In short. An indoor generator installation turns the room into part of the machine. The room must admit enough cool air for the radiator and for combustion, discharge hot radiator air outdoors without letting it return to the inlet, carry the exhaust safely outside, contain fuel and noise, and leave space for service. Most indoor failures are overheating under load caused by recirculation or undersized openings. Design the air path from manufacturer airflow data, never from kVA alone.
What the room has to do
A diesel generator converts roughly a third of its fuel energy into electricity. The rest leaves as heat through the exhaust, through the radiator, and as radiated heat from the engine, alternator and exhaust pipe into the room. A 500 kVA set at full load can reject 400 kW or more through its radiator and another 30 to 50 kW directly into the room air (illustrative). Indoors there is no breeze to carry it away. The room has to supply cool air to the radiator face, let the fan push the heated air out through a wall or duct, provide combustion air, and keep the room within the temperature at which the alternator and controller are rated, usually 40 to 50 °C. It also has to hold the day tank safely, keep exhaust gas out of occupied spaces and attenuate noise to the level the neighbours and the pollution control board accept.
Cooling air: the calculation that matters
Two airflows are needed and must not be confused. The radiator airflow is fixed by the fitted fan and stated on the data sheet, often 8 to 15 cubic metres per second for a set of 400 to 600 kVA. The room ventilation airflow is the extra air that carries away heat radiated into the room without the room rising more than a chosen amount above outdoor ambient.
Q = H ÷ (ρ × cp × ΔT)
Here Q is the airflow in cubic metres per second, H is the heat radiated into the room in kW, ρ is the air density (about 1.13 kg per cubic metre at 40 °C), cp is the specific heat of air (about 1.005 kJ per kg per kelvin) and ΔT is the permitted room temperature rise in kelvin. As an illustrative example, a set radiating 40 kW into the room with a permitted rise of 10 K needs about 40 ÷ (1.13 × 1.005 × 10), which is roughly 3.5 cubic metres per second of room air in addition to the radiator flow (illustrative). On a 44 °C Hyderabad afternoon a 10 K rise puts the room at 54 °C, above the rating of most alternators and controllers, so permitted rise, derating and ventilation must be decided together. The generator room ventilation and heat rejection pages work through the full method.
Openings, louvres and recirculation
A large hole in the wall is not an airflow. Louvres, bird mesh, acoustic splitters and ducts each add resistance, and a radiator fan can overcome only a limited static pressure, typically 125 to 250 Pa. The free area of a weather louvre is often only 40 to 50 percent of its face area, and an acoustic louvre less, so the gross opening must be sized from the free area needed.
| Element | Good practice | What goes wrong |
|---|---|---|
| Inlet opening | Low on the wall opposite the radiator, free area sized for radiator plus room flow at low velocity | Inlet behind the set or beside the outlet draws hot air back in |
| Outlet opening | Radiator discharge ducted directly to the outside with a flexible connection | Radiator blows into the room and hopes the air finds the exit |
| Louvres and mesh | Free area and pressure drop confirmed against fan static pressure | Decorative louvre with 30 percent free area strangles the fan |
| Outside the outlet | Discharge faces away from the inlet and from any wall within a few metres | Hot air hits a compound wall and curls back to the inlet |
| Multiple sets | Each set has its own inlet and outlet path or the room is sized for all running together | Room sized for one set, two run during a long outage |
Recirculation is the most common cause of a set that passed its no-load test and overheats after an hour on load. Hot discharge air returns to the inlet, the radiator sees 55 °C air instead of 40 °C, and coolant temperature climbs until the controller trips. The cure is geometry: inlet and outlet on opposite sides, the outlet ducted and sealed to the radiator, and nothing outside that deflects the air back.
Exhaust, fuel, noise and access
The exhaust pipe should take the shortest route outdoors with the fewest bends, be sized so that back pressure at full load stays under the engine manufacturer’s limit, have a flexible bellows at the engine, be insulated where people can touch it, and terminate above roof level clear of windows and intakes. Every added metre and elbow raises back pressure, which costs power, raises exhaust temperature and shortens turbocharger life; the exhaust back pressure page gives the check and the exhaust system page the components.
The day tank should be the only fuel in the room, bunded so that a leak cannot run under the engine, with a fill point that does not require carrying cans past a hot exhaust. Noise is attenuated with acoustic louvres and lined ducts, and these add exactly the resistance the airflow calculation must include; a quiet room that overheats has failed. Finally, leave at least a metre around the set, headroom to lift the cylinder head, and a route to remove the radiator and alternator without demolishing a wall.
Basements and rooms within buildings
Basement generator rooms in Hyderabad apartment towers and IT buildings concentrate every difficulty: inlet air comes down a shaft, exhaust rises through the building, the radiator discharge is ducted a long way against high resistance, and fuel sits under occupied floors. Remote radiators, forced ventilation fans on a secure supply, and fire detection are usually required, and an installation planning review before civil work starts saves rebuilding the room. Where the choice exists, a ground-level room or an acoustic enclosure outdoors, described on the outdoor installation page, is simpler to make reliable.
Related photographs and videos
Common mistakes
- Sizing the wall openings by a rule of thumb per kVA; the louvre free area falls short and the set derates in summer.
- Letting the radiator discharge into the room instead of ducting it out; recirculation raises the inlet air temperature and the set trips on high coolant temperature under load.
- Adding acoustic louvres after a noise complaint without recalculating airflow; the fan cannot overcome the extra resistance.
- Running a long exhaust with several elbows to reach a convenient wall; back pressure exceeds the limit and the turbocharger fails early.
What to send us
Send a sketch or drawing of the room with dimensions, the positions of the proposed inlet, outlet and exhaust route, photographs of the room and of the outside wall where the openings will be, the generator data sheet or nameplate if a set has already been chosen, and any noise limit the site must meet. Use the generator sales enquiry and we will review the ventilation, exhaust and access before the civil work is fixed.
Questions engineers ask
How big should a generator room be?
Allow at least one metre clear on all sides, more at the radiator end for the duct and at the alternator end for withdrawal, plus space for the day tank and panel. For a 500 kVA set that typically means a room around 7 metres by 4 metres with 3.5 metres clear height, but the openings matter more than the floor area.
Can a generator room be cooled with an exhaust fan instead of ducting the radiator?
Only for small sets with low heat output. For anything above about 100 kVA the radiator fan moves far more air than a wall exhaust fan, and if its discharge is not ducted out the hot air mixes in the room and returns to the inlet. Ducting the radiator is the standard solution.
Why does the generator overheat under load but run fine on no load?
At no load the engine rejects little heat and any ventilation seems adequate. Under load the radiator must reject its full rating, and if the inlet is undersized or hot discharge air recirculates, the coolant temperature climbs over 20 to 60 minutes until the set trips. The fix is almost always in the air path, not the engine.
How high must the exhaust stack be?
The Central Pollution Control Board specifies the minimum stack height for diesel generators as a function of the set’s kVA rating, measured above the building or ground as applicable, and the state pollution control board applies it at consent stage. The pipe must also clear windows and air intakes regardless of the formula.
Generators by rating
- Under 15 kVA generators
- 15–50 kVA generators
- 50–125 kVA generators
- 125–250 kVA generators
- 250–500 kVA generators
- 500–1000 kVA generators
- 1–2 MVA generators
- 2 MVA and above generators
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.