Planning RECD Protection Against Rainwater and Condensation

Here is a fact that surprises many generator owners: burning one litre of diesel produces roughly one litre of water. Diesel is a hydrocarbon, and when the hydrogen in it burns, it becomes water vapour. In a hot exhaust that vapour stays invisible. When it meets a cold pipe wall, it turns back into liquid, often carrying acids formed from sulphur and nitrogen compounds in the exhaust.

So an exhaust system has two sources of water. One comes from outside, as rain falling into the outlet or running down the stack. The other comes from inside, as condensate from the engine’s own combustion. Both can reach a retrofit emission control device, and both can do harm: soaking the filter, corroding the housing, washing deposits into the substrate and producing clouds of steam that look worrying at start-up.

This article traces the six routes water takes into and around an RECD, then sets out the design features and operating habits that block each one.

Two Kinds of Water, Different Behaviour

Property	Rainwater	Exhaust condensate
Where it comes from	Sky, through the outlet or down the outside of the system	Water vapour from combustion condensing on cold surfaces
When it appears	Wet weather, especially monsoon storms with wind	Every cold start; worst on short runs and in cold, humid weather
Where it collects	Base of the stack, first low point downstream of the outlet, inside cladding	Low points, the first cold sections after the engine, inside the device on cold starts
Chemistry	Mostly clean; may carry dust and salt near the coast	Acidic, carrying sulphur and nitrogen compounds and soot
Main damage	Soaked filter, rusted pipe, wet insulation	Internal corrosion, sludge, deposits in the filter
Main defences	Rain cap, outlet design, cladding detail, drains	Slopes, drains, insulation, running habits, fuel quality

Treating these as one problem leads to half-solutions. A rain cap does nothing for condensate. A drain at the engine end does nothing for rain at the stack base. The plan needs to address both.

Why Water Matters to an RECD in Particular

A plain exhaust pipe tolerates a fair amount of water. It rusts more quickly, but it keeps working. An RECD is more sensitive.

• The filter substrate is a porous ceramic with fine channels. Water can soak into it and carry soot and ash deeper into the walls.
• Heating a soaked substrate quickly can stress it, because water turning to steam inside the walls expands sharply.
• Catalyst coatings can be affected by contaminants dissolved in water, such as salts and acids.
• Wet soot forms a paste that is harder to burn off than dry soot.
• Steam at start-up can make a healthy device look like it is producing white smoke.

Our explainer on what an RECD is and how it works describes the filter structure, and the article on why a generator emits white smoke explains how steam is often mistaken for a fault.

Route 1: Rain Falling Into the Outlet

The most obvious route. An open vertical stack is a rain gauge with a pipe attached. In a heavy monsoon storm, a 150 mm stack can collect a surprising amount of water in an hour, and all of it runs down towards the device.

Defences

Rain caps. Several designs exist, each with pros and cons.

Outlet design	How it keeps rain out	Watch out for
Hinged flapper cap	A flap closes under gravity when the set is off and opens when exhaust flows	Flap can seize with soot or rust; can rattle; must open fully
Fixed cap with side discharge	A cover sits above the outlet; exhaust leaves sideways	Adds some backpressure; can direct exhaust towards nearby buildings
Rain-trap stack section	An inner pipe inside a larger outer sleeve, with a drain at the bottom of the outer sleeve	Needs a drain line; more fabrication
Angled or mitred outlet	Outlet cut at an angle or turned to face sideways	Partial protection; wind-driven rain still enters
Downward-facing gooseneck	Outlet turned downwards	Directs exhaust at the ground or people; generally avoided

Whichever design is used, the backpressure it adds must be counted in the system total. Our explainer on how exhaust backpressure affects a DG set explains why every fitting matters once an RECD has used most of the margin.

Horizontal outlets. A side-discharging horizontal outlet, sloped slightly downward away from the system, lets rain run out rather than in.

Outlet height and direction. Rooftop stacks often need to discharge vertically to disperse exhaust well. That makes a good rain cap or rain-trap section more important. Our rooftop RECD installation guide covers stack design on roofs.

Route 2: Rain Running Down the Outside

Water does not need to enter the outlet to cause trouble. Rain running down the outside of a stack follows the pipe to every bracket, flange and penetration.

Where it causes problems

• Roof and wall penetrations, where it runs into the building
• Brackets and clamps, where it sits in crevices and corrodes the pipe and fixings
• The top of the device housing if the device sits beneath a vertical section
• Flanges, where it can creep along gasket faces

Defences

• Storm collars at roof penetrations, clamped to the stack above the flashing, shed water outward. The collar must allow the stack to move with thermal growth.
• Drip rings on vertical pipes above flanges and devices throw water clear.
• Sealed bracket details with no horizontal ledges that collect water.
• A canopy or roof over an outdoor device, where the layout puts it directly under a long vertical run.

Route 3: Rain Getting Under the Cladding

Outdoor insulation is clad with metal sheet. If the cladding is detailed poorly, rain gets between the sheet and the insulation and stays there.

How it gets in

• Laps facing upward, so water runs into the joint
• Unsealed gaps where brackets, sensors or drains pass through
• Open ends at the termination of insulation
• Damage from ladders, tools or impact

Why it matters

Wet insulation stops insulating, which means cooler exhaust and poorer regeneration. Worse, it holds water against the pipe, causing corrosion that cannot be seen. The corrosion under insulation article describes how serious this can become.

Defences

• Laps arranged so water runs over them, like roof tiles
• Longitudinal seams on horizontal pipe placed below the horizontal centre line, facing down
• Sealed collars or boots wherever something pierces the cladding
• End caps on insulation terminations
• Drain holes at the lowest points of the cladding, so water that does enter can leave

Our companion guide on RECD insulation and heat shields covers the wider insulation plan.

Route 4: Condensate From Cold Starts

Every time a cold engine starts, the first minutes of exhaust meet cold pipe. Water vapour condenses on the walls and runs downhill. On a long system in a cool, humid room, this can be a noticeable amount of liquid.

What makes it worse

• Short runs. The system never gets hot enough to dry out before the engine stops. The next start adds more water.
• Frequent starts. Each cold start adds a new dose.
• No-load or light-load tests. Exhaust stays relatively cool, so condensation continues for longer.
• Long uninsulated pipe. Cold walls condense more.
• Cold, humid weather. Winter mornings and monsoon nights increase condensation.

The temperature at which vapour turns to liquid is its dew point. Acidic components in exhaust can condense at temperatures above the dew point of plain water, which is why the first sections to cool are often the first to corrode.

Our articles on whether an RECD can work reliably during cold starts and whether an RECD suits a DG set that runs only occasionally explain how this pattern affects the device.

Defences

Slope horizontal sections so condensate runs to a drain, not towards the device or back to the engine. A gentle fall is enough, as long as it is consistent and has no dips.

Drain at every low point. Each low point needs a drain: a small pocket below the pipe with a valve or plug. The most important drains are:

• Just upstream of the device inlet
• At the base of every riser
• After a silencer, if the silencer sits low

Insulate upstream sections so the pipe warms faster and less vapour condenses.

Improve running habits. Longer runs at meaningful load dry the system out. Our guide on choosing an RECD for a generator running at low load explains why loaded runs help.

Route 5: Moist Air Breathing In While Idle

When a hot exhaust system cools after shutdown, the gas inside contracts and draws air in through the outlet. On humid nights, that air carries moisture which condenses on the cooling walls and inside the filter. Over weeks of idleness, this adds up, particularly in coastal and monsoon conditions.

Defences

• A rain cap that closes when the set is off reduces the air exchange
• Periodic loaded runs dry the system out
• Drains checked before starting after long idle periods

Coastal sites face this route plus salt-laden air. Our guide on choosing an RECD for coastal salt-laden environments covers material choices for those conditions.

Route 6: Splash, Wash-Down and Flooding

Some water arrives sideways or from below.

• Wash-down hoses in food plants, cold stores and process areas can hit a device or its cladding directly
• Roof runoff from a building gutter that discharges near the device
• Splash from vehicles in yards during heavy rain
• Flood water in low-lying plant rooms

Defences

• Position devices away from wash-down zones, or fit splash guards
• Redirect gutters and downpipes away from the device
• Raise devices above splash height in yards
• For flood-prone rooms, set a design water level and keep the device above it; our companion guide on designing an RECD installation in a flood-prone plant room explains how

The Drain Itself: Small Detail, Big Effect

Drains appear in almost every defence above. A badly designed drain can be worse than none.

Good drain features

• A pocket or sump below the pipe, so water collects away from the gas stream
• A valve or plug that can be opened to drain and then closed, so exhaust does not leak out during running
• An accessible position, reachable without a ladder where possible
• A drain line leading to a safe discharge point, not onto a walkway or into an electrical trench
• Material suited to acidic condensate, usually stainless steel

Common drain mistakes

Mistake	Result
No drain at the riser base	Water collects and is blown into the device at the next start
Drain left open permanently	Exhaust leaks out; soot staining; smell
Drain blocked with soot	Water stays in the pipe
Drain placed above the actual low point	Water collects below the drain
Condensate discharged onto a floor or into a stormwater drain	Oily, acidic water reaches places it should not

Exhaust condensate is not clean water. It carries soot, acids and traces of oil. Collect and dispose of it sensibly and record how, as part of your environmental compliance records.

Fuel Quality and Acid Condensate

The sulphur in diesel becomes sulphur oxides in the exhaust, which form sulphuric acid in condensate. Lower-sulphur fuel produces less acid. Our article on how ultra-low sulphur diesel improves emission control explains the link, and fuel quality also matters for the catalyst in the RECD itself. The broader composition of diesel exhaust explains why condensate is rarely just water.

A Seasonal Calendar

Water risks change through the year. A simple calendar keeps the right checks in front of the team.

Season	Main water risks	Checks and actions
Pre-monsoon	Blocked drains and seized caps from the dry season	Free and test rain caps; clear drains; inspect cladding seals; check gutters near the device
Monsoon	Heavy rain into outlets; runoff; flooding; humid idle periods	Check drains weekly; inspect for water staining after storms; run the set at load regularly; follow flood procedure if needed
Post-monsoon	Hidden water in cladding and low points	Inspect insulation for dampness; open inspection windows; check for corrosion
Winter (north India)	Heavy condensate on cold mornings	Drain before starting after cold nights; prefer longer loaded runs; see our guide on winterizing a diesel generator
Summer	Low water risk; dust may block caps and drains	Clean caps and drains; check flapper movement

Add these to the site’s routine; the monthly maintenance checklist for diesel generators and the RECD maintenance guide are natural homes for them.

Canopy Sets and Open-Frame Sets

Packaged canopy sets often have their exhaust outlet on the canopy roof, sometimes with a short tailpipe. When an RECD is added outside the canopy, the connection may introduce a new low point or an exposed horizontal section. Open-frame sets in sheds have their own exposure patterns. Our comparison of open frame vs enclosed canopy generators explains the differences. In either case, check that the new connection drains away from both the engine and the device.

A Walkthrough: Cold Storage Facility in Mangaluru

This is an illustrative case.

The site

• A 160 kVA canopy set in an open yard beside a seafood cold store
• RECD fitted outside the canopy on a floor frame, with a horizontal connection from the canopy tailpipe and a 5 m vertical stack after the device
• The set ran for short periods during frequent grid interruptions and a weekly 15-minute no-load test
• Heavy monsoon, salt-laden air and wash-down water from the loading bay

The symptoms

During the first monsoon after the retrofit, staff reported clouds of white vapour at start-up lasting several minutes, rust streaks on the device cladding and a backpressure reading that climbed faster than the vendor expected.

What the inspection found

Route	Finding
1. Rain into outlet	Open vertical stack with no cap; water found pooled at the device outlet cone
2. Rain down the outside	Stack bracket had a flat ledge where water sat; rust streaks below
3. Rain under cladding	Cladding laps on the device faced upward; insulation wet on the top surface
4. Cold-start condensate	Horizontal connection sloped slightly towards the device, with no drain upstream
5. Moist air while idle	Long idle gaps between short runs; damp air drawn in nightly
6. Splash and wash-down	Loading bay wash-down hose reached the device frame

The corrections

1. A hinged flapper rain cap was fitted, with its backpressure included in a revised calculation
2. The stack bracket was replaced with a sloped design and a drip ring was added above the device outlet flange
3. The cladding was refitted with laps facing downward and sealed boots at sensor penetrations
4. The horizontal connection was re-supported to fall towards a new drain pocket just upstream of the device
5. The weekly test was changed to a 30-minute run with the cold store’s compressors as load
6. A splash screen was fitted between the loading bay and the device
7. A pre-start drain check was added to the operator’s routine

The start-up vapour reduced to a brief puff, rust streaks stopped and the backpressure rise returned to the expected pattern. Cold storage loads and running patterns have their own effect on an RECD; our guide on RECDs for cold storage explains them. The 160 kVA DG set RECD page covers this rating.

A Water Protection Checklist for the Site Plan

• Outlet has a rain cap or rain-trap section, included in the backpressure calculation
• Storm collars at roof penetrations, allowing thermal movement
• Drip rings above flanges and devices on vertical runs
• Brackets detailed without water-holding ledges
• Cladding laps facing downward and sealed at penetrations
• Drain holes at cladding low points
• Horizontal pipe sloped consistently towards drains
• Drain pocket and valve just upstream of the device inlet
• Drain at the base of every riser
• Drains accessible and discharged to a suitable point
• Device positioned away from wash-down, gutters and splash zones
• Running pattern includes regular loaded runs
• Seasonal checks added to the maintenance routine

Many of these items appear on our list of common RECD installation mistakes, which shows how often water details get missed.

Compliance Note

The device is approved under CPCB’s System and Procedure for Emission Compliance Testing of RECDs. Water protection is part of good installation practice rather than an approval item, but a device that keeps getting wet will load faster, corrode sooner and may produce start-up vapour that draws attention. Keep the drain positions, cap details and seasonal checks in your installation records.

Frequently Asked Questions

Why does water come out of my generator exhaust?

Burning diesel produces water vapour, roughly a litre for every litre of fuel. When the exhaust meets cold pipe, the vapour condenses. Rain entering the outlet adds more. Some water is normal; a lot of it, or water that never stops, needs investigation.

Is a rain cap enough to protect an RECD?

It stops rain falling into the outlet, which is one of six routes. Drains, slopes, cladding details and running habits handle the rest.

Where should the condensate drain be?

At every low point, and most importantly just upstream of the device inlet and at the base of every riser.

Will water damage the RECD filter?

A soaked filter can be damaged if heated quickly, and wet soot is harder to burn off. Keeping water away is far better than recovering from it. If a device has been soaked, ask the vendor before running.

Does white vapour at start-up mean the RECD is faulty?

Not necessarily. It is often steam from condensate in a cold system. If it lasts a long time, or appears when the system is warm, investigate.

How do running habits affect condensation?

Short, light runs leave the system cold and wet. Longer runs at meaningful load dry it out.

Close Every Route

Water reaches an RECD by six routes, from above, along the outside, through the cladding, from the exhaust itself, through the air breathed in while idle and from splash or flood. Each one has a simple, inexpensive defence. Put a cap on the outlet, a collar on the roof, a drip ring above the flange, laps the right way round on the cladding, a slope on every horizontal and a drain at every low point, and then run the set long enough to dry out. The filter will stay dry and the device will do the job it was bought for.

Aceget designs drainage, outlet protection and cladding details into every RECD layout and includes seasonal checks in the handover pack. To review water protection on your installation, reach out to Aceget, or see the full retrofit emission control device range.