Horizontal vs Vertical RECD Mounting: What Determines the Layout?

Look at a few dozen RECD installations across Indian factories, offices and housing societies and you will see two broad shapes. In some, the device lies on its side, a long cylinder or box on a skid next to the generator or along a pipe rack. In others, it stands upright, like a short chimney section, against a wall or at the base of a stack. Both work. Both are approved, installed and serviced every day.

So why does one site end up with a horizontal device and another with a vertical one? It is rarely a matter of taste. The layout is usually decided by a handful of practical factors: which way the exhaust already travels, how much floor or wall space is free, how the device will be lifted in and serviced, where water collects, and what the vendor’s design allows.

This guide sets out eight determinants, explains how each pushes the decision one way or the other, and then gives a simple scoring sheet you can fill in for your own site. Two short worked examples show the sheet in use.

First, What Is Actually Being Oriented?

An RECD typically contains an inlet section, a catalyst, a particulate filter and an outlet section, all inside an insulated housing. The filter is commonly a wall-flow monolith: a ceramic or metal block with thousands of parallel channels, alternately plugged at each end, so that exhaust must pass through the porous walls. DieselNet’s explanation of wall-flow monoliths describes the structure. Our article on what an RECD is and how it works gives the plain-language version.

When people talk about horizontal or vertical mounting, they mean the direction in which exhaust flows through these channels:

• Horizontal: flow travels sideways through the device; the housing lies along the ground, a frame or a pipe rack
• Vertical upflow: exhaust enters at the bottom and leaves at the top
• Vertical downflow: exhaust enters at the top and leaves at the bottom; less common in retrofit work

The filter itself does its job in any of these directions. What changes with orientation is everything around it: supports, water, lifting, access and space.

Determinant 1: The Direction the Exhaust Already Travels

The simplest layouts follow the exhaust’s existing path. Fewer bends mean lower backpressure, less cost and fewer joints that could leak.

Existing exhaust path	Orientation that usually fits
Long horizontal run from the engine to a wall or stack	Horizontal, placed in the run
Short connection from the engine straight up to a stack or riser	Vertical, placed at the base of the stack
Canopy tailpipe exiting through the roof	Horizontal on the canopy roof, or vertical beside it, depending on space
Basement exhaust entering a vertical shaft	Vertical at the shaft foot, or horizontal before the shaft

Each change of direction adds resistance. Our explainer on how exhaust backpressure affects a DG set shows why bends add up quickly. If one orientation needs two extra elbows and the other needs none, that alone may settle the matter.

Determinant 2: The Space Available, Measured in Three Dimensions

Floor area and height trade against each other.

• Horizontal devices need length and width but little height. They suit long, narrow spaces, pipe racks, canopy roofs and rooms with low ceilings.
• Vertical devices need little floor area but more height, plus clearance above for removing internal modules. They suit tight yards, narrow side setbacks and walls where floor space is precious.

Measure the candidate spaces in length, width and height, including the clearance needed for service. A vertical device that fits under a beam but cannot release its filter upwards is not really a fit. Our companion guide on service clearance around an RECD explains those clearances.

Determinant 3: How the Filter Will Be Removed

Particulate filters need periodic cleaning to remove ash that cannot be burned away. DieselNet’s overview of ash accumulation in diesel particulate filters explains why. Removal direction differs by orientation:

Orientation	Usual removal direction	What you need
Horizontal	Sideways, along the device axis, after unbolting a clamp or flange	Clear length beside the device at least equal to the module, plus handling room
Vertical	Upwards, out of the top section, or downwards on some designs	Overhead height and a lifting point or hoist above the device

In a room with a low ceiling, horizontal withdrawal is often easier. In a crowded yard with open sky above, a vertical device lifted out by a small crane or hoist may be simpler. Some vendors offer housings that split at a mid-flange so the filter section can be lifted out sideways from a vertical device; ask whether that option exists.

Determinant 4: Where Water Will Go

Water reaches an exhaust system in two ways. Condensate forms when hot, moist exhaust meets cold metal during cold starts and short runs. Rain enters through an unprotected outlet. Both run downhill.

Horizontal devices

Water can pool along the bottom of the housing and at the inlet cone if the device sits perfectly level or slopes backwards. A slight fall towards a drain, and a drain point at the low end, solve this.

Vertical upflow devices

The inlet is at the bottom. Any water running down the stack, or condensing inside the device, collects at the inlet section, where the hot exhaust then has to pass through it. A drain at the base is essential, and a good rain cap on the stack outlet matters more than ever.

Vertical downflow devices

Water entering from the top runs straight onto the catalyst and filter faces. This arrangement needs particularly good rain protection and is less common for that reason.

Why it matters

Repeated wetting can carry soot and ash into places they should not be, encourage corrosion of steel parts and, in severe cases, damage the substrate when water flashes to steam on a hot surface. Sites in high-humidity areas need extra care; our guide on RECDs for water treatment plants discusses corrosion in wet environments.

Determinant 5: Support, Weight and Wind

Weight is the same whichever way the device faces. How that weight reaches the structure is not.

Factor	Horizontal	Vertical
Footprint	Long and narrow; load spread over two or more saddles	Compact; load concentrated on a small base
Centre of gravity	Low	High, especially with a stack above
Overturning risk	Low	Higher; needs anchoring or wall bracing
Wind exposure outdoors	Low profile	Taller profile, more wind load
Typical support	Skid, saddles on a frame, pipe rack	Base plate with anchors, wall brackets, guyed stack
Thermal growth	Along the length; one saddle should slide	Upwards; top connections must allow vertical movement

On rooftops and in earthquake-prone zones, the high centre of gravity of a vertical device with a stack needs proper bracing. On a slab with limited capacity, a vertical device’s concentrated base load may need spreading. On the other hand, a horizontal device needs a sliding support at one end to allow thermal expansion, or it will push against the engine connection every time it heats up. The vibration isolation principles apply to both, and our guide on how to prevent generator vibration issues covers the engine side.

Determinant 6: Flow Distribution at the Inlet

A filter works best when exhaust spreads evenly across its whole face. If flow concentrates in the centre, that part loads with soot faster, regenerates differently and may see higher thermal stress.

The inlet section of the device is designed to spread flow. That design assumes a certain length of straight pipe before the inlet and a certain inlet geometry. Orientation changes what the layout can offer:

• A horizontal device placed in a long straight run usually receives well-developed, even flow.
• A vertical device fed by an elbow immediately below its inlet may receive flow skewed to one side.

Ask the vendor how much straight pipe they need before the inlet and whether their inlet design tolerates an elbow close to it. If the layout cannot provide the recommended straight length, choose the orientation that can.

Determinant 7: Heat and Its Effect on Surroundings

A hot device warms whatever is near it. Orientation decides where that heat goes.

• Vertical devices release heat upwards in a column. Indoors, that heat rises to the ceiling, where cable trays, sprinkler heads and light fittings sit. Outdoors, it rises harmlessly.
• Horizontal devices spread heat along their length, at whatever height they are mounted. At floor level, people walking past may be exposed; overhead, heat reaches the ceiling services.

Inside rooms and canopies, think about what is above and beside the device. Outside, think about nearby windows, air intakes, plastic tanks and walkways.

Insulation reduces surface heat in both orientations. Our guides on common causes of generator overheating and the basement installation companion article cover heat in enclosed spaces.

Determinant 8: What the Vendor’s Design Permits

Finally, the device has to be designed for the orientation you choose. Some RECDs are built to be mounted only one way; others are offered in both. The vendor’s installation manual may also set limits on tilt, the position of sensors and the direction of drain points.

Ask:

1. Which orientations is this model designed for?
2. Does orientation affect the filter’s warranty or service interval?
3. Where must drains and sensors be placed in each orientation?
4. How is the filter removed in each orientation?
5. Is there a minimum straight pipe length before the inlet?

Approval of the device comes through CPCB’s System and Procedure for Emission Compliance Testing of RECDs. Installation in line with the vendor’s instructions keeps that documentation valid. Our guide on choosing CPCB approved RECD vendors suggests how to compare vendors’ installation flexibility.

Sensors and Monitoring by Orientation

Backpressure and temperature sensors behave slightly differently depending on orientation.

• Pressure tapping points should not sit at the bottom of a horizontal pipe, where condensate collects and can block them. Side or top positions are better.
• In vertical devices, pressure lines running down from the device to a gauge need a slope that lets condensate drain back, or a trap that can be emptied.
• Temperature sensors should sit where they measure the main flow, not a stagnant corner.
• Displays and loggers should be readable from a safe standing position in either orientation.

The RECD maintenance guide explains how monitoring data supports service decisions.

A Scoring Sheet for Your Site

Rate each determinant for your site. Give 2 points to the orientation that suits that factor clearly, 1 point to each if both are workable, and 0 to an orientation that would cause real problems. Add the totals.

Determinant	Horizontal score (0 to 2)	Vertical score (0 to 2)	Notes
1. Existing exhaust direction
2. Space in three dimensions
3. Filter removal route
4. Water and drainage
5. Support, weight and wind
6. Inlet flow distribution
7. Heat and surroundings
8. Vendor’s design
Total

A score of 0 on determinant 8 rules that orientation out regardless of the total. Elsewhere, a clear difference in totals usually points to the right answer. When the totals are close, the deciding factor is normally service access, because that is the one you will live with for the life of the device.

Worked Example One: Forging Unit in Ludhiana

This is an illustrative case.

The site

• A 400 kVA set in an open yard beside a forging shop, with a canopy and a tailpipe that bends through 90 degrees and runs horizontally for about 6 metres to a stack at the boundary wall
• Plenty of ground space along the run; heavy dust and scale in the air from the forging operations
• The set runs daily for several hours during peak tariff periods, at moderate to high load

Scoring

Determinant	Horizontal	Vertical	Notes
Existing exhaust direction	2	1	Long horizontal run already present
Space	2	2	Open yard
Filter removal	2	1	Sideways withdrawal easy at ground level; a vertical unit would need a hoist
Water and drainage	2	1	Simple drain at the low end of a sloped run
Support, weight and wind	2	1	Low skid on ground; vertical unit would need anchoring near the wall
Inlet flow distribution	2	1	Straight pipe available before the inlet
Heat and surroundings	1	2	Horizontal unit at ground level needs a guard for passing workers
Vendor’s design	2	2	Both offered
Total	15	11

Result

The device was placed horizontally on a ground skid in the existing run, with a sliding saddle at the stack end to allow expansion, a drain at the low point and a mesh guard against contact. Heavy-dust sites have their own needs; our guide on RECDs for foundries covers dust and heat. The 400 kVA DG set RECD page shows the rating in this example.

Worked Example Two: Office Building in Bengaluru

This is an illustrative case.

The site

• A 250 kVA set in a canopy in a narrow side setback between the building and the boundary wall, about 2.4 metres wide
• Tailpipe exits through the canopy roof and runs straight up a stack fixed to the building wall to above the parapet
• No spare ground space along the setback; the walkway beside the canopy must remain clear for fire access
• The set runs for weekly tests and outages of a few hours a month

Scoring

Determinant	Horizontal	Vertical	Notes
Existing exhaust direction	1	2	Exhaust already rises vertically
Space	0	2	No length available along the setback without blocking the walkway
Filter removal	1	1	Horizontal would need side clearance that does not exist; vertical needs a lifting point above
Water and drainage	1	1	Vertical needs base drain and a good rain cap
Support, weight and wind	1	2	Vertical unit can be bracketed to the building wall
Inlet flow distribution	1	1	Elbow from the canopy roof in both cases; vendor confirmed inlet tolerance
Heat and surroundings	1	1	Wall nearby; heat shield needed in either case
Vendor’s design	2	2	Both offered
Total	8	12

Result

The device was mounted vertically, bracketed to the building wall above the canopy, with a mid-flange split so the filter section could be lowered by a small chain hoist from a beam fixed above. A drain at the base and a rain cap at the top dealt with water. A heat shield protected the wall finish. Because the set runs lightly, the team also scheduled monthly loaded runs to help regeneration, as our guide on choosing an RECD for a generator running at low load recommends. See the 250 kVA DG set RECD page for this rating.

What About Angled or L-Shaped Layouts?

Some sites cannot offer a clean horizontal or vertical position. The exhaust may leave the canopy at an angle, or the only free space may be a corner where the device must turn the flow through 90 degrees. Two compromises come up often.

Inclined mounting. Tilting a device a few degrees from horizontal can help drainage and fit it under a sloping roof or beam. Larger tilts are a different matter: internal supports, drains and sensors are positioned for a specific orientation, and the vendor must confirm what angle the design tolerates.

Elbow-inlet or elbow-outlet housings. Some vendors offer devices with the inlet or outlet on the side of the housing rather than the end, so a vertical device can accept a horizontal pipe, or the reverse. This saves a separate elbow and its backpressure, but the inlet design must still spread flow evenly across the filter face.

In both cases, ask for written confirmation that the layout is within the device’s design limits, and keep it with the installation records.

Orientation Myths

“Vertical devices work better because hot gas rises.”

Exhaust is pushed through the system by the engine, not drawn up by buoyancy. The filter works the same way in either orientation. Orientation matters for layout and water, not for filtration.

“Horizontal devices collect more ash.”

Ash collects at the closed ends of the filter’s inlet channels, whatever the orientation. The rate depends on engine oil consumption and running hours, not on whether the device lies down or stands up.

“You can mount any device in any direction.”

Not always. Some designs are made for one orientation only, with drains, sensors and internal supports placed accordingly. Check the manual.

“Vertical saves space, so it is always better indoors.”

It saves floor space but needs overhead clearance for service. In a low-ceilinged room, a horizontal device may be the better fit.

Questions About RECD Orientation

Does it matter whether an RECD is mounted horizontally or vertically?

The filter works in either direction. Orientation matters for backpressure, space, service access, drainage, support and heat. The best choice depends on the site layout and the vendor’s design.

Which orientation is easier to service?

It depends on the space. Horizontal devices usually need side clearance for filter withdrawal; vertical devices need overhead clearance and a lifting point. Choose the one your site can support.

How do I stop water collecting in a vertical RECD?

Fit a good rain cap on the stack, provide a drain at the base of the device and slope pressure sensing lines so condensate can drain.

Can I change the orientation of an existing RECD?

Only if the vendor confirms the device is designed for both orientations, and drains and sensors are relocated accordingly. Ask before moving it.

Is a vertical RECD suitable for rooftops?

Yes, with proper anchoring and bracing for wind and seismic loads, and a plan for lifting the filter out during service.

Does orientation affect backpressure?

Not directly, but the layout it leads to can. An orientation that needs extra bends raises backpressure; one that follows the existing exhaust path keeps it low.

Let the Site Decide

Horizontal and vertical RECDs are not rivals. They are two answers to different site questions. Follow the exhaust’s existing direction, measure space in three dimensions, plan the filter’s way out, manage water, support the weight sensibly, give the inlet a clean flow, keep heat away from what it can harm and stay within the vendor’s design. Score the options honestly and the layout usually chooses itself.

Aceget offers CPCB type-approved devices in horizontal and vertical configurations for engines from 25 kVA to 2500 kVA, and plans the orientation as part of every site survey. To score your own layout with an engineer, contact Aceget, or compare ratings on the retrofit emission control device page.