In short. The exhaust pipe is the part of a generator installation most often done wrong — and the mistakes are expensive: excess back-pressure that steals power and raises exhaust temperature, a stack below the CPCB height rule that fails inspection, smoke nuisance that turns neighbours into complainants, and rain or condensate running back into a silent engine. This guide covers sizing, routing, the silencer, the CPCB stack-height formula and the faults we find on service calls, for any make of set.
Back-pressure: the number that rules the design
Every engine maker publishes a maximum allowable exhaust back-pressure; exceed it and the engine loses power, burns more fuel, runs hotter and smokes. Back-pressure comes from pipe length, bends and the silencer. The working rules: keep the run as short and straight as the site allows; every 90° bend counts as several metres of extra pipe, so two gentle 45° bends beat one tight 90°; and when the run grows long, go up one pipe size — a larger diameter costs little against the fuel an engine wastes pushing into a throttled pipe for ten years. On long runs, increase the diameter rather than accepting the engine flange size as fate: the flange is where the pipe starts, not what it must remain.
The CPCB stack-height rule
For diesel generators, the Central Pollution Control Board formula sets the minimum stack height as H = h + 0.2 × √kVA, where h is the height of the building where the set is installed, in metres. Worked examples: a 125 kVA set beside a 10 m building needs a stack of at least 10 + 0.2 × 11.2 ≈ 12.3 m; a 500 kVA set at the same building needs 10 + 0.2 × 22.4 ≈ 14.5 m; a 1,000 kVA set, 10 + 0.2 × 31.6 ≈ 16.3 m. The point of the rule is dispersion — exhaust released above the building envelope instead of into windows. State boards apply this at consent and renewal, so build it in at installation rather than welding an extension under notice later.
Routing, support and the details that age well
- Flexible bellows at the engine — the engine moves on its mounts; rigid pipe transmits that movement into cracked welds and broken turbo flanges.
- Slope away from the engine with a condensate drain at the low point. A vertical stack collects rain and condensation; without a drain cock and a rain cap (or a 45° cut facing away from the wind), that water finds the turbocharger.
- Lagging on indoor runs — an exhaust pipe runs hot enough to injure and to heat the generator room; insulate indoor sections and anywhere a person can touch.
- Independent supports — the pipe hangs from the structure, never from the engine or the silencer lugs alone.
- Clear of openings — terminate away from windows, fresh-air intakes and balconies; a compliant stack that still fumigates the third floor is a failed design.
Choosing the silencer
Silencers trade attenuation against back-pressure. An industrial grade suits open yards; a residential grade is the default for town sites working with the canopy’s own treatment; a critical grade buys the most quiet at the most back-pressure, for hospitals, hotels and offices where the set sits near occupied rooms. Match the grade to the neighbours and the duty, and check its loss is inside the engine’s back-pressure budget together with the pipe run — the silencer and the pipe share one allowance, not an allowance each.
Faults we keep finding on service calls
Undersized pipe on a lengthened run after a set was shifted; stacks shortened during building work and never restored; missing condensate drains with turbochargers rusted from standing water; bellows replaced with rigid pipe “temporarily”, years ago; lagging removed for a repair and never refitted. Most of these take an hour to put right and save an engine. If your installation has grown or moved since commissioning, have the exhaust surveyed with the next service — ours or anyone’s.
Exhaust work across Telangana and AP
We size, fabricate, route and certify exhaust systems for new installations and correct old ones — including stack-height compliance for pollution-board consent — as part of generator installation or as a standalone job on any make. Send a photo of the present run and the set’s nameplate to WhatsApp 098852 01608, or use the enquiry form.
Questions engineers ask about generator exhausts
What is the stack height formula for a DG set?
H = h + 0.2 × √kVA — building height plus 0.2 times the square root of the rating. A 250 kVA set beside a 10 m building needs about 13.2 m.
What size exhaust pipe does my generator need?
Start from the engine flange, then check the full run — length, bends and silencer — against the maker’s back-pressure limit, and go up a size on long runs. Send us the set’s nameplate and the routing; we will size it to the engine’s budget.
Why does my generator smoke more after it was shifted?
Usually the new exhaust run: more length and bends on the same pipe diameter means more back-pressure. Have the run measured before suspecting the engine.
Does the exhaust really affect fuel consumption?
Yes. Back-pressure beyond the limit raises fuel use and exhaust temperature while cutting available power — the engine pays for the pipe, every running hour.
Can you fix only the exhaust without an AMC?
Yes — exhaust correction is a standalone job on any make, though most customers add it to a maintenance contract visit.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 1 November 2026.