The thin fiberglass filters (usually blue in color) are referred to as "rock catchers" by some of my fellow construction friends. They don't do much to catch dust, but they'll stop anything large from damaging the blower. It's entirely possible that it was changed before you moved in and just hasn't had enough time to catch the little dust that it will catch.
If you really want to filter the air, you'll want to upgrade this to the pleated filters. Just remember that these do require changing on a more regular basis.
The other possibility is that you may have filters at your return vents doing a majority of the air filtering, and the filter at the blower is simply redundant.
You are quite correct to note that pressure drop across an element within a dynamic fluid system is related to volumetric flow…this has certain implications.
To return to your original question, “Can I sense filter replacement need based on pressure drop?” the short answer is, not really.
This is for the reason you state, that such a differential pressure value is dependent upon the volumetric flow, and if the flow is indeterminate or unknown, the value of the pressure differential does not reveal anything quantitatively as to the state of the filter element.
By way of example, a brand new filter element could indicate need of replacement if the fan speed were increased, resulting in a pressure differential increase to whatever level deemed to constitute the service limit of the filter.
The volumetric ‘nameplate’ capacity of the fan or blower purchased cannot be assumed to prevail within the target system, as the impeller speed and volumetric flow will depend upon many variables, not least those of the filters employed.
In commercial and industrial scale systems incorporating multiple elements, pressure differential measurements prevailing across individual elements may be monitored in addition to volumetric flow, which is controlled at specified process levels, as well motor energy required to sustain process levels, and these parameters and other cost factors integrated in calculations to arrive at maintenance schedules yielding least cost operation.
In a domestic DIY situation, the first and second filters could be changed alternately to determine which one, or whether both require replacement to restore desired system performance.
‘Filter capacity’ has been reached when the filter(s) in whatever combination do no longer yield the desired performance (basically CFM/min) which can be quantitatively determined.
I hope this explanation will be of assistance.
Best Answer
You don't. You just remove the fan and build a new 4 filter base. You could remove the bottom and vacuum all the filters to remove some of the dirt but but just replacing would be better. Don't count on this stopping COVID-19.