges, clamps and bolting faces
Support mats, sealing rings and gaskets that hold the core in place and stop exhaust bypassing it
The core (or cores) themselves
Sensors, wiring and the controller, where the design includes them
Internal and external supports, brackets and mounting feet
Insulation, heat shields and cladding

Not every design has a field-replaceable core

Some RECDs use cartridges that slide out after unbolting a section. Others have the core permanently canned, so “replacing the core” means replacing the can, or allow it only at the supplier’s works.

Before you plan anything, ask your supplier in writing whether the core is field-replaceable on your specific model and capacity, what tools and lifting are needed, and whether a replacement core keeps the device in its type-approved configuration.

Why RECD filter core replacement becomes necessary

Cores fail through a handful of recognisable mechanisms. Identify which one, because several point to an engine problem that will destroy the next core too.

Ash that cleaning cannot remove

Soot burns off; ash does not. Ash, mainly from lubricating oil additives, accumulates over thousands of hours and steadily raises back-pressure. DieselNet’s page on ash accumulation in particulate filters explains how it packs into the rear of the channels and why it eventually limits filter life.

Many designs can be ash-cleaned, but once post-cleaning back-pressure no longer returns close to baseline, the core has reached the end of its useful life.

Thermal cracking and melting

Ceramic substrates crack when they experience steep temperature gradients. The classic cause is an uncontrolled soot burn: too much soot accumulates (often after long periods at low load), then the engine goes to high load, the soot ignites all at once and the local temperature spikes. DieselNet’s discussion of filter regeneration and its risks covers why this happens.

A cracked filter core lets exhaust bypass the filtration walls. Back-pressure may actually fall, which can fool an untrained operator into thinking the device is healthy.

Oil and coolant contamination

An engine burning oil sends unburnt lubricant and additive ash into the device far faster than normal. A head gasket or cooler leak can send coolant, which can glaze and chemically attack some substrates.

Catalyst poisoning

Where the core carries a catalytic coating, sulphur and oil-additive compounds can deactivate it over time. The benefits of ultra-low sulphur diesel for emission control are directly relevant here. A poisoned catalyst may not show up as a back-pressure problem at all; it shows up as harder regeneration and more soot loading.

Mechanical and transit damage

Cores are surprisingly fragile in shock. A device dropped during unloading, transported without adequate restraint, or subjected to a severe engine backfire can suffer cracks or a displaced core. If your device has been moved between sites or returned to the works, read our sibling guide on packaging and transporting an RECD without damaging its internals.

End of design life

Every core has a finite design life in hours, cleaning cycles or both. Ask your supplier for the figure and track running hours.

Why the whole assembly sometimes has to go

The harder case is when the problem lies around the core.

Housing corrosion and cracking

RECD housing corrosion is common in coastal sites, humid plant rooms, outdoor installations without proper rain protection, and on sets that run so rarely that condensation sits inside the can. Look for flaking scale, pinholes, bulging, rust staining around welds, and soot tracks on the outside of the housing. Sites near the sea face particular risk; our article on RECD selection for coastal, salt-laden environments explains why.

Distorted flanges and failed welds

Thermal cycling, misaligned exhaust connections or missing bellows distort flanges and crack welds, and a distorted flange will not seal however hard it is torqued. If you see this, also check whether the connecting pipework has adequate flexible bellows; otherwise a new assembly will suffer the same fate.

Mat and seal damage that cannot be renewed

The support mat around the core holds it in place and seals the gap between core and housing. If the mat has eroded or been overheated, exhaust bypasses the core around its edge. In some designs the mat can be renewed along with the core; in others it is part of a canning process that only the factory can perform.

Obsolete design or unavailable parts

If the supplier has discontinued the model, a replacement core of the original specification may simply not be available. A substitute core of different dimensions or material could change the device’s performance and its type-approved configuration.

The engine or duty has changed

Where the generator has been replaced, re-rated or moved to a very different duty cycle, the original device may no longer be sized correctly. Our post on whether an RECD can be transferred to another diesel generator covers the sizing questions that arise.

Compliance and type approval

CPCB’s system for RECD compliance requires devices to be type approved, with at least 70% particulate matter capture efficiency, through recognised testing agencies. A device that has been significantly altered may no longer match the configuration that was approved. If a replacement core is not an approved part for your model, a complete approved assembly may be the only way to keep your documentation straight. Confirm with your supplier and, where your consent conditions require it, your State Pollution Control Board.

Core only vs complete assembly: side-by-side comparison

How the two options typically differ (specifics depend on design and site):

Factor	Core only	Complete assembly
Purchase cost	Typically a fraction of a new device	Full device cost, plus any new transition pieces
Downtime	Usually shorter; device can often stay in place	Longer; removal and re-installation of the whole unit
Lifting needs	Hoist or chain block for the core module	Crane or larger lifting arrangement, especially on rooftops
Labour	Unbolt section, extract, fit new core and seals	Disconnect exhaust, remove, install, align and support new unit
Gaskets and seals	Section gaskets and, where applicable, mat or seal kit	Full set of flange gaskets for both ends
Re-commissioning and testing	Leak check and new back-pressure baseline	Full commissioning, leak check, baseline, sensor checks
Compliance documentation	Confirm part number matches approved configuration	New serial number and certificate to file and update
Warranty on new part	Typically covers the core only	Typically covers the whole new device
Residual life of old housing	Housing age, corrosion and fatigue carry forward	Housing life resets to zero
Sensors and controller	Retained, so must be checked separately	Usually new or re-verified as part of the package

The residual life of the old housing is the factor most often ignored in a “cheapest now” decision, and warranty scope differs: a new core in an old housing invites arguments about whether a later leak came from the new part or the old can. Read RECD warranty terms on parts, labour and exclusions before you commit.

Diagnostic steps before you decide on RECD repair vs replace

Do not decide on a single back-pressure reading. Run these checks with a competent engineer present, with the set isolated under lockout/tagout (the OSHA guidance on controlling hazardous energy is a useful reference for the principles). Exhaust components stay hot for a long time after shutdown, so allow adequate cooling.

1. Compare back-pressure against baseline at the same load

Back-pressure only means something at the same load and exhaust temperature as the commissioning baseline.

Higher than baseline after cleaning: likely ash loading, glazing or oil contamination
Lower than baseline: suspect a crack, mat failure or bypass
Erratic: suspect the sensor or its tapping before you blame the core

Our guide on how exhaust back-pressure affects a DG set explains why sustained high back-pressure also harms the engine.

2. Rule out false sensor readings

A blocked pressure tapping, cracked sensor hose or drifted transducer can make a healthy core look clogged, or a cracked one look fine. Verify with an independent calibrated gauge. If a sensor needs replacing, follow our sibling guide on specifying replacement RECD sensors without losing compatibility.

3. Inspect the inlet face

Where the design allows access, look at the inlet face of the core with a torch. Note:

Even dry black soot (normal)
Wet, oily or glossy deposits (engine oil or fuel problem)
White or grey crusting (coolant or ash)
Plugged channels or debris (upstream damage, such as turbo or exhaust lining fragments)

4. Inspect the outlet face

The outlet face of a healthy wall-flow core should be clean. Soot streaks or patches on the outlet face, or soot inside the outlet cone, indicate that exhaust is bypassing filtration through a crack, a melted zone or a failed seal. Map where the streaks are: a ring around the edge points to mat or seal failure, while a localised patch points to a crack in the core.

5. Inspect the housing

Check the outer can and cones for corrosion, pinholes, weld cracks, bulging and soot staining. Measure wall thickness with an ultrasonic gauge where corrosion is visible.

6. Check flange flatness

Lay a straight edge across each flange face. Gaps, warping or elongated bolt holes mean it will not seal reliably.

7. Review the history

Gather running hours, cleaning cycles, alarms, repairs and engine faults. A well-kept RECD maintenance record makes this step quick.

A decision tree: replace RECD element or whole unit?

Work through these questions in order.

Is the core field-replaceable on this design?
- No: you are choosing between factory core replacement (device goes back to the works) and a complete new assembly. Compare downtime and cost, then go to Q4.
- Yes: go to Q2.
Is the housing sound? (no structural corrosion, flanges flat, welds intact, mat or seal renewable)
- No, and the damage is structural: replace the complete assembly. Go to Q3 before ordering.
- Marginal (surface corrosion, minor flange wear): note the expected remaining housing life and go to Q3, then carry that estimate into Q5.
- Yes: go to Q3.
Has the engine root cause been identified and fixed? (oil consumption, injectors, coolant leak, chronic low load, fuel quality)
- No: stop. Fix the engine or operating problem first, or any new core or assembly may fail the same way.
- Yes, or there was no engine root cause (normal end of life): go to Q4.
Will the replacement maintain the type-approved configuration?
- Unclear: get written confirmation from the supplier of the part number, dimensions and approval status.
- No: do not fit it. Choose an approved replacement or a complete approved assembly.
- Yes: go to Q5.
What is the life-cycle cost over your planning horizon?
- Estimate both options over five years, including expected housing replacement, downtime and re-commissioning (see the worked example below).
- If core-only is cheaper even allowing for a likely housing failure in the horizon, choose core-only.
- If the housing is likely to fail within the horizon and the second shutdown costs are high, the complete assembly usually wins.
Does the timing fit your operations and compliance calendar?
- In Delhi NCR, check whether CAQM directions under GRAP may restrict DG use during the period you plan to have the set down or need it for testing. Schedule accordingly.
- Confirm whether your consent conditions require you to inform the SPCB of a change to the control device.

If your first answer at Q1 was “no”, also ask whether a refurbished core is an option. Our sibling post asks whether a refurbished RECD filter core can be reused and what evidence you should demand before accepting one.

Fix the root cause first

This is the step most often skipped, and the reason some sites go through several cores in quick succession.

Engine oil consumption

High oil consumption delivers ash and unburnt oil directly to the core. Measure consumption over a fixed number of hours and compare with the engine maker’s guidance. Our post on engine oil consumption and RECD suitability explains the thresholds to discuss with your service provider.

Injectors and combustion quality

Worn or dribbling injectors, a restricted air filter and incorrect timing all raise soot output. More soot means more frequent and hotter regeneration events, which is exactly what cracks ceramic cores.

Wet stacking and chronic low load

Sets that idle or run at very light load for long periods load the core with wet, oily soot, then risk an uncontrolled burn when load rises. If your set’s duty matches this pattern, read about choosing an RECD for a generator running at low load and consider scheduled load-bank exercise.

Fuel quality

Contaminated or high-sulphur fuel poisons catalysts and adds ash. Buy from reliable sources and drain tank water regularly.

RECD substrate replacement cost: what to expect in quotes

When comparing quotations, make sure each states:

The core part number and confirmation that it is an approved part for your model
Seal, mat and gasket kit included or extra
Labour, lifting equipment and scaffolding
Disposal of the old core
Re-commissioning, leak test and new baseline reading
Warranty period and what it covers
Lead time for the core

A complete-assembly quote should additionally list removal of the old unit, new transition pieces, support modifications, insulation and sensor re-fitment.

If you are using a service contractor other than the original supplier for either job, check their access to genuine parts and approval documents. Our sibling guide on whether a different service company can maintain an installed RECD covers the questions to ask.

Re-commissioning after RECD filter core replacement or a new assembly

Whichever path you take, the device is not back in service until it has been re-commissioned and documented.

Mechanical checks

Fit new gaskets of the correct material and rating at every disturbed joint. Never reuse old gaskets.
Torque bolts in a cross pattern to the supplier’s stated values, and re-check after the first heat cycle.
Confirm alignment and that supports carry the weight, not the engine exhaust flange or turbocharger.
Re-fit insulation and heat shields.

Leak test

Run the set and check every joint for soot traces or exhaust escape, and check the outlet for soot breakthrough.

New back-pressure baseline

Record a fresh baseline at a stated load, with the load, exhaust temperature, ambient temperature, date and running hours noted. Without this, you have nothing to compare against at the next inspection. Use an unambiguous date format in your records, such as the ISO 8601 standard.

Sensors and controller

Where fitted, confirm sensor readings are plausible, alarms work and core-dependent settings are updated.

Records and notifications

File the new core or device serial number, the supplier’s confirmation of approved configuration, the commissioning sheet and photographs. If the old part failed early, keep it and document its condition in case of a claim; our sibling post on what to record when raising an RECD warranty claim lists the evidence. If your consent conditions require notification of changes to pollution control equipment, inform your SPCB.

Illustrative 5-year life-cycle cost example: 625 kVA DG set

The following worked example is illustrative. The rupee figures are not quotations and real costs vary widely with design, capacity, site and supplier. The point is the method, not the numbers. For capacity-specific information, see the 625 kVA DG set RECD page.

The situation

A 625 kVA standby set in a humid industrial location has an RECD with a field-replaceable core. Back-pressure after the last ash clean stayed well above baseline, and the outlet face shows a localised soot patch indicating a crack. The housing shows surface corrosion near the lower weld and one slightly worn flange. The service engineer estimates the housing will probably need replacing in about year 3.

The engine root cause (an overdue injector service) has already been fixed.

Assumptions (illustrative)

Item	Cost
Replacement core	₹3,20,000
Core fitting labour, seal kit, gaskets, re-commissioning	₹45,000
Downtime and hire cover for core job	₹30,000
Complete new assembly	₹8,60,000
Assembly removal, installation, crane, gaskets, re-commissioning	₹1,10,000
Downtime and hire cover for assembly job	₹60,000
Routine maintenance (both options)	₹15,000 per year
Extra housing upkeep (patching, extra inspections) while old housing remains	₹20,000 per year

Option A: core replacement now, housing replaced in year 3

Year 0 core job: ₹3,20,000 + ₹45,000 + ₹30,000 = ₹3,95,000
Extra housing upkeep, years 1 and 2: 2 × ₹20,000 = ₹40,000
Year 3 complete assembly: ₹8,60,000 + ₹1,10,000 + ₹60,000 = ₹10,30,000
Routine maintenance, 5 years: 5 × ₹15,000 = ₹75,000

Option A total: ₹3,95,000 + ₹40,000 + ₹10,30,000 + ₹75,000 = ₹15,40,000

Per year: ₹15,40,000 ÷ 5 = ₹3,08,000

Note that the core bought in year 0 is replaced along with the housing in year 3, so most of its value is lost.

Option B: complete assembly now

Year 0 complete assembly: ₹8,60,000 + ₹1,10,000 + ₹60,000 = ₹10,30,000
Routine maintenance, 5 years: 5 × ₹15,000 = ₹75,000

Option B total: ₹10,30,000 + ₹75,000 = ₹11,05,000

Per year: ₹11,05,000 ÷ 5 = ₹2,21,000

Comparison

	Option A (core now, housing year 3)	Option B (assembly now)
5-year total	₹15,40,000	₹11,05,000
Per year	₹3,08,000	₹2,21,000
Shutdowns for replacement	2	1
Device age at end of year 5	Assembly 2 years old	Assembly 5 years old

If the housing really does fail in year 3, Option B saves ₹15,40,000 − ₹11,05,000 = ₹4,35,000 over five years and avoids a second shutdown.

What if the housing survives?

Now suppose the housing lasts beyond year 5 with continued patching. Option A then costs:

₹3,95,000 + (5 × ₹20,000) + ₹75,000 = ₹3,95,000 + ₹1,00,000 + ₹75,000 = ₹5,70,000

That is ₹11,05,000 − ₹5,70,000 = ₹5,35,000 cheaper than Option B.

Break-even reasoning

The decision therefore hinges on how likely the housing is to fail within five years. Call that likelihood p. The expected cost of Option A is:

₹5,70,000 + p × (₹15,40,000 − ₹5,70,000) = ₹5,70,000 + p × ₹9,70,000

Setting this equal to Option B’s ₹11,05,000:

p × ₹9,70,000 = ₹11,05,000 − ₹5,70,000 = ₹5,35,000

p = ₹5,35,000 ÷ ₹9,70,000 ≈ 0.55

In this illustration, if you judge the chance of housing failure within five years to be above roughly 55%, the complete assembly is the better buy. Below that, core-only is cheaper on expected cost. Where the engineer’s honest view is “it will probably need replacing in about year 3”, the likelihood is well above 55%, so Option B wins.

Discounting future cash and the residual value of Option A’s newer assembly both nudge the result slightly towards Option A, but rarely overturn a ₹4,35,000 gap. Run the numbers with your own figures.

FAQ

How do I know if my RECD filter core is cracked?

Look for soot streaks or patches on the outlet face of the core or inside the outlet cone, and for back-pressure that has dropped below the commissioning baseline at the same load. Visible smoke at the chimney outlet is another sign. Confirm with the supplier before deciding.

Is RECD filter core replacement always cheaper than a new unit?

The purchase price is usually lower, but not always the life-cycle cost. If the housing is likely to fail within a few years, the cost of a second shutdown, lifting and re-commissioning can make the complete assembly cheaper over five years.

Can any compatible-size core be fitted into my RECD?

No. The core must be the part approved for your model and capacity, so that the device stays in its type-approved configuration. Get written confirmation of the part number from your supplier.

Can I run the DG set without the core while waiting for a replacement?

No. Running without the core, or bypassing the RECD, defeats the emission control and is a compliance risk that can lead to sealing and penalties. Arrange hire cover or schedule the work to avoid it.

Do I need to inform the pollution control board after replacing the core or device?

It depends on your consent conditions. Check the consent document and ask your SPCB if it is unclear. Either way, keep the new serial number, approval confirmation and commissioning records on file.

Talk to Aceget before you order a replacement

Aceget supplies CPCB type-approved retrofit emission control devices for DG sets from 25 kVA to 2500 kVA. If you are weighing core replacement against a complete assembly, share your back-pressure history, inspection photographs and set details with our team, and we will help you work through the options for your installation. You can review our type approval certifications or contact us to start the conversation.

Replacing an RECD Filter Core vs the Complete Assembly