What this page will not do
It will not tell you a clearance, it will not tell you a ventilation rate, and it will not tell you whether an enclosure is acceptable. Those figures exist, they are specific to a model, and they are in the installation manual for the unit in front of you. Every clearance field here starts at a placeholder purely so the arithmetic runs, and every one of them is wrong for your machine until you replace it.
The reason for the caution is not legal tidiness. Carbon monoxide has no colour and no smell, the early symptoms read as tiredness and a headache, and by the time anyone is worried it is often too late to act. People die in enclosures, in attached garages, near open windows, and under decks. Where a set may run and how its exhaust leaves is a licensed installer question, and it is not one a calculator is competent to answer.
Where 1.08 comes from
Sensible heat carried by an airstream is BTU/h = 1.08 × CFM × ΔT, and the constant is not magic. A cubic foot of air at ordinary conditions weighs about 0.075 pounds and holds 0.24 BTU per pound per degree F. Sixty minutes in an hour: 60 × 0.075 × 0.24 = 1.08. That is all of it.
Turn it round and the shape of the problem appears. Airflow is inversely proportional to the rise you allow: at 20 degrees of rise, 12,000 BTU/h needs 556 CFM of cooling air; at 10 degrees it needs 1,111. Halving the acceptable rise doubles the air and roughly doubles the opening area. That trade is the whole ventilation question in one line, and the table on this page runs it across a range of rises so you can see where it turns steep.
The constant is a sea-level, ordinary-temperature figure. Thin air at altitude carries less heat per cubic foot, so the same CFM removes less — another reason the manufacturer figures, which account for their own machine and its derating, outrank anything a general calculator says.
Gross area is not free area
A 24 × 24 inch louvre is 576 square inches of hole in the wall and nothing like 576 square inches of air path. Blades take some, the frame takes some, and an insect screen behind it can take half of what is left. The free area percentage is published for the specific louvre; with a fine screen it can fall below a third. If you size an opening on gross area you can end up with less than half the air you expected, which shows up as a temperature rise nobody planned for.
The same distinction runs through the attic vent net free area audit and register and grille sizing, where free area against face area is the recurring trap.
The envelope multiplier is the surprise
A 60 × 28 inch set is 11.7 square feet. Put three feet round it on every side and the envelope becomes 132 × 100 inches — 91.7 square feet, nearly eight times the footprint of the machine. Clearance dominates enclosure size the way the aisle dominates a pump house, and it is why purpose-built generator housings look absurdly large next to the set inside them.
Related
Sizing the machine itself is a different question: generator sizing covers running and surge watts, and run time and fuel covers what a long outage actually consumes. If the enclosure is a small building rather than a factory housing, the layout and heat-loss arithmetic on the pump house size calculator is the nearest companion.
Questions people ask
Can I build a shed around my generator?
That is not a question a calculator can answer, and the honest reason is that the answer depends on the machine, the fuel, the exhaust arrangement and the rules where you are. Manufacturers publish installation instructions that address enclosures directly, sometimes permitting them with specific provisions and sometimes not at all, and an installation that departs from those instructions can void the warranty and worse. The hazard that makes this different from a shed over a pump is carbon monoxide, which is why the decision belongs to the manufacturer and a licensed installer rather than to arithmetic.
What heat rejection figure should I use?
The one on the specification sheet for your set, under a heading like radiated heat to ambient or heat rejection to room. It is not the fuel input, it is not the exhaust heat, and it is not the electrical output. A rule of thumb about what fraction of fuel energy lands in the room can be out by a factor of several, because the fraction depends on the cooling arrangement and on whether the radiator discharges inside the space or through the wall. If the sheet gives kilowatts, multiply by 3,412 for BTU per hour.
Why is combustion air listed separately from cooling air?
Because it leaves through the exhaust pipe rather than through the discharge opening, so it does not carry heat out of the room the way cooling air does — but it still has to get in through the intake. Size the openings on cooling air alone and the engine and the cooling flow compete for the same hole. Both numbers come from the specification sheet. On a small set combustion air is a modest share of the total; on a large one it is not.
What face velocity should I use through the louvres?
Your own figure, informed by the louvre data rather than by a general number. Higher velocity means smaller openings and more pressure drop, and pressure drop matters because the fan has only so much of it to give. Louvre manufacturers publish pressure drop against face velocity for each product, and that curve is the real constraint. Where the airflow is driven by an engine-mounted fan rather than a separate powered fan, the allowable resistance is usually small and the openings correspondingly large.
Does a bigger enclosure need less ventilation?
No, and this is a common misreading. Volume buys time, not steady-state capacity. In the first minutes a large space heats more slowly because there is more air to warm, but once it reaches equilibrium the heat going in has to equal the heat going out, and the only thing carrying it out is airflow. A large enclosure with small openings ends at the same too-high temperature as a small one, just later. The air changes per minute line here is a sense of scale, not a sizing criterion.