Most homeowners change a filter when it looks gray. The filter is doing its job when it catches dust, but the same loading that makes it look dirty also makes the blower work harder to move the same air. You can measure that load yourself with a manometer or a digital differential pressure gauge. A single reading taken at the start of the heating season gives you a number to compare against next year.
What the Number Means
Pressure drop is the difference in static pressure on the two sides of the filter, usually read in inches of water column (in. w.c.). A clean filter at its rated airflow has a low drop. As dust loads the media, the drop climbs. The reading describes restriction, not filter quality, so compare it to the filter manufacturer's rated value at a stated airflow. A number you saw on a forum from a different cabinet tells you very little about your house.
Find Your Measurement Points
Many air handlers have no test ports, so you need two points: one upstream of the filter in the return, and one downstream of the filter on the blower side or in the supply plenum before the coil. Small plastic tubing from the gauge can pass through a drilled hole in the return duct, and you seal that hole with foil tape afterward. Keep the hole away from wire runs and from insulation you cannot reach. The fan must run to take a live reading, so keep hands and tools clear of the blower wheel and wiring. If you are not comfortable working inside a cabinet with power on, stop at the filter change and let a technician take the readings.
Take the Baseline
Install a new filter of the same size and rating you normally buy. Set the fan to the speed you use most in heating and let the system run for a few minutes so the reading settles. Record the reading, the filter size and MERV rating, the date, and the fan setting. Write the numbers on a card taped inside the cabinet door or in the notebook you keep with the system paperwork. A baseline is only useful if you can reproduce the conditions, so the fan setting matters as much as the number.
Reading the Trend
Check the reading again near the middle of the season and once more near the end. If the drop has risen sharply while the filter is still within its change interval, look at what else changed. Check for a closed or blocked register, a new return grille layout, a recent duct modification, or a filter with a higher rating than the system was designed to carry. A higher-MERV filter can add a meaningful amount of restriction on a system that was not sized for it, so treat a grade change as something to measure, not something to assume. A fouled evaporator coil also adds downstream restriction, which is one reason a rising trend deserves a closer look.
Where the Distributor News Fits
ACHR News reported on HVAC distributor consolidation, and its summary says that growth opportunities come with "changes to their day-to-day experiences" for contractors. Read ACHR News on HVAC distributor consolidation for the industry view. For a homeowner, the practical takeaway is narrower. Whatever shelf your filters come from, the measurement belongs to your house. A filter bought from a new source still needs its reading checked against its rated value, and the baseline you record tells you whether the swap changed anything.
When to Call a Technician
If the baseline reads high at startup, or the reading jumps between checks without a filter change, bring your numbers to the technician. Ask them to verify the fan speed and the airflow, because a restriction problem can come from several causes and a filter is only one of them. Your log gives them a starting point, which often shortens the visit. Keep the card with the system records so the next season's reading has something to be compared against.