- September 21, 2026
- Posted by: Aceget
- Category: Uncategorized
Every so often a site manager comes up with an idea that sounds like an economy. The generator already has a chimney. Extending it by a few metres costs much less than a retrofit emission control device. If the smoke goes higher, surely fewer people are affected, so why not raise the stack and skip the RECD?
The idea is understandable, and part of it is true. A taller chimney does reduce the pollution that people breathe in the immediate neighbourhood. But it does that by spreading the same emissions over a larger volume of air. It does not remove a single gram of particulate matter from the exhaust. An RECD works the opposite way: it reduces what leaves the pipe in the first place.
Instead of arguing that in the abstract, this article puts you in the shoes of three neighbours and a regulator, all standing at different points around one diesel generator. Each of them tells you what a taller chimney changes for them, and what an RECD changes for them. Once you have heard all four, the answer to the title question becomes hard to miss.
The Short Answer
No, raising the chimney is not an alternative to an RECD. It is a separate requirement that solves a separate problem. Stack height governs where the exhaust is released and how well it disperses. An RECD governs how much pollution is in the exhaust. A generator that is covered by the retrofit requirement needs the device whatever its stack height, and a generator with an RECD still needs a stack of the correct height.
We found no public rule that allows a taller stack to be counted as a substitute for a retrofit device. If you have been told otherwise, ask for the rule and the reference in writing, and confirm with your State Pollution Control Board before spending money on either option.
What the Chimney Rule Actually Says
Before meeting the neighbours, it helps to know what the height requirement is for. In India, a generator’s exhaust stack is expected to rise above the building on which the set is installed. The commonly quoted formula is:
H = h + 0.2 x √kVA
where H is the total stack height in metres, h is the height of the building in metres where the DG set is installed, and kVA is the generator’s rating. That formula is given, along with the reason for it, on this stack height explainer, which says a greater height is needed so that pollutants are dispersed at elevated levels and ground-level concentrations stay within safe limits. A record of DG set rules reproduced in a National Green Tribunal document shows the same stack height rule alongside noise limits and notes that some state boards publish ready-made values for small sets, for example a building height plus a fixed margin for the smallest ratings.
Notice what the rule is designed to do. It is a dispersion rule. It says nothing about emission mass or concentration in the exhaust. It is one leg of a compliance stool, and the RECD requirement is another leg. Sitting on one leg does not make the stool stand.
The formula in numbers
The part that surprises people is how small the required addition is. Here is the extra height above the building for typical ratings, worked out from the formula.
| Generator rating | Extra height above building (0.2 x √kVA) |
|---|---|
| 25 kVA | 1.0 m |
| 62.5 kVA | 1.6 m |
| 125 kVA | 2.2 m |
| 250 kVA | 3.2 m |
| 500 kVA | 4.5 m |
| 1000 kVA | 6.3 m |
These are calculated values rounded to one decimal place, given only to show the scale. Your board’s own table and the exact building height on your approved drawing are what count.
Now to the neighbours.
Neighbour One: The Shop at Ground Level Next Door
Meet the owner of a small shop next to the generator room. When the set runs, she smells exhaust at her door. Her question is simple: which fix helps her more?
What a taller chimney does for her
Quite a lot, in the near field. Lifting the outlet moves the plume away from her doorway and gives it more time and space to mix before it reaches street level. If the current chimney is short and discharges near her window, extending it can bring a real, noticeable improvement in what she breathes. The reasoning is dispersion, which is exactly what the height rule is meant to secure.
What an RECD does for her
It changes what is in the plume. Particulate matter and, depending on the design, some gases are reduced before the exhaust leaves the pipe. Cummins India, for example, describes its RECD as capturing more than 70 percent of particulate matter, according to its launch announcement. Whatever height the plume rises to, there is less pollution in it.
Her verdict
For the shop owner, both help, and they help in different ways. If the chimney is already at the required height, more height gives diminishing returns. If it is below the required height, correcting it is compulsory anyway and should be done alongside the retrofit.
Neighbour Two: The Resident on the Twelfth Floor Next Door
Now move to the tall building next to the site. A family lives on the twelfth floor, at roughly the same level as the top of the DG set chimney after it has been raised.
What a taller chimney does for them
This is the counterintuitive part. Raising the stack lifts the plume up and away from ground level, which is good for the shop, but it can also bring the plume closer to the windows of neighbouring high-rise flats. Wind direction, the shape of the surrounding buildings and the time of day decide whether the plume passes over, beside or into that building’s air intakes and balconies.
Building shape matters too. CPP Wind’s explanation of good engineering practice stack height describes downwash as the stagnant, recirculating eddy of air that forms on the leeward side of a building, which can trap emissions near the surface and cancel the benefit of height. A chimney that is tall by the formula can still be short relative to a large neighbouring block, and the flow around such blocks is complex.
What an RECD does for them
It reduces the emissions wherever the plume ends up. A cleaner plume is a smaller problem at the twelfth floor, at the shop, or anywhere else the wind carries it. Because the RECD works at the source, it does not depend on wind direction or building geometry to deliver its benefit.
Their verdict
For the resident on the twelfth floor, height alone can move the problem rather than solve it. Cleaning the exhaust is the fix that helps at every height.
Neighbour Three: The Community Two Kilometres Downwind
The third neighbour is not one person. It is a housing colony a couple of kilometres away in the direction the wind usually blows. Nobody there can see the generator. They only share the air.
What a taller chimney does for them
Almost nothing, and possibly slightly the opposite. A tall stack spreads the plume over a bigger volume of air near the source, but the emitted mass continues downwind. The Center for Public Integrity, writing about the US experience with tall industrial stacks, put it plainly: a tall stack may reduce pollution concentrations locally, but it does not reduce total emissions, which travel downwind and can hurt air quality elsewhere. That article, on EPA’s efforts to limit pollution from tall smokestacks, also notes that under US rules a source cannot count dispersion from excess stack height toward its emission limit. That is a US example and not Indian law, but the principle is a useful one: dispersion is not the same as control.
What an RECD does for them
It reduces the mass that enters the shared air in the first place. That is what matters to a community that lives downwind of many generators at once. Across a city, thousands of sets adding a little each is a large total, and that is why the retrofit requirement exists. Our explainer on why an RECD is mandatory in India covers the background, and our note on PM2.5 versus PM10 explains why fine particles matter so much for health.
Their verdict
For the downwind community, only the RECD makes a real difference. The chimney shifts the exposure around. The device removes some of it.
The Regulator: Standing at the Stack Outlet With a Probe
The fourth person is the one whose opinion decides your paperwork. She arrives with a sampling probe and a notebook.
What a taller chimney does for her
Nothing at all about the measurement. Emission standards for engines are written around what leaves the source: mass per unit of energy or concentration in the exhaust. She samples at the outlet. If the concentration in the stack is above the limit, it is above the limit whether the stack is 5 metres tall or 25 metres tall. Height cannot change the number in the sampling probe. Our piece on whether a smokeless DG set still needs particulate measurement explains why measured data matters more than appearance.
What an RECD does for her
It changes the measured value, and it comes with a test certificate that documents its performance for a particular engine band. For a set covered by the retrofit requirement, the device and its paperwork are the evidence she is looking for. Her checklist on inspection day is described in our guide to what happens during a pollution control board inspection.
Her verdict
For the regulator, the chimney is one item on a checklist and the RECD is another. A tall chimney will not rescue a failed emission requirement.
One Generator, One Arithmetic Example
To see the difference without any technical jargon, use round numbers. These are illustrative and not measured values.
Suppose a generator emits 100 units of particulate matter an hour. Now compare two upgrades.
| Upgrade | Units leaving the pipe per hour | What changes |
|---|---|---|
| Nothing | 100 | Baseline |
| Raise the chimney by 10 metres | 100 | Same 100 units, released higher, mixing into more air near the source |
| Fit an RECD with a 70 percent reduction | 30 | Only 30 units released in the first place |
| Raise the chimney and fit the RECD | 30 | 30 units released higher up |
The 70 percent figure is used only because it is commonly quoted as a minimum for retrofit devices, and Cummins’ announcement uses the same “more than 70 percent” language for its own unit. Your certificate will show the actual number for your device.
Look at the second row. A taller chimney leaves the mass unchanged. It changes the place where the mass arrives. If the receiving air is calm or capped, as it often is on winter mornings in northern India, that mass does not spread far and can build up. Our articles on how weather patterns trap pollution across north India and why Delhi’s AQI rises in winter explain why dispersion cannot be counted on in those conditions. In cities under episode-based restrictions, our guide to GRAP rules for DG sets in Delhi NCR shows how the rules tighten precisely when the atmosphere stops mixing.
The Hidden Engineering Cost of a Taller Chimney
Even where a taller chimney is useful, it is not free of consequences. Three of them matter for a generator owner.
More pipe means more back pressure
Every metre of extra pipe adds friction and, with bends, adds more. The engine has a maximum exhaust back pressure set by its maker, and the silencer, any RECD and the pipe all share it. A very tall chimney fitted without checking the exhaust budget can push the total closer to the limit. Our article on how exhaust back pressure affects a DG set explains the effect, and the piece on whether an RECD changes usable power output shows how to check it. If you plan both a taller chimney and an RECD, add the pressure drops together before either goes in.
More structure means more load
A taller pipe needs supports, guy wires or a bracket that can handle wind loading, thermal expansion and vibration from the engine. Poorly supported pipe can crack, leak exhaust into the enclosure or stress the turbocharger outlet. The engineering for that is not a casual add-on.
More height may mean more permissions
Depending on the building, the site and the local authority, a taller stack could have implications for structural safety, building rules, aviation clearance near airports or the terms of the pollution consent. Do not assume that raising it is a free choice. Ask the relevant offices, and see how the wider set of permissions fits together in our guide to how to get a pollution NOC for a DG set.
When a Taller Chimney Is Exactly Right
None of this says chimneys are unimportant. There are good reasons to look at height, alongside an RECD.
- The current stack is below the required height. This is a compliance gap and must be corrected regardless.
- A sensitive receptor is close. A hospital ward, a school, a busy outdoor area or an air intake for another building is within a few metres of the outlet.
- The stack discharges sideways or downwards. Some improvised arrangements point the exhaust toward a wall or the ground. Fixing that is a priority.
- The plume is being pulled back down by a nearby structure. A dispersion review by a qualified person can identify downwash risk.
- The site is in a bowl or a courtyard. Poor natural ventilation means every metre matters.
In each of these cases the height is being fixed for its own reasons. The RECD question is separate and has to be answered separately.
Decision Matrix: What Each Option Solves
| Problem you have | Taller chimney helps? | RECD helps? |
|---|---|---|
| Exhaust smell at ground level nearby | Yes, if current stack is short | Some, because the plume is cleaner |
| Set fails a stack emission test | No | Yes, that is its purpose |
| Set is covered by the retrofit requirement | No | Yes |
| Board inspection asks for the RECD certificate | No | Yes |
| Neighbouring high-rise complains | Maybe, could worsen | Yes, at any height |
| Community-wide fine particle load | No | Yes |
| Stack is below the legal height | Yes, must be corrected | Not relevant |
| Noise complaints | No | Not designed for this, see below |
On the last row, if noise is the complaint, neither a taller chimney nor an RECD is the tool. The silencer is. Our sibling article on whether an RECD can replace a generator’s exhaust silencer explains why.
Cost Thinking: The Comparison That Actually Matters
Owners who ask about chimney height are usually really asking about cost. Here is a clearer way to compare.
- List what you must do anyway. A stack at the required height is a requirement, not an option. Its cost belongs to the baseline in both scenarios.
- Add the retrofit cost if your set is covered. Our guide to RECD cost in India explains what goes into the price.
- Compare with the alternative you are actually facing. For many owners the real alternative is not “chimney versus RECD”. It is “RECD versus replacing an older set”. The post on whether retrofitting an old DG set with an RECD is worth it walks through that comparison.
- Count the risk. A stack raise that does not satisfy the retrofit requirement leaves the penalty, sealing and operational risks in place.
The cheapest option that does not solve the problem is still an expense.
A Word on Other Shortcuts
Chimney height is one of several ideas that circulate as substitutes. The others include treating the fuel with additives and reading a clear exhaust as proof of compliance. We look at additives in Can diesel fuel additives replace an RECD?, and at appearance versus measurement in the smokeless DG set article linked above. If you would like a broader comparison of how states treat these rules, see our overview of state-wise pollution control norms for DG sets.
Questions to Ask Before Anyone Proposes Raising the Stack
- What problem is the taller chimney meant to solve: smell, complaints, a failed test, or a compliance requirement?
- Where is the rule that says it satisfies that requirement? Please put it in writing.
- What is the required stack height for my kVA and building, according to my board?
- How will the extra pipe affect back pressure with the silencer and RECD included?
- Who will design the support and check wind loading?
- Will the extension affect my consent, my building permissions or my fire and structural clearances?
- If I also fit an RECD, where should it sit, and does the taller stack change the recommended position?
Quick Answers
Can a taller chimney replace an RECD? No. The chimney disperses, the RECD reduces emissions at the source. We found no public rule treating height as a substitute for the retrofit requirement.
What is the standard stack height for a DG set? The commonly quoted rule is the building height plus 0.2 times the square root of the kVA rating, subject to your State Pollution Control Board’s own table. Confirm the exact requirement with your board.
Does a taller stack reduce PM in the exhaust? No. It reduces ground-level concentration near the source by dilution. The particulate mass leaving the pipe is unchanged.
Should I raise the stack and fit an RECD? If the current stack is below the required height, or if a sensitive receptor is close, yes. Otherwise the RECD is what your compliance requirement needs, and height should be set at the required value.
The Bottom Line
A taller chimney is a good answer to the question “where should the exhaust go?” It is not an answer to the question “what should be in the exhaust?” The shop owner, the resident, the downwind colony and the regulator all agree on that, each from their own vantage point. Fix the stack to the required height, fit the RECD if your set is covered, and keep the paperwork for both.
For help with a specific set, send your kVA rating, engine details and a photo of the exhaust arrangement through the Aceget contact page, and browse the RECD product range to see which certified bands apply.