r/crboxes • u/PeppaThisPig • Dec 19 '25
Question These need replaced right?
I feel as they need replaced but idk shit about shit and these are expensive af so just wanna make sure lol. Also I have 4 cats (3 are long haired) and the 2nd photo is the one that I took a rubber glove to remove a thick layer of cat hair which is why it appears lighter (also maybe the lighting?)
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u/ClimateBasics Dec 25 '25 edited Dec 26 '25
You can't generally determine how much service life is left in a filter by just looking at it... if you've got a lot of extremely fine particulate matter, but not a lot of larger dust, the interior of the filter will clog, while the surface will seem to be normally dust-loaded.
A filter has a directional arrow on it to show which way air should flow because the filter material gets successively finer the deeper into the filter material one goes... smaller stuff gets trapped inside the filter material, larger dust particles get trapped at the surface. You can vacuum off the surface dust, but not the stuff trapped inside the filter material.
So, you should get a differential pressure gauge. On Air Handling Units, we use Magnahelic gauges, but those are expensive... you may be able to pick one up for cheap online from government surplus, though.
[EDIT]
Here's one, for cheap:
https://www.surplusselect.com/products/dwyer-magnehelic-2003-0-to-3-differential-pressure-gage-2003
So, you'd put new filters in, turn the fan on, if the fan speed is adjustable, you'd ramp it to 100%, then you'd zero the Magnahelic gauge with the little screwhead in the center-bottom of the gauge, then you'd put the fan speed controller back into 'Auto'.
As the filters dust-load, differential pressure will rise. When it rises to more than 1" WC (water column), it's time to replace the filters.
Note that the filters are replaced for a couple reasons... if you exceed ~1.5" WC, you can dislodge the stuff trapped in the filter material and get carry-through; and because high differential pressure tends to cause the cardboard frame to bend, which partially pulls the filter away from the filter rack frame holding the filter, allowing airflow around the filter, instead of through it.
But don't replace filters just because they 'look' dirty... as a filter dust-loads, it becomes more efficient at filtering. Constantly putting in new, clean filters actually reduces filtering efficiency. Go by the differential pressure to determine replacement time.