A single-board computer drawing about 20 W can live in a fully sealed box if you conduct the heat into the enclosure wall. Conduction to a metal body keeps the seal intact, needs no service parts, and holds a predictable temperature rise: roughly 15 to 25 C inside for a 20 W load in a small stainless or aluminum enclosure in still air. A fan with a vent cools better per dollar of hardware, but it opens the box to water, dust and condensation, which is exactly what you sealed it against. For a board that must run unattended in a wet or dirty spot, conduction wins; forced air is the fallback only when the wall alone cannot carry the load.
If you are new to sealed-box thermals, start with the general method in removing heat from a small sealed IP66 enclosure; this article compares the two specific paths for one hot board.
Conduction versus fan and vent, side by side
| Conduction to enclosure wall | Internal fan + breather vents | |
|---|---|---|
| Seal integrity | Unbroken; rating unchanged | Openings must each be IP-rated |
| Heat capacity, still air | 10–30 W practical in a small box | 30–100 W in the same box |
| Moving parts | None | Fan bearings and filter clogging |
| Water and dust ingress risk | None beyond the door gasket | Vents can blind, splash or wick |
| Condensation behavior | Breather optional, drain optional | Air exchange brings moist air in |
| Noise and vibration | None | Present, matters on vehicles |
| Wall material required | Metal (stainless, aluminum) | Any, including polycarbonate |
| Cost of failure | Throttling only | Water ingress plus throttling |
The honest summary: a fan moves more heat than a passive wall, always. The question is whether you need it, and what the openings cost you. The trade-offs of active cooling paths are laid out in enclosure cooling options compared.
Where conduction to the wall wins
Conduction is the right call when the box must stay sealed and the load is moderate, which describes most single-board installs.
The seal never breaks. Heat leaves through the metal itself. There are no louvers, no membranes to blind with road grime, no fan to fail. The door gasket is the only sealing element, and it is the one you can inspect.
The wall is a big heat sink. A 16 x 12 x 8 in stainless enclosure has roughly 5 sq ft (0.5 m²) of surface. A bare steel surface in still air sheds on the order of 1 W per square foot per degree C of rise, so 20 W into that wall produces a wall rise near 4 C. The inside air runs hotter than the wall, typically 10 to 20 C above ambient for a 20 W load once you account for the board-to-wall path, but the physics is comfortably on your side. For the theory of how a sealed box reaches equilibrium, see PCB heat in a sealed enclosure.
Nothing sucks in moist air. A vented box breathes: warm humid air in, condensation at night. A conduction-cooled box can stay closed, optionally fitted with a breather and drain as described in breather vent vs drain for a sealed enclosure. On a vehicle or outdoors, that matters as much as the temperature.
Vibration and altitude do not touch it. No bearings, no fan blade resonance, no derating in thin air. It also works at any mounting angle.
The weak point of conduction is the board-to-wall interface. Heat must cross a thermal pad or gap filler into the wall. A soft pad, correctly compressed, adds only a few degrees at 10 to 20 W over a 40 x 40 mm footprint. A hard, dry or under-compressed pad adds tens of degrees and quietly ruins the design.
Where the fan and vent path wins
High power density. If the box is small and the load is 40 W or more, the wall alone may not hold the rise you need. Forced air through filtered vents multiplies the heat you can dump from the same surface.
One component far hotter than the rest. A fan stirs the air and evens out hot spots, though note that an internal fan alone, with no vent, does not remove heat from the box at all. It only spreads heat to the walls, where conduction still has to carry it out.
Big, cheap boxes. In a large indoor panel, filtered fans are the economical answer. For the sizing logic, read enclosure cooling: fan vs air conditioner vs heat exchanger.
Against that, every vent is a maintenance item. Filters clog; a blinded filter turns cooling into insulation, a failure mode covered in monitoring enclosure air filters for clogging. On a vehicle, spray, grit and vibration make vents the least reliable part of the system. For a deeper look at whether any opening is acceptable outdoors, see enclosure vent or sealed box: which stays dry outdoors.
The numbers for a 20 W board
Rough planning values, still air, no sun, 35 C ambient, assuming the board's heat reaches the wall through pads covering its hottest chips:
| Enclosure (H x W x D, in) | Surface (sq ft) | Wall rise at 20 W | Realistic inside-air rise |
|---|---|---|---|
| 10 x 8 x 6 | 2.8 | ~7 C | 20–30 C |
| 14 x 12 x 8 | 4.4 | ~5 C | 15–25 C |
| 16 x 12 x 8 | 5.0 | ~4 C | 15–20 C |
| 20 x 16 x 10 | 8.0 | ~3 C | 10–15 C |
These are estimates for planning, not guarantees. The inside-air rise depends on how well the chips are coupled to the wall and on internal layout; component layout inside a sealed enclosure covers that half of the design. The takeaway is that going one size up buys more cooling than any fan-less trick in a small box.
If the box sits in the sun, add the solar load before judging conduction; solar heat load on an outdoor enclosure gives those numbers, and a shield helps more than a bigger box.
How to couple the board to the wall
The interface is the whole design. Do it in this order:
- Identify the hot chips: the processor package and, on a board with NVMe storage, the drive controller. Get their maximum case temperatures from the board documentation.
- Choose a metal enclosure and mount the board within a short reach of a wall or the back panel, not floating mid-box on standoffs.
- Fit soft thermal gap pads, 2 to 3 mm thick, between each hot chip and the wall or a bracket tied to the wall. Pads tolerate tolerance stack-up; grease does not.
- Hold the pads compressed. Use the board's own mounting holes plus a stiff backplate so the pads stay squeezed at their rated compression.
- Keep the pad area generous: bigger footprint means lower drop. A 40 x 40 mm pad at 10 W with a 3 W/m-k pad drops only about 4 C across the pad.
- Verify with a thermocouple or the board's own sensors at worst-case load and worst-case ambient before you trust the design.
If one chip still runs hot after all that, add an internal fan to stir air toward the walls. That is the one fan that does not break the seal, and it is usually enough.
What to specify
A conduction-cooled build is a natural custom enclosure job: you set the material, the size and the cutouts, and the board mounts, pad lands and gland positions go on the drawing.
- Material: 304 stainless for general outdoor and washdown use; 316L near salt or brine; aluminum where weight matters, as on a vehicle. All conduct well enough for this duty once heat is spread.
- Size: pick from the numbers above; one size larger than you need is cheap insurance.
- Rating: rated NEMA 4X / IP66 by the manufacturer; not UL listed. That covers rain and hose-directed water. IP65 alone is a lower bar and fine only for splash, not washdown.
- Cutouts: glands on the bottom face where spray cannot run in; sealed connectors for anything through the wall.
- Thermal hardware: gap pads sized to the hot chips, a compression scheme, and an internal temperature sensor so throttling never surprises you.
- Delivery: made to order and delivered in 10 business days, counted from your approval of the drawing.
If you are still sizing the box itself, the enclosure selector filters by environment and usable inside dimensions; a dimensional match does not establish thermal suitability, so run the heat numbers separately.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 10 × 8 × 6 in | 304 stainless | 10 x 8 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 30 × 24 × 10 in | 304 stainless | 30 x 24 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
How many watts can a sealed metal enclosure dissipate?
It depends on wall area and allowable temperature rise, not on the electronics inside. As a planning number, a bare steel or stainless box sheds roughly 1 W per square foot of surface per degree C of rise above ambient in still air. A 16 x 12 x 8 in box has about 5 sq ft of surface, so 20 W raises the wall about 4 C and the inside air somewhat more. Verify against your board's limits before committing.
Do thermal pads really work through a stainless wall?
Yes, if the pad is compressed correctly and the heat path is short. A 3 mm soft pad has a thermal conductivity of roughly 2 to 6 W/m-K, and a 40 x 40 mm footprint carries 10 W with only a few degrees of drop across it. The wall then spreads the heat. The weak links are air gaps and mounting hardware that does not hold the pad compressed, not the stainless itself.
Won't the enclosure wall get too hot to touch?
It gets warm, and that is the design working. Twenty watts spread over a few square feet of steel is a mild rise above ambient. If the wall is too hot to hold a hand on, the heat is not spreading: check that the pad is compressed, the board is mounted near the wall, and the box is not sitting in direct sun.
Can I add a fan inside a sealed enclosure to help?
An internal fan does not remove heat from the box; it only stirs the air. It helps when one component is much hotter than the rest, by spreading heat to the walls so conduction can do its job. It does nothing for the total temperature rise, and it adds its own watts. Size the wall and pads first, then add the fan only if one chip still overshoots.
Is an IP65 rating enough for rain and washdown spray?
IP65 means the box resists dust and low-pressure water jets from any direction, per IEC 60529. It suits rain and splashing but not high-pressure hose-down, where IP66 or better is the safer call. Our enclosures are rated NEMA 4X / IP66 by the manufacturer and are not UL listed, which covers both cases.
