Short answer: blow in. Mount your fan on a washable filter, low on the coolest wall, aimed across the hottest board, and leave the exhaust as a plain opening high on the opposite side. Filtered intake holds the box slightly above room pressure, so dust and mist can only leave. Direction itself barely moves the average temperature — the same delivered airflow gives the same rise either way — so air quality and layout decide it for you. The number that settles the job is airflow: a typical 80 mm fan quotes 25-30 CFM [42-51 m³/h] free air on its datasheet, delivers about 12-15 CFM [20-26 m³/h] through its filter and grille, and holds a 10 °C rise with roughly 70-85 W inside.
How much air the heat needs
Air leaves the enclosure warmer than it went in, and every cubic metre of it carries about 1.2 kJ per kelvin — that is the entire cooling mechanism. Turn it into a planning line: CFM = 1.76 × watts ÷ allowed rise in °C, or in metric m³/h ≈ 3 × watts ÷ °C. Sixty watts with a 10 °C budget needs 1.76 × 60 ÷ 10 ≈ 11 CFM [19 m³/h] of delivered air. The budget comes from what is inside: drives and most electronics carry a 40 °C ambient rating on their datasheets unless a number there says higher, so a 30 °C shop floor leaves you 10 °C to spend.
For scale, seal that same 60 W inside a 12 × 10 × 6 in [305 × 254 × 152 mm] stainless box with no fan. The six faces give about 0.33 m² of surface, and stainless walls pass about 3.7 W/m²K — the planning figure this site uses — so the box sheds 1.2 W per kelvin. The air climbs 60 ÷ 1.2 ≈ 50 °C above the room: 80 °C inside on a 30 °C day. That is why the fan exists, and why internal fans alone change so little, because the heat still has to cross a wall.
Why filtered intake wins in dirty air
Make the fan the only way in, and push through it. The box then sits slightly above room pressure the whole time it runs, and every other gap — exhaust holes, glands, the door seam — leaks outward. The dust and coolant mist in the room ride the outgoing stream instead of settling on your drive. Reverse the fan and every one of those gaps becomes an intake for whatever is floating around. Filter-fan makers sell intake kits on exactly this mechanism: filtered air in, a slight positive pressure, and only filtered air entering the enclosure.
Aim the jet at the hottest board while you are at it. Intake air arrives at room temperature, so the component working hardest meets the coldest air in the system — a small edge no exhaust layout gives you, since its stream has already crossed everything else. In coolant mist, keep the filter in place and mount the fan where spray cannot land on it; for what dies first when a fan runs in heavy mist, the mist failure checklist covers it.
The filter charges rent. Plan delivered flow at 50-60 % of the free-air rating with a clean washable element, and less as it loads — that is the planning figure used in the worked example below, so swap in your fan's curve against the filter's pressure drop when you are near the limit.
When an exhaust fan is the right call
In a clean, dry electrical room the two directions tie on physics: 11 CFM in or 11 CFM out, the same 10 °C rise. Exhaust then earns its keep two ways. A large louvered inlet low on the door adds almost no resistance, so the fan runs near its free-air rating instead of surrendering half of it to a filter. Filter-fan installation guides suggest at least twice the fan's open area for that inlet; the grille pressure-drop article has the numbers. The exhaust side is also where a duct attaches when hot air has to go somewhere, out of a machine shroud or up into a plenum.
Two costs ride along. The box runs slightly negative, so every gap inhales unfiltered room air, and the fan's bearings sit in the hottest air leaving the box. Bearing life roughly halves for each 10 °C hotter, the same rule of thumb as electrolytic capacitors. For the fan-side arguments in full, the enclosure fan direction article collects them. Choose exhaust for clean rooms and ducted hot air; choose intake anywhere dust, powder or mist is in the air.
Where the intake and the exhaust go
Air should enter low, sweep the boards, and leave high on the opposite wall. Cool air pools at the bottom, hot air stratifies at the top, and a diagonal path makes the stream cross the hot parts instead of the empty corners. Two openings on the same wall short-circuit: air crosses at that height and the far end stagnates. Keep the intake where the room air is coolest, which usually means away from the supply transformer and the machine next door.
An enclosure that already has vent slots on the bottom and the back fits the pattern with no new holes. Mount the intake fan over the bottom slots and leave the back slots as the exhaust. The fan placement article works through spacing, clearances and the awkward cases where a wall is already full of components.
What a second fan actually buys you
A fan pushing at the inlet and a fan pulling at the outlet sit in series on one path, and fans in series move the same air — about single-fan flow, nowhere near double. What the pair adds is pressure. When the filter loads and resistance climbs, the two-fan curve still has head to spare, so your delivered CFM holds up longer than a lone fan's would. That is the honest case for push-pull: pressure margin against a dirty filter, plus limp-home airflow if one fan stops.
To genuinely double the airflow, give the second fan its own path — its own filtered inlet and its own exhaust, on a separate diagonal. Two fans sharing one hole fight over the same openings and land well short of twice the air. Sizing each path to the heat it sweeps beats one big stream that skips half the box.
What the fan does to the box's rating
Cut a fan into a wall and the assembly carries the fan kit's rating, not the door gasket's. A typical 148 × 148 mm [5.8 × 5.8 in] filter fan is rated 65-85 m³/h [38-50 CFM] at IP54 on its datasheet. Under IEC 60529, the first digit 5 means dust-protected: fine dust can enter, though not enough to disturb operation. The second digit 4 means splashing water from any direction; hose jets sit well outside its scope. Your vented enclosure is therefore a dust-protected, splash-proof assembly — right for a clean plant floor, wrong for a washdown hose.
The weakest part sets the number: a Type 4X / IP66 door with an IP54 fan kit in the wall is an IP54 box. In hose-down areas keep the walls solid and move the heat closed-loop — the cooling method comparison says where each option wins — and the vented enclosure article shows what each kind of opening does to a Type 4 rating. All ratings on this site are manufacturer ratings: our stock boxes are rated NEMA 4X / IP66 by the manufacturer and not UL listed.
Worked example: 60 W in a small drive box
Take a compact CNC control box — a four-axis stepper drive, a 48 V supply and two relays — in a 12 × 10 × 6 in stainless enclosure on a 30 °C shop floor. The dissipation figures are planning assumptions from typical mid-size parts; swap in the numbers from your datasheets before you order the fan.
| Item | Heat |
|---|---|
| Four-axis stepper drive | 30 W |
| 48 V power supply | 20 W |
| Relays and wiring losses | 8 W |
| 12 V fan, its own motor heat | 2 W |
| Total | 60 W |
- Allowed rise: 40 °C rating − 30 °C ambient = 10 °C.
- Airflow needed: 1.76 × 60 ÷ 10 ≈ 11 CFM [19 m³/h].
- Fan picked: an 80 × 80 × 25 mm axial quoting 27 CFM free air. Through filter and grille at 50 %, it delivers about 13 CFM [22 m³/h].
- Steady rise: 1.76 × 60 ÷ 13 ≈ 8 °C, so the enclosure air sits near 38 °C — 2 °C under the drive's limit.
Now the failure case. Let the filter load until delivery falls to 30 % of free air, about 8 CFM, and the rise becomes 1.76 × 60 ÷ 8 ≈ 13 °C, putting 43 °C air on a 40 °C-rated drive. That is your maintenance signal: wash the element on a schedule, or spend the second fan as push-pull pressure margin exactly here.
Proof takes one thermometer. After a day of running, exhaust temperature minus room temperature should read about 8 °C; 13 °C at the exhaust says the filter is eating your flow.
What to put on the order
Order the openings with the box and you mount the fan and filter onto finished cutouts instead of drilling stainless in the field. Send the fan's datasheet cutout and the filter outline, mark the intake low on the cool wall and the exhaust high and opposite, and custom enclosures come from one piece, delivered in 10 business days counted from your approval of the drawing. On the drawing, list:
- the fan model with its free-air CFM and its cutout template;
- the filter part and media, washable, sized to the fan;
- the exhaust vent, rated IP54 like the intake, with open area at least the fan's;
- a stall signal — a tach wire on a DC fan or a run LED on an AC kit — so a stopped fan announces itself before the drive does; the two-wire vs four-wire wiring article covers the hook-up;
- the assembly rating you expect, IP54 with a filter fan, written next to the cutouts so nobody treats the box as sealed.
If the enclosure size is still open, the enclosure selector checks panel fit and DIN module count before you fix the fan position.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 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
Does an intake fan cool better than an exhaust fan?
The box average sits within about a degree at the same delivered airflow — air entering at 30 °C leaves 10 °C hotter from either side. Direction matters for what the air picks up: filtered intake holds the box slightly positive so dust and mist can only exit, and the intake jet meets the hottest board at room temperature. In a clean electrical room, either direction works and exhaust suits ducting.
Will one fan in and one fan out double my airflow?
About the same, and that surprises people: the two sit in series on one path, so they move single-fan flow — your 13 CFM stays about 13 CFM — with extra pressure to shove it through a loaded filter. That pressure margin, plus airflow when one fan dies, is what the pair really buys. Twice the air needs a second inlet and exhaust on a separate path across the box.
How much airflow does the filter cost?
Plan on 50-60 % of the fan's free-air rating with a clean washable filter and grille in place — about 12-15 CFM from an 80 mm fan quoting 25-30 CFM. A loaded element can pull delivery down near 30 % of free air, which is when the internal temperature climbs past the limit. Wash the media on a schedule and prove the flow with a thermometer: an 8 °C rise at the exhaust says clean, 13 °C says wash it.
Can I fit a filter fan to a NEMA 4X washdown enclosure?
Keep the box sealed for washdown. A fan opening sets the assembly to the fan kit's rating, typically IP54 under IEC 60529 — dust-protected and splashing water only, with hose jets outside its scope. Move the heat with a closed-loop heat exchanger or an air conditioner instead, and save the filter fan for clean indoor air where dust is the enemy rather than the hose.
How hot can the room be before a fan stops being enough?
Work back from the 40 °C ambient rating most drives and supplies carry. At a 30 °C room you have 10 °C to spend and 60 W needs about 11 CFM delivered. At a 35 °C room the same load gets a 5 °C budget, so the number climbs to 1.76 × 60 ÷ 5 ≈ 21 CFM delivered — a 120 mm fan quoting 60-80 CFM free air covers it through a filter. Past that, closed-loop cooling takes over.
