Short answer: blow in, through a filter, whenever the air around the box carries dust. One filtered intake fan holds a gasketed box near 0.1 in H2O [25 Pa] above the room, so the dust lands on media you swap instead of on your boards. Blow out when the room is clean and air-conditioned, and mount that fan high, beside the heat. Past about 100 W of heat load in a wall-mount box, run both: filtered intake low, exhaust high, with the intake moving 10-20% more air so the box stays positive. Direction sets where the dust lands and how long the fan lives; where the holes go is a separate question, answered in enclosure fan placement: low intake or high exhaust.
Direction side by side
| Filtered intake (blow in) | Exhaust only (blow out) | Push-pull (both) | |
|---|---|---|---|
| Pressure in the box | 0.05-0.1 in H2O [12-25 Pa] positive | Slightly negative | Positive when intake CFM is 10-20% higher |
| Where dust lands | On the filter media | At every seam, gland and louver | On the one filter |
| Filter to service | One, monthly in a dusty plant | None | One |
| Air through the fan | Room air at 25-30 °C | The hottest air in the box, 40-50 °C | Intake fan cool, exhaust fan hot |
| Airflow you get | About 50-60% of free-air CFM after filter and grille | About 80-90% after a louver | Near the sum of both fans |
| Best home | Dusty plants: grain, flour, sawdust, powder | Clean, cooled electrical rooms | High heat loads in a single box |
Read the table as a pressure story. With a filtered intake, every seam blows a slow stream of filtered air outward, so the only way dust enters is the media. With an exhaust fan, every seam becomes an inlet, and dust builds wherever air creeps in: around glands, at hinge gaps, inside the door channel.
Where blowing in wins
In a plant with grain, flour, sawdust or mineral dust, blow in and filter it. Size the fan at about twice the airflow your heat load needs and the box holds 0.05-0.1 in H2O above the room. Read that pressure at commissioning with a manometer at a spare 1/2 in gland port, door closed; the positive pressure check with a manometer walks through the reading and what a passing number looks like.
The second win is the jet. Right at the faces, air leaves a fan 3 to 4 times faster than it drifts toward the intake side, so an intake fan gives you an aimable stream: a 55 CFM unit on a 4 in [102 mm] hole throws about 630 ft/min [3.2 m/s] onto a drive heatsink, while the same fan as an exhaust only pulls a lazy drift across it. Aim the jet across the heat sinks rather than straight at the exhaust louver, or the top third of the box never sees cool air.
The cost is airflow and service. Clean media costs about 0.1-0.15 in H2O at rated flow, and a loaded one two to three times that, so figure the fan delivers about half its free-air CFM in service. Match the media to the dust as covered in enclosure air filter selection for dusty environments, and put the swap on the PM round: monthly in a flour mill, every six months in a clean electrical room. A loading filter is the first suspect when the fan still runs but the box gets warmer every month — the signs of a clogging filter tell you it is due.
One more quiet win: the intake fan breathes 25-30 °C room air instead of the 40-50 °C layer at the top of the box. Fan bearing life roughly halves per 10 °C, so that difference alone is worth 2 to 4 times the life on the same fan.
Where blowing out wins
In a clean, air-conditioned electrical room at 22-25 °C, blow out and skip the filter. Air drifts in through a low louver slowly and evenly, so the whole backplate gets flow instead of one corridor from fan to hole, and the fan at the top removes the hottest layer first. There is nothing to service and nothing to forget.
Mount the exhaust fan beside your biggest heat source so it sweeps the hot boundary layer off it, and send the outlet away from where people stand: 40 °C air on an operator's wrist all shift is a complaint you will hear. The reasoning on distance and height is in enclosure fan placement around hot components.
Accept two costs. The fan bathes in the hottest air in the box, and bearing life halves about every 10 °C: a fan rated 40,000 h in 25 °C air gives roughly 10,000 h at 45 °C, so buy ball bearings when the air through the fan tops 50 °C. And every gap is now a dust inlet, so keep exhaust-only for clean rooms; one dusty summer will coat the boards.
When to run push-pull
Above about 100 W in a wall-mount box, one filter hands a single fan too much pressure drop. Split the work: a filtered intake low and an exhaust high, each carrying part of the flow, each running slower and quieter than one big unit.
Keep the intake fan 10-20% larger in CFM than the exhaust and the box stays positive, so the dust story of the intake section still holds: everything stops at the one filter. Put the exhaust beside the hottest device so the flow is aimed at what needs it.
Switch both from one thermostat set about 5 °C below your allowed inside temperature — 35 °C for a 40 °C drive limit — so the filter loads only while the box is actually warm. The wiring and setpoint logic are in thermostat control for an enclosure fan, and choosing the units themselves starts with filter fan selection for panel cooling.
A worked example: 150 W in a 20 x 16 x 8 in box
Take a 20 x 16 x 8 in 304 stainless wall-mount enclosure in a flour-packaging room at 30 °C, with 19.88 x 15.88 in of panel behind the door. Inside: a 5 hp [3.7 kW] drive at 80% load passing about 3 kW at 97% efficiency, which sheds 3% of it as 90 W, plus a 35 W supply and 25 W of relays and IO — 150 W total. The drive is rated to 40 °C ambient, so your rise budget is 10 °C [18 °F].
Airflow comes from the standard sizing formula, CFM = 3.16 x watts ÷ rise in °F: 3.16 x 150 ÷ 18 = 26 CFM, or in metric 3 x 150 ÷ 10 = 45 m³/h. The full method, including what changes when room air sits close to the allowed inside temperature, is in enclosure fan airflow calculation near ambient.
Flour dust, so the direction is filtered intake low on the door and a louvered exhaust high on the side. Choose a filter fan rated 55 CFM [93 m³/h] free air: at 0.15 in H2O across clean media and grille it delivers about half, 28 CFM, which is your 26 plus a hair of margin. Commissioning tells you it worked: the manometer at a spare gland reads at least 0.05 in H2O [12 Pa] with the door shut, and the box settles near 39 °C in the 30 °C room.
What the fan does to the rating
With the door solid, our boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed. Cut a hole for a fan and the sealed rating ends at that hole: a filter-fan opening holds the assembly to roughly IP54, dust-tight and drip-proof, fine in a dry dusty plant and no match for a hose.
For washdown or outdoor rain, keep the door solid and reject the heat through a sealed route: an air-to-air heat exchanger or an enclosure air conditioner, sized with the same 150 W arithmetic. The trade-offs between the three routes are laid out in enclosure cooling: fan vs air conditioner vs heat exchanger. If your specification quotes code types, Table 110.28 in the NEC matches enclosure types to environments, and the fan opening decides which row you can claim.
The decision by environment
| Where the box lives | Fan direction | Why |
|---|---|---|
| Clean, air-conditioned room at 22-25 °C | Exhaust only, top or rear | No filter to service; slow even inflow cools the whole panel |
| Dusty plant: grain, flour, sawdust, powder | Filtered intake, low | 0.05-0.1 in H2O positive; dust stops at the media |
| Same plant, heavier heat load | Push-pull, intake 10-20% larger | Enough air at low pressure drop, box still positive |
| Corrosive coastal or chemical air | Filtered intake, 316L box | Media catches salt and mist before the boards see it |
| One dominant heat source | Intake jet aimed at it, louver above | The jet runs 3-4x the drift speed of the intake side |
| Washdown or outdoor rain | No fan: sealed box plus heat exchanger or A/C | Any fan opening ends the hose-down rating |
What to specify
Have the fan and louver cutouts made with the box rather than field-cut. Send the fan maker's cutout template with your drawing and the hole edges, gasket seat and door stiffening all come out right; the dimensions to check are in the enclosure cutouts, glands and HMI windows guide. We cut to your drawing, and the box is delivered in 10 business days, counted from your approval of the drawing.
On the order, write the size and grade: 304 indoors, and near salt or washdown overspray a 24 x 20 x 8 in 316L wall-mount box, which takes a 4 in filter fan and matching louver with panel room for the same 150 W of gear. Add the fan, the louver, the thermostat, and one set of spare filter media per fan per year in a dusty plant.
Wall-mount enclosures run from 6 x 4 x 4 in junction-box sizes to 60 x 36 x 16 in, so direction, not box availability, is your real constraint. If the enclosure itself is still open, the enclosure selector narrows it by environment and heat before you commit. Then pick direction from the table above: blow in through a filter wherever dust lives, blow out in clean rooms, and when the load outgrows one fan, do both.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 316L stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
How much positive pressure can one filtered intake fan hold?
Plan on 0.05 to 0.1 in H2O [12 to 25 Pa] with the door closed, for a gasketed wall-mount box with a small louvered exhaust and a fan sized about twice the heat-load airflow. A slack door gasket or an oversized louver pulls it toward zero, so read it with a manometer at a spare gland port during commissioning.
Does an exhaust fan really wear out faster than an intake fan?
Yes, from the air it moves. Fan bearing life roughly halves for every 10 °C rise in the air passing through it. An exhaust fan at the top of a box with 45 °C air leaving runs about 20 °C hotter than the same fan blowing cool room air in, so a 40,000 hour bearing lasts nearer 10,000 hours. Choose ball bearings above 50 °C air.
Can an enclosure keep a washdown rating with a fan in the door?
A filter-fan opening seals to about IP54, which is dust-tight and drip-proof, and a hose goes straight through it. Keep a washdown box sealed; our enclosures are rated NEMA 4X / IP66 by the manufacturer, not UL listed, with the door solid. Move the heat instead through an air-to-air heat exchanger or an enclosure air conditioner sized on the heat load.
Is push-pull worth a second fan at modest heat loads?
At loads a single fan carries comfortably, usually not: one filtered intake fan and a louver do the job, and a second fan only adds a filter and a failure point. Go to push-pull once the heat load passes what one filter-fan handles, or whenever your calculated temperature rise lands within 2 °C of a component limit, and keep the intake fan 10 to 20 percent larger in airflow than the exhaust so the box stays positive.
