A generator room needs about 50 to 80 m³/h of ventilation air per kW of generator rating, roughly 30 to 47 CFM per kW. That air splits three ways: combustion air, radiator airflow, and dilution air. The room should hold its temperature rise to about 10 °C above design ambient.
Fixed at about 5–7 m³/h per kW. This air must be supplied directly to the engine and must not be ducted away.
The largest share: 30–50 m³/h per kW, moved by the set's own fan. This is the primary airflow for heat rejection.
15–25 m³/h per kW to keep the room, not just the engine, cool. Prevents heat buildup and recirculation.
First: Size the intake louvers for the total
airflow (combustion + radiator + dilution), not just combustion air.
A common failure is a room that has plenty of combustion air but recirculates
hot radiator air because the openings are too small.
Second: For standby installations, the room must meet this
demand at the generator's maximum ambient rating even on the hottest day,
because NFPA 110 requires the room to hold the temperature the manufacturer specifies.
Size louvers for the sum of combustion, radiator, and dilution air. Undersized openings lead to hot-air recirculation.
Keep room temperature rise ≤10 °C above design ambient. This ensures the generator does not derate on hot days.
Standby installations must maintain manufacturer’s specified maximum ambient temperature, as required by NFPA 110.
Adjust ventilation rates for altitude and local temperature extremes to maintain adequate airflow and cooling.
A requirements table only bites when it is translated into your room. This calculator takes your genset size, cooling type, design ambient, and altitude, and returns every airflow component plus the intake and exhaust louver free areas, in m³/h and CFM.
Enter the generator rating and site conditions. Cooling type and design margin are applied automatically.
Select cooling type, design velocity, and margin for a more precise louver sizing.
This table summarizes typical ventilation airflows for a turbocharged diesel genset at 30 °C ambient. Values are design ranges drawn from OEM guides and NFPA 110, and the exact figures for your engine come from its data sheet.
| Component | Typical flow per kW | Purpose |
|---|---|---|
| Combustion air | 5–7 m³/h per kW | Air consumed by the diesel engine |
| Cooling / radiator air | 30–50 m³/h per kW | Radiator fan airflow, the largest component |
| Room heat removal (dilution) | 15–25 m³/h per kW | Removes heat radiated by engine, alternator, exhaust |
| Total ventilation rate | 50–80 m³/h per kW | Roughly 30–47 CFM/kW, or 24–38 CFM/kVA |
| Genset size | Total ventilation airflow (typical) | Intake louver free area* |
|---|---|---|
| 50 kW | ~3,100 m³/h (~1,830 CFM) | ~1.8 ft² (0.17 m²) |
| 250 kW | ~15,600 m³/h (~9,170 CFM) | ~9.2 ft² (0.85 m²) |
| 500 kW | ~31,250 m³/h (~18,400 CFM) | ~18 ft² (1.7 m²) |
| 1,000 kW | ~62,500 m³/h (~36,800 CFM) | ~37 ft² (3.4 m²) |
Proper airflow is as important as the total volume. Follow these five steps to ensure combustion, cooling, and dilution air reach the right places – and to avoid hot‑air recirculation and premature shutdowns.
Air should enter low, at the alternator end, flow across the whole set, and leave through the radiator ducted directly outside. This layout stops hot‑air recirculation, cools the alternator and engine, and keeps clean air at the engine air filter.
Minimum room height is about 3.5 m to the highest point, and 4.0 m is preferred above 500 kW. Keep at least 1.0 m clearance around the set, 1.5 m on the radiator discharge side, and never stack equipment against the intake wall.
For a radiator‑ducted install, connect the discharge to the exhaust louver with a sheet‑metal plenum and flexible joint, and keep no obstructions within 1 m of the radiator face. For remote radiators, install mechanical fans sized at 1.15–1.2× the calculated flow.
Use the manufacturer’s published free‑area figure (typically 50–70 % of gross opening). Size the intake free area to about 1.5× the radiator face area (2.0× in hot climates) and keep design velocity near 1,000 ft/min (305 m/min).
Install a carbon monoxide alarm set at ≤50 ppm, linked to the ventilation system. In cold climates, add a motorised recirculation damper to prevent freezing between runs. Test the system on the same schedule as the generator maintenance.
Using typical total ventilation of 62.5 m³/h per kW, a 500 kW set requires ~31,250 m³/h (~18,400 CFM). At a design velocity of 1,000 ft/min, the required louver free area is ~18 ft² (1.7 m²). Always verify against the engine data sheet and apply altitude correction (+10 % per 760 m).
The required airflow and louver size depend on more than just the generator rating. These six factors often determine whether the room stays within the manufacturer’s maximum ambient rating.
Air entering low at the alternator end and exiting through a ducted radiator prevents hot air from looping back into the room. The intake and exhaust grilles must be on opposite walls, separated by at least 3 m.
Minimum height of 3.5 m (4.0 m preferred above 500 kW) and adequate working space (1.0 m around set, 1.5 m on radiator discharge side) are essential for proper airflow and maintenance access.
Ducted radiator installations require a smooth plenum and flexible joint. Remote radiator systems need mechanical supply/exhaust fans sized at 1.15–1.2× the calculated flow, with thermostatic control and back‑draft dampers.
Louver free area is the open part that actually passes air – typically 50–70 % of gross area. Size the intake to ~1.5× radiator face area (2.0× in hot climates) and keep velocity near 1,000 ft/min to minimise pressure drop.
Total pressure drop across intake and exhaust should stay ≤0.5 in. water column (125 Pa). Higher losses reduce fan performance and can derate the radiator airflow, leading to overheating.
NFPA 110 requires the room to stay at or below the manufacturer’s maximum ambient rating (7.7.1), prohibits fire dampers in ventilation openings (7.7.2.3), and demands two‑hour‑rated construction (7.2.1.1) and proper ducting (7.7.4.2).
Most room‑ventilation problems come from a few repeated oversights. Avoiding these errors helps maintain proper airflow, prevent nuisance shutdowns, and extend engine life.
Many designers size only for combustion air and forget the radiator and dilution components. The total ventilation rate (50–80 m³/h per kW) is what actually keeps the room cool – not just the engine.
Using the full frame area instead of the published free‑area figure leads to undersized openings. A louver with 50 % free area needs twice the gross size to pass the same airflow – a common cause of high pressure drop.
Long ducts, sharp bends, and undersized louvers increase system resistance. When total pressure drop exceeds ~0.5 in. WC, the engine fan and auxiliary fans cannot deliver their rated airflow, even if the louver area appears sufficient.
In freezing weather, a standby room can drop below 0 °C between runs, causing coolant and battery issues. Motorised recirculation dampers and thermostatic controls are essential to keep the room above freezing without wasting heat.
A CO alarm (set at ≤50 ppm) linked to the ventilation system is required for safety, yet often omitted. Regular testing – on the same schedule as the generator maintenance – ensures the room can supply its own air and avoids repeated shutdowns.
Final ventilation design should be checked against the engine and genset data sheets, the site’s altitude and ambient conditions, and the applicable NFPA 110 requirements. When in doubt, consult the manufacturer for the exact airflow and pressure‑drop specifications.
Use our interactive calculator to get your airflow and louver sizes in minutes, or download the complete ventilation requirements table as Excel, CSV, or PDF for your HVAC contractor.
A: A generator room needs about 50 to 80 m³/h per kW of rating, roughly 30 to 47 CFM per kW, split among combustion air, radiator airflow, and dilution air, sized for a room rise of about 10 °C. That combined figure is the generator room ventilation rate to design around.
A: Add three terms: combustion air at 5 to 7 m³/h per kW, radiator air at 30 to 50 m³/h per kW, and dilution air equal to the room heat load divided by (air density × specific heat × allowable rise). Add 10% per 760 m of altitude.
A: No. A diesel generator consumes thousands of litres of air per hour and dumps deadly carbon monoxide; running one in an unventilated room is a carbon monoxide hazard and will overheat the set. Ventilation air must come from outside.
A: Size the intake louver free area for about 1.5× the radiator face area (2.0× in hot climates) and a velocity near 1,000 ft/min, and keep the exhaust grille on the opposite wall at 1.5× the radiator face.
A: Yes. NFPA 110 requires air from the exterior, no fire dampers in the ventilation openings, a ducted radiator discharge, and a room temperature at or below the manufacturer’s maximum ambient rating (7.7.1, 7.7.2, 7.7.2.3, 7.7.4.2).
A: A common design target for an emergency generator room is about 15 air changes per hour when the set is off; basement designs often use around 6 air changes per hour, with the running requirement set by heat removal.
A: Combustion air feeds the engine and must reach it directly; cooling air moves through the radiator and carries heat away. Ducting the radiator air outside does not reduce the engine’s need for combustion air.
A: The room heats past the manufacturer’s ambient rating, the set derates or shuts down on high coolant temperature, hot air recirculates, and engine life shortens. Undersized ventilation is a leading cause of unexplained generator failures.
