RECD Installations Near Fresh-Air Intakes: What Should Be Reviewed?

The complaint usually reaches the facility manager in a roundabout way. Someone in a fourth-floor meeting room says the air smells “like a bus depot” during a power cut. A receptionist gets a headache every time the generator runs. A cleaner notices a fine grey film on the supply diffusers in one wing. Nobody connects these to the generator for weeks, because the generator is on the other side of the roof.

Then someone stands beside the air handling unit during a generator test and smells it immediately.

Diesel exhaust being drawn back into a building through its own fresh-air intakes is called re-entrainment. It is an old problem, and an RECD retrofit is a good moment to deal with it, because the exhaust route is being redesigned anyway. It is also a moment when the problem can be made worse without anyone noticing, if the new device moves the outlet, changes the plume or adds a new source of leaks near an intake.

This article sets out nine questions a review should answer whenever an RECD or its outlet sits anywhere near a fresh-air intake. Each question explains why it matters, what to look at and what a good answer looks like.

Why an RECD Does Not Make the Intake Problem Go Away

It is tempting to assume that once the exhaust passes through an emission control device, it is safe to breathe. It is not.

An RECD is designed mainly to capture particulate matter. Depending on its design, it may also reduce carbon monoxide and hydrocarbons through a catalyst. It does not remove the exhaust’s nitrogen oxides in most designs, it does not remove carbon dioxide and it does not stop the gas being hot, low in oxygen and unpleasant. The composition of diesel exhaust explains why even treated exhaust does not belong inside an occupied building.

There are also periods when an RECD’s performance is lower: during cold starts before the catalyst warms, and in some designs during regeneration, when collected soot is burned. Our explainer on what an RECD is and how it works describes these stages, and our article on whether an RECD can reduce diesel exhaust odour explains what to expect about smell.

So the intake question is not “is the exhaust clean enough?” It is “is the exhaust kept away from the air people breathe?”

The Nine Review Questions at a Glance

No.	Question	Main concern
1	What intakes are there, and where exactly?	Completeness of the inventory
2	How far is the outlet from each intake, in three dimensions?	Separation
3	Which way does the wind usually blow, and what does the building do to it?	Plume path
4	How high is the outlet, and which way does it point?	Plume rise and escape
5	Which intakes are running when the DG is running?	Operating modes
6	Does the RECD change the outlet position or the plume?	Retrofit effect
7	Where are the leak points on the RECD and pipework?	Fugitive releases
8	What happens during maintenance and regeneration?	Occasional releases
9	How will we know if it goes wrong?	Detection and response

The rest of the article takes each in turn.

Question 1: What Intakes Are There, and Where Exactly?

Most reviews miss intakes because they only look for the obvious large louvres on the main air handling units. A complete inventory is the foundation of everything that follows.

What counts as an intake

• Fresh-air louvres on air handling units and treated fresh-air units
• Rooftop packaged units and their outdoor air hoods
• Kitchen make-up air units
• Ventilation fans for plant rooms, stairwells and car parks
• Pressurisation fans for fire escape stairs and lift lobbies
• Openable windows, especially on upper floors facing the roof
• Balcony doors and terrace doors
• Cooling tower air inlets, which can carry contaminants into water systems
• The DG room’s own combustion and cooling air inlets
• Neighbouring buildings’ intakes and windows

How to record them

Mark each intake on the roof or site plan with its height above roof or ground, its orientation and its airflow if known. A photograph of each helps. Do not forget intakes on the next building over; exhaust does not respect boundary walls.

The stair pressurisation fans deserve special mention. They run during fire alarms, which may coincide with a power cut and a running generator. A pressurisation fan drawing diesel exhaust into an escape stair is a serious failure.

Question 2: How Far Is the Outlet From Each Intake, in Three Dimensions?

Distance is the simplest protection. Exhaust disperses and dilutes as it travels. Double the distance and the concentration reaching an intake falls substantially.

Measure properly

Measure the straight-line distance from the outlet to each intake, including the height difference. An intake five metres away horizontally but three metres below the outlet is in a different position from one five metres away at the same level.

Reference values

Ventilation standards set minimum separations between exhaust outlets and outdoor air intakes based on how contaminated the exhaust is. ASHRAE Standard 62.1 classes the most noxious exhausts in its highest category with a minimum separation of about 30 feet, roughly 9 metres, according to ASHRAE’s guide on exhaust re-entrainment. The same guide notes that doubling the separation increases dilution roughly fourfold. Engine exhaust is commonly treated with considerable caution in these schemes. Indian building services design generally follows the National Building Code and good practice from such standards; your HVAC consultant decides which values apply.

A practical note: minimum distances in standards are minimums, not targets. On a crowded roof, aim for as much separation as the layout allows.

A separation table for the review

Intake	Horizontal distance from outlet	Height difference	Straight-line distance	Running when DG runs?	Concern level
AHU-1 fresh air	6 m	outlet 2 m higher	6.3 m	Yes	High
AHU-2 fresh air	18 m	outlet 2.5 m higher	18.2 m	Yes	Low to medium
Stair pressurisation fan	9 m	outlet 1 m higher	9.1 m	During fire alarm	High
Kitchen make-up air	25 m	outlet 4 m higher	25.3 m	Partly	Low
Neighbour’s windows	14 m	outlet 3 m lower	14.3 m	Not controlled	Medium

The values are illustrative. The table makes it obvious where to focus.

Question 3: Which Way Does the Wind Usually Blow, and What Does the Building Do to It?

Distance alone does not tell the whole story. An intake 20 metres directly downwind can receive more exhaust than one 8 metres upwind.

Prevailing winds

Check the site’s wind pattern through the year. In much of India, the monsoon wind and the winter wind come from quite different directions. A layout that works in June may fail in December. Local meteorological data or simple long-term observations from site staff help.

Building wake and recirculation

When wind blows over a building, it separates at the roof edge and forms a zone of swirling, recirculating air on the roof and on the downwind side. Exhaust released inside that zone can be carried in any direction, including back towards intakes that are nominally upwind. Taller buildings next door create their own wakes that can push plumes down onto a lower roof. The general behaviour of plumes is described in the atmospheric dispersion modeling article.

Rooftop screens and enclosures

Acoustic screens around rooftop plant are common in cities. They are good for neighbours’ ears and can be bad for air. A screen enclosing both the DG outlet and an air handling unit traps the plume in a box with the intake. ASHRAE’s guide makes the same observation about screened enclosures.

What a good answer looks like

The outlet discharges outside the roof recirculation zone, the main intakes are upwind for the most common wind directions, and no screen encloses both. Where that is impossible, the review should say so and move to stronger measures under Questions 4 and 9.

Question 4: How High Is the Outlet, and Which Way Does It Point?

The outlet’s height and direction decide whether the plume escapes the building’s air or lingers around it.

Height

A taller outlet releases exhaust above the roof’s recirculation zone, so it has a better chance of being carried away. DG stack heights in India are set by formula linked to the building height and the set’s rating, and those heights are partly intended to help dispersion. Our article on whether increasing chimney height is an alternative to an RECD explains the role of height.

Direction

Outlet direction	Effect near intakes
Vertical, open or with a flapper that opens fully	Plume rises with its own momentum and heat; best for dispersion
Vertical with a fixed cap that deflects sideways	Plume pushed sideways and down; can reach nearby intakes
Horizontal	Plume travels along the roof or wall; risky near intakes
Downward-facing	Plume driven towards the roof, ground or people; generally avoided

Rain protection should not cost dispersion. Our companion guide on rainwater and condensation protection discusses rain caps; for outlets near intakes, choose designs that keep a vertical discharge.

Exit velocity

A plume leaving the stack quickly rises further before the wind bends it over. Reducing the outlet size can increase velocity, but every change adds backpressure, which the RECD has already used some of. Our explainer on how exhaust backpressure affects a DG set explains the trade-off.

Question 5: Which Intakes Are Running When the DG Is Running?

This question is easy to skip and often decides the answer.

Map the operating modes

During a grid failure, the DG supplies some loads but not others. The building management system may start, stop or change the speed of air handling units. Write down, for each intake:

• Is it running during a grid failure?
• Is it running during the weekly or monthly DG test?
• Is it running during a fire alarm with a power cut?
• Can its outdoor air damper be closed or reduced while the DG runs?

Use the modes to your advantage

If an air handling unit near the outlet can switch to full recirculation, or reduce outdoor air, for the period the DG runs, the risk falls sharply. This is a controls change, often cheap. It should be agreed with the HVAC consultant, because reducing outdoor air affects indoor air quality in other ways.

Watch for the opposite effect

Some buildings increase fresh air during an outage because cooling is reduced. Some run exhaust fans on DG power while leaving supply fans off, which pulls the building into negative pressure and draws air in through every gap, including gaps near the exhaust. Our guide on RECDs for printing facilities explains how building pressure can draw exhaust indoors.

Test schedules

Weekly DG tests often happen at the same time every week. If that time is mid-morning on a working day, every intake is running and the building is full. Moving the test to early morning or a weekend, where possible, reduces exposure.

Question 6: Does the RECD Change the Outlet Position or the Plume?

The retrofit itself can shift the risk.

Outlet moved

Fitting the device sometimes means re-routing the exhaust and moving the outlet. A new outlet a few metres from the old one can land beside an intake that was previously safe. Review the new position as if the building had never had a generator.

Device placed outdoors near an intake

If the RECD sits on the roof, its housing, flanges and drain points are new sources of hot surfaces and small leaks right beside whatever is nearby. A device mounted under an AHU fresh-air hood is a poor arrangement even if the stack is far away. Our guide on rooftop RECD installation covers roof layouts.

Plume temperature

An RECD can change exhaust temperature at the outlet. A cooler plume rises less, so it stays closer to the roof. The change is usually modest, but on a marginal layout it is worth asking the vendor.

Rain cap or silencer changes

If the retrofit adds or changes a cap or silencer at the outlet, check the discharge direction under Question 4 again. Our article on whether an RECD can replace a generator’s exhaust silencer covers the case where the silencer is removed.

Question 7: Where Are the Leak Points on the RECD and Pipework?

Not all exhaust leaves through the outlet. Some escapes through joints, gaskets and fittings.

Typical leak points

• Flanges at the RECD inlet and outlet
• Clamps on the filter access section
• Flexible bellows, especially if damaged
• Sensor bosses and pressure tappings
• Condensate drains left open
• Wall and roof penetrations with poor seals
• Cracks at welds that have been stressed by restrained expansion

Why location matters

A small leak in an open yard is a maintenance issue. The same leak beside an AHU intake, or inside a plant room that shares air with occupied space, puts exhaust directly into the building. If any part of the RECD or pipe runs inside a room or shaft that connects to occupied areas, leaks there are the most important to prevent.

Review checks

• No RECD flange or clamp located within the immediate approach to an intake
• Gaskets rated for exhaust temperature and replaced when flanges are opened
• Penetrations sealed and fire-stopped
• Drains closed during running
• A leak check with the set at load during commissioning

Our article on common RECD installation mistakes includes several leak-related errors, and the flexible bellows guide explains how bellows fail.

Question 8: What Happens During Maintenance and Regeneration?

Some releases happen only occasionally, which is exactly why they get missed.

Filter removal and cleaning

When a filter module is removed for cleaning, loose soot and ash can be released. If this happens on a roof beside a running AHU, some of it goes indoors. The maintenance plan should say where filters are removed and cleaned, and whether nearby AHUs should be paused. Our RECD maintenance guide describes the tasks.

Regeneration

In some RECD designs, collected soot is burned off periodically. During that time the exhaust temperature and composition can change, and some designs may produce more odour. Ask the vendor what happens during regeneration, how often and for how long, and whether it can be scheduled when occupancy is low.

Cold starts and low-load running

The first minutes after a cold start, before the catalyst warms, are often the period of strongest smell. Light-load running keeps the system cool for longer. Our articles on whether an RECD can work reliably during cold starts and choosing an RECD for a generator running at low load explain these conditions.

Question 9: How Will We Know If It Goes Wrong?

Even a careful review relies on assumptions about wind, operation and maintenance. Detection closes the loop.

Carbon monoxide detection

A carbon monoxide detector in the supply air of an AHU near the outlet, or in occupied spaces it serves, gives early warning. Carbon monoxide is odourless and harmful at elevated levels, as the carbon monoxide poisoning article explains. Detection can be wired to the building management system to close outdoor air dampers or raise an alarm.

Complaint logging

Log every smell complaint with date, time, location and whether the DG was running. Patterns appear quickly. A complaint log that shows smells only in winter, or only when a particular AHU is running, points straight to the cause.

Periodic smoke tests

A simple smoke test at the outlet with the DG running, observed from the roof, shows where the plume goes under that day’s wind. Repeat it in different seasons.

Commissioning observation

During RECD commissioning, stand at each high-concern intake with the set at load and note any smell. It is low-tech and effective.

Mitigation Options If the Review Finds a Problem

Problem	Options, roughly from least to most disruptive
Intake too close to outlet	Change AHU control during DG running; raise the outlet; move the outlet; relocate the intake
Intake downwind in common wind	Raise the outlet above the recirculation zone; move the outlet to the other side of the roof
Screened enclosure traps plume	Open the screen near the outlet; extend the stack above the screen
Outlet cap pushes plume sideways	Change to a vertical discharge design
Leaks near intake	Relocate the RECD away from the intake; improve gaskets and seals
Maintenance releases	Schedule filter work with AHUs paused; clean filters off the roof
Unknown risk	Carbon monoxide detection; complaint logging; seasonal smoke tests

For complex buildings, an airflow study by a specialist, using wind tunnel testing or computer modelling, can answer questions that simple rules cannot. ASHRAE’s guide describes a hierarchy from simple observation to expert analysis.

A Walkthrough: IT Office Building in Bengaluru

This is an illustrative case.

The site

• A 500 kVA set in a ground-level DG room, with the exhaust rising through a shaft to the roof of a six-storey office building
• The roof carried three AHUs with fresh-air hoods, a stair pressurisation fan, cooling towers and an acoustic screen enclosing the AHUs and the DG stack together
• The RECD was planned for the roof, beside the stack, because the DG room was full
• Staff on the sixth floor had complained of diesel smell during outages for two years

The review

Question	Finding
1. Intakes	Three AHU hoods, one stair pressurisation fan, cooling tower inlets, and a neighbouring building’s openable windows
2. Distances	Stack 5.5 m from AHU-3 hood, 8 m from the stair fan
3. Wind	Monsoon westerlies carried the plume away; winter easterlies carried it across the AHUs; the screen trapped it in both cases
4. Outlet	Vertical outlet with a fixed cap deflecting exhaust sideways, about 1.5 m above the AHU hoods
5. Operating modes	All AHUs ran on DG power at full outdoor air; the weekly test was at 11 am on Wednesdays
6. RECD effect	Proposed device position was directly beside AHU-3, 2 m from its hood
7. Leaks	Device flanges and filter clamps would sit inside the screened area
8. Maintenance	Filter cleaning planned on the roof
9. Detection	None

What was changed

1. The RECD position moved to the opposite corner of the roof, outside the acoustic screen, with a short horizontal run from the shaft
2. The stack was extended so the outlet sat well above the AHU hoods and above the screen, with a vertical flapper outlet replacing the fixed cap
3. The section of acoustic screen nearest the stack was replaced with an open louvred panel
4. The BMS was changed so AHU-3 switches to minimum outdoor air whenever the DG runs
5. The weekly test moved to 7 am on Saturdays
6. Carbon monoxide detection was added in AHU-3 supply air, linked to the BMS
7. Filter cleaning was moved off the roof to a ground-level service area

The outcome

In the following winter, the complaint log recorded no diesel smell complaints during outages. A smoke test in December showed the plume clearing the screen and moving away above the AHUs. Office buildings on shared campuses often have several DG sets and many intakes; our guide on RECDs for data centres covers high-density plant roofs, and the 500 kVA DG set RECD page shows the rating used here.

Building Types That Need Extra Care

Hospitals. Patients are vulnerable and the building runs around the clock. Our guide on RECDs for hospitals covers retrofit planning.

Schools. Children are more sensitive to air pollution, as our article on how air pollution affects children’s health explains. Our guide on RECDs for schools discusses timing and location.

Housing societies. DG sets often sit close to flats with openable windows. Our purchase planning guide for RWAs discusses placement.

Food plants. Fresh air for production areas must be clean. Our guide on RECDs for food processing units covers hygiene considerations.

Compliance Note

The RECD’s approval is covered by CPCB’s System and Procedure for Emission Compliance Testing of RECDs. Intake separation is a building services and occupant safety matter rather than an approval item, but repeated smell complaints from occupants or neighbours can lead to scrutiny from the pollution control board. Record the review, the operating mode changes and any detection settings with your environmental compliance records.

Frequently Asked Questions

How far should a DG exhaust be from a fresh-air intake?

As far as the layout allows. Ventilation standards set minimum separations for contaminated exhausts, with the most noxious class at around 9 metres, but wind, height and building shape matter as much as distance. Your HVAC consultant decides the value for your building.

Does an RECD make it safe to put the outlet near an intake?

No. An RECD reduces particulate and, in some designs, carbon monoxide and hydrocarbons, but treated exhaust is still not fit to breathe.

Why do we smell diesel only in some seasons?

Because wind direction changes through the year. A layout that carries the plume away in one season can carry it across intakes in another.

Can the AHU be controlled to reduce the risk?

Often, yes. Switching nearby AHUs to minimum outdoor air or recirculation while the DG runs reduces intake of exhaust. Agree the change with your HVAC consultant.

Should the RECD itself be kept away from intakes?

Yes. Its flanges, clamps and drains are possible leak points, and filter maintenance can release soot.

Is a carbon monoxide detector worth installing?

On buildings where the outlet is close to intakes, it provides a useful early warning and can trigger damper changes automatically.

Review the Air, Not Only the Exhaust

An RECD retrofit is a rare opportunity to fix where exhaust goes, not only what is in it. Take the chance. List every intake, measure the distances, think about the wind and the roof, check what is running when the generator runs, look at what the device and its maintenance add and put a way of detecting problems in place. Nine questions, answered on paper before installation, can prevent years of complaints in the rooms below.

Aceget reviews outlet and intake positions as part of every RECD layout and coordinates with HVAC consultants where intakes are close. To have your roof or site reviewed, speak with Aceget, or explore the retrofit emission control device range.