October 25, 2026
Posted by: editor
Category: RECD & Emission Control Devices

If you run six, ten or twenty diesel generators, the idea of keeping one spare Retrofit Emission Control Device (RECD) on the shelf is attractive. One unit fails, you lift it out, drop the spare in, and the generator is back in compliant service before anyone at the pollution control board notices.

The reality is more conditional. A shared spare RECD can work, but only for a narrow set of fleets, and only if you plan the swap as carefully as the original installation. For most sites, a well-chosen kit of critical components plus a clear supplier response agreement gives better value for money, with less capital sitting idle in a store room.

Below: when a full spare makes sense, what a swap involves, and an illustrative cost comparison for six 500 kVA sets.

The Short Answer

Yes, a shared spare RECD can support a fleet, but only when all of the following are true:

The generators it is meant to cover are identical or near-identical in engine model, rating, exhaust flow and exhaust connection details.
The spare is the same model and revision as the installed devices, with the same flanges, mounting points, sensor ports and controller.
Your compliance records and any consent conditions allow a device change on a set, and you will document every swap.
You have the lifting access, labour, gaskets and test equipment to complete a swap and verify it within a sensible time.
The spare is stored properly and checked periodically, so it is actually serviceable on the day you need it.

If any of these fails, the spare cannot be fitted quickly, or at all, and a component kit plus a service agreement is usually better.

“Shared” here means a spare moved to whichever set needs it, never two generators running through one device at once. On that, see whether two diesel generators can share one RECD.

Why an RECD Is Not a Generic Part

To understand why a spare is conditional, it helps to remember what an RECD is matched to. As explained in what an RECD is and how it works, the device is sized to a particular engine’s exhaust conditions. The main parameters are:

Exhaust gas flow at rated load, set by displacement, speed, aspiration and rating.
Back-pressure budget: how much extra restriction the engine tolerates. See how exhaust back-pressure affects a DG set.
Exhaust temperature profile, which affects how the element behaves and regenerates (see DieselNet on DPF ash for how elements age in service).
Connections and mounting: flange sizes and patterns, orientation, frames and brackets.
Instrumentation and controls: sensors, the controller, its power supply, and any link to a building management system (BMS).

Two sets with the same kVA rating can still differ on several of these points. Our article on why same-kVA DG sets may need different RECDs explains how. A spare that fits one engine perfectly may restrict another or simply refuse to bolt on.

Decision Tree: Can a Shared Spare RECD Work for Your Fleet?

Work through these questions in order. A “no” at any step tells you where the spare plan breaks down.

Question 1: Are the covered sets genuinely identical?

Check engine model, kVA, prime or standby rating, aspiration and OEM exhaust flow. Same nameplate kVA is not enough.

Yes: go to Question 2.
No, but there are two or three identical sub-groups: a spare can only cover one sub-group. Decide whether that sub-group is large enough to justify it.
No, it is a mixed fleet: a single shared spare will not work. See our guide to RECD fitment in a mixed make and age DG fleet and skip to the alternatives section below.

Question 2: Are the exhaust connections and mounting the same on every set?

Even identical engines can have different exhaust routes. One set may have the RECD on a floor frame inside the enclosure, another on a wall bracket outside, a third on a rooftop stand with a long vertical run. Flange sizes, bellows positions and support points all need to match, or the spare needs adapter pieces that you have made and tested in advance.

Yes: go to Question 3.
No: either standardise the connection details across the fleet (often a good idea anyway) or accept that the spare covers only the sets that match.

Question 3: Is the back-pressure margin similar on each set?

Total back-pressure includes the RECD, the silencer, pipework length and bends. A set with a long exhaust run may already be near its limit, and a spare that is fine elsewhere could push it over. Check the commissioning back-pressure readings for each set.

Yes, all sets have comparable margin: go to Question 4.
No: exclude the tight sets from spare coverage, or resolve the exhaust route first.

Question 4: Are the controllers, sensors and firmware compatible?

The spare must have the same sensor types and ranges, the same controller hardware and firmware, and the same wiring connectors. If your RECD controller offers data output to a BMS, the spare’s register map and addressing must also be compatible, or you will need to remap after a swap.

Yes: go to Question 5.
No: plan to keep the existing controller and sensors in place and swap only the mechanical body, if the design allows that. Confirm with your supplier.

Question 5: Do your records and consent conditions allow a swap?

Your compliance file links a specific device, with its serial number and test documentation, to a specific generator. Moving devices around changes that link. Check what your State Pollution Control Board (SPCB) consent says and what your supplier’s documentation covers.

Yes, with documentation: go to Question 6.
Unclear: ask the supplier and, where needed, the SPCB before buying the spare, not after a failure.

Question 6: Can you physically complete a swap in an acceptable time?

Think about lifting gear, access routes, crew availability at night or on Sundays, gasket stock and test equipment.

Yes: a shared spare is technically viable. Now test it against the cost comparison below.
No: the spare will sit unused when you need it most. Fix access and logistics first, or choose an alternative.

When a Shared Spare Works Well

Large, standardised fleets bought in one programme with the same exhaust layout everywhere, as some telecom and retail chains are.
Critical sites with no spare capacity, where losing one set for several weeks is genuinely costly.
Owners with in-house mechanical teams who can perform and document a swap without waiting for external crews.

When a Shared Spare Does Not Work

Mixed fleets with different engine makes, ratings or ages.
Small fleets of two or three sets, where the spare is a large share of device spend and may never be used.
Awkward access, such as basements or rooftops where every lift needs a crane permit. See basement DG room access planning.
Fleets with N+1 redundancy, where one set out for a few weeks does not affect supply.
Designs with field-serviceable parts, so most faults are fixed with components.

Compliance and Records

This is where shared spares most often go wrong: the swap takes a day, but a paperwork gap can last much longer.

The certificate and consent tie to the installed set

CPCB type approval covers the device model and the engine range it was tested for. Your site records, and often your SPCB consent file, then link a specific device serial number to a specific DG set. When you move the spare onto a set, the serial number on that set changes. If an inspector arrives and the serial on the device does not match your records, you have a problem to explain.

The related question of moving a device from one set to another permanently is covered in can an RECD be transferred to another diesel generator. A shared spare is essentially a planned, repeatable version of that transfer, so the same cautions apply.

Record every swap

For each swap, record at minimum:

Date and time, in a consistent format such as ISO 8601.
DG set ID, serial number removed and serial number fitted.
Reason for the swap, and gaskets and fasteners used.
Leak test result and back-pressure reading at a stated load.
Controller settings loaded and BMS mapping checked.
Who did the work, who verified it, and whether the SPCB or supplier was informed.

These entries belong in your RECD asset register. If you do not have one yet, see the sibling post on how to build an RECD asset register for a multi-site DG fleet.

What not to do

Never treat “run without the RECD while we wait” as a contingency plan. Bypassing or removing an RECD from an in-use set is a compliance risk, particularly in Delhi NCR during GRAP restrictions. If a failure leaves a set without a working device, discuss the situation with your supplier and the SPCB, and use other compliant sets where possible.

Swap Logistics and Time

A realistic swap has more steps than “unbolt and bolt”. Here is a typical sequence for a floor or frame-mounted unit.

Step	Typical activity	Illustrative time
1. Permit and isolation	Work permit, DG locked out, exhaust allowed to cool	2–4 hours (cooling dominates)
2. Disconnect	Remove sensor cables, controller connections, insulation and cladding	1–2 hours
3. Lift out	Unbolt flanges, rig and lift the failed unit with chain block or crane	1–3 hours
4. Lift in	Position spare, fit new gaskets, align and torque flanges	1–3 hours
5. Reconnect	Refit sensors, controller, insulation and cladding	1–2 hours
6. Leak test	Run the set, check every joint for exhaust leaks	30–60 minutes
7. Back-pressure check	Measure at a stable load and compare with baseline	30–60 minutes
8. Controls and BMS	Load settings, confirm alarms and data points	1–2 hours
9. Records	Update asset register, compliance file, notify as required	1 hour

Add these up and a smooth swap on an accessible, ground-level set takes roughly 9–19 hours including cooling time, so plan for a full working day at least. A rooftop or basement set may take two days or more once crane hire, permits and building management approvals are included.

Points that are easy to miss

Lifting: confirm the device weight and check that lifting points, beams or crane access exist at every covered location. Crane booking lead time can exceed the swap itself.
Gaskets: never reuse exhaust gaskets. Keep new gaskets, bolts and clamps for at least two swaps with the spare. See our RECD spare parts list.
Leak and back-pressure checks: leaks upstream of the device release untreated exhaust and can damage nearby cabling. The back-pressure reading, compared with that set’s commissioning baseline, confirms correct fitment.
BMS remapping: if your RECD controller offers Modbus or similar output, the spare may come up with a different address or default settings. Check the register map before signing off.

Storage and Condition of the Spare

A spare that has sat in a damp corner for three years may not be serviceable when you need it.

Filter elements and insulation can absorb moisture, and corrosion is a real concern in coastal or humid locations. Even unused, rubber seals and cable insulation age, sensors may drift, firmware can fall behind the installed fleet, and gaskets have a shelf life.

A simple storage checklist

Indoors, dry, off the floor, covered.
Flanges capped, sensor ports plugged.
Controller and sensors stored in sealed bags with desiccant.
Inspection every six months: visual check, desiccant change, connector check.
Firmware version checked against the fleet once a year.
Gasket and fastener stock checked and replaced when past shelf life.
Location and condition logged in the asset register.

Alternatives to a Full Spare RECD

Alternative 1: Critical component spares kit

Most RECD faults do not need a whole device. Sensors, controllers, cables, gaskets and clamps fail more often than the main body, and a kit of these at a central store restores most faults in hours. Where the design allows, add a spare filter element, substrate or cartridge, which covers the core element at a fraction of a full device’s cost. Ask your supplier which parts are field-replaceable on your model.

Alternative 2: Supplier service agreement

A service agreement with defined response times, as described in RECD service response commitments, shifts much of the risk to the supplier. The key terms are response time, attendance on weekends and holidays, whether the supplier stocks critical parts, and the lead time for major components.

Alternative 3: Kit plus agreement

For many fleets, the practical answer is both: a component kit on site and a service agreement behind it. You might also negotiate for the supplier to keep a core element or a complete device in stock against your fleet, so you are not paying for it up front.

Worked Example: Six Identical 500 kVA Sets (Illustrative)

The numbers below are illustrative only. They are not quotations and do not represent Aceget pricing. Use them to structure your own comparison with real figures from your supplier and site.

The fleet

A manufacturing group runs six identical 500 kVA sets across two plants, three sets per plant. All have the same engine, the same RECD model and the same floor-mounted exhaust layout. Each plant normally needs two sets during a long outage, so the third set at each plant is reserve.

Assumed events over five years

3 minor events: one pressure sensor failure, one controller fault, one gasket leak.
1 major event: the core element on one device is damaged and needs replacement, with a supplier lead time of about four weeks.

Assumed costs

Item	Illustrative cost
Complete spare RECD	₹14,00,000
Storage and condition checks for spare	₹30,000 per year
Component kit (sensors, controller, cables, gaskets, clamps)	₹3,00,000
Kit upkeep (calibration checks, gasket replacement)	₹15,000 per year
Parts for a minor event	₹40,000
Labour for a minor event (no agreement)	₹20,000
Core element replacement for the major event	₹4,50,000
Full swap labour (crane, gaskets, leak test, back-pressure, BMS)	₹75,000
Labour for major repair in place (no agreement)	₹40,000
Service agreement (covers labour and call-outs)	₹1,20,000 per year

Option A: Full spare RECD

Spare: ₹14,00,000
Storage over five years: 5 × ₹30,000 = ₹1,50,000
Minor events: 3 × (₹40,000 + ₹20,000) = ₹1,80,000
Major event: swap labour ₹75,000, plus repairing the failed unit so it becomes the new spare, ₹4,50,000 = ₹5,25,000
Five-year total: ₹14,00,000 + ₹1,50,000 + ₹1,80,000 + ₹5,25,000 = ₹22,55,000

Downtime: the minor events still wait for parts because the spare should not be stripped for components, say 5 days each, so 15 set-days. The major event is covered by a swap of about 1.5 days. Total about 16.5 set-days out of compliant service.

Option B: Critical component kit

Kit: ₹3,00,000
Upkeep over five years: 5 × ₹15,000 = ₹75,000
Minor events: labour 3 × ₹20,000 = ₹60,000, plus restocking the kit 3 × ₹40,000 = ₹1,20,000, so ₹1,80,000
Major event: ₹4,50,000 + ₹40,000 = ₹4,90,000
Five-year total: ₹3,00,000 + ₹75,000 + ₹1,80,000 + ₹4,90,000 = ₹10,45,000

Downtime: minor events fixed in about 1 day each, so 3 set-days. Major event waits about 28 days for the core element. Total about 31 set-days.

Option C: Service agreement only

Agreement over five years: 5 × ₹1,20,000 = ₹6,00,000
Minor event parts: 3 × ₹40,000 = ₹1,20,000 (labour covered)
Major event parts: ₹4,50,000 (labour covered)
Five-year total: ₹6,00,000 + ₹1,20,000 + ₹4,50,000 = ₹11,70,000

Downtime: minor events take about 2 days for response plus 3 days for parts, so 5 days each, 15 set-days. Major event about 28 days. Total about 43 set-days.

Comparison

Option	Five-year cost	Set-days out of service	Main risk
A: Full spare RECD	₹22,55,000	~16.5	Spare not serviceable or does not fit when needed; capital tied up
B: Component kit	₹10,45,000	~31	Long wait on a major failure
C: Service agreement only	₹11,70,000	~43	Slow response on minor faults; dependence on supplier stock

Reading the numbers

Option A costs ₹22,55,000 − ₹10,45,000 = ₹12,10,000 more than Option B over five years. It saves 31 − 16.5 = 14.5 set-days of downtime. That works out to ₹12,10,000 ÷ 14.5 ≈ ₹83,448 per set-day avoided.

For this fleet, each plant has a reserve set, so one set out for four weeks does not cut supply, provided the other two are compliant and healthy. Paying around ₹83,000 for each day of avoided downtime on a reserve set is hard to justify.

A hybrid of B and C is worth checking. Take Option B, remove its separately paid labour (₹60,000 + ₹40,000 = ₹1,00,000), and add the agreement (₹6,00,000): ₹10,45,000 − ₹1,00,000 + ₹6,00,000 = ₹15,45,000. Minor events still take about a day, and the agreement gives you a contractual response if the kit is not enough. If the supplier also agrees to hold a core element in stock against the fleet, the major-event wait could drop from four weeks to a few days, closing most of the gap with Option A at a much lower cost.

The answer changes if the fleet has no reserve, if outages are long and frequent, or if one set out of service means a process stops. Then the value of a lost day can be far higher than ₹83,000, and Option A may pay for itself. Run the comparison with your own figures.

Rental Fleets: A Special Case

Rental companies are the most likely to benefit from a shared spare. Their fleets are often a few standard models bought in quantity, and yards have lifting gear and crews on hand. The catch is records: device serial numbers must follow each set through every hire, and customers may hold their own consent. Our guide on fitting an RECD on a rented diesel generator covers the practical side.

Keep spares only for the highest-volume model, and only at the main yard.
Do swaps in the yard between hires, not at customer sites, wherever possible.
Update the hire paperwork and asset register with the new serial number before the set leaves the yard.
Give the customer a copy of the device documentation that matches the fitted serial.

FAQ

Can one spare RECD cover generators of different kVA ratings?

Generally no. An RECD is sized to a specific exhaust flow and back-pressure budget. A device sized for a larger set may not achieve the right operating conditions on a smaller one, and a device sized for a smaller set may restrict a larger one. Check with your supplier, but plan on one spare per identical group.

Do I need to inform the SPCB every time I swap a spare RECD?

It depends on your consent conditions and state practice. Check the consent document and ask the SPCB if it is unclear. At the very least, record every swap in your compliance file so the serial number on each set always matches your records.

Is it acceptable to run a set without its RECD until the spare is fitted?

No. Running an in-use set without its RECD is a compliance risk. Use other compliant sets to carry the load where possible, and discuss the situation with your supplier and the SPCB if no compliant option exists.

Is a component kit enough if my fleet has N+1 redundancy?

Often yes. With a reserve set at each location, a long wait on a major failure usually does not cut supply. A kit for quick fixes and a service agreement for major work tend to give the best balance of cost and risk.

Can I use parts from the spare RECD to fix minor faults?

You can, but every part you take makes the spare less ready. If you do, log it and restock the spare quickly. A separate component kit avoids this problem.

Plan Your RECD Spares Strategy with Aceget

Aceget supplies CPCB type-approved Retrofit Emission Control Devices for DG sets from 25 kVA to 2500 kVA. If you are deciding between a shared spare RECD, a component kit and a service agreement for your fleet, our team can help you work through the fit, records and logistics for your sets. Review our certifications or contact our team to discuss your fleet.

CPCB compliance DG fleet management emission control generator maintenance RECD spares service agreements

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