A CR box is simple, so when something is off it is usually one of a handful of causes. Work through these in order.
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- No shroud. Without a cardboard shroud over the fan corners, air recirculates and output drops. Add one — it's the biggest single fix.
- Filter arrows wrong. Arrows must point inward. A reversed filter chokes flow.
- Loaded filters. If it started strong and faded, the filters are full — see when to replace filters.
- Blocked intake. Give the filter faces clear space; don't push the box against a wall.
Noise and rattles
- Tape any loose cardboard that's vibrating against the fan.
- Stand the box on a rug or mat to damp low-frequency hum.
- Run the fan a speed lower — often barely reduces clean air but noticeably cuts noise. For more, see noise-reduction tips.
Air leaks
With the fan running, run your hand around every seam. Any draught you feel at a joint is unfiltered air bypassing the filters — tape it. A leaky box can lose a large share of its effectiveness.
Filters loading too fast
Very dusty homes, pets or heavy smoke will load filters quickly. Vacuum the intake surfaces gently every few weeks to extend life, and consider a five-filter build for more area.
Still not right?
If a build simply won't run quietly or effectively, a ready-made engineered kit removes the variables — sealed enclosure, tuned fans, under 35 dB.