AHU filter pressure drop tells you when to change filters; replacing on a gauge reading beats replacing on a fixed calendar.
Reading the Gauge
A differential-pressure gauge across the filter measures the resistance the air meets. A clean filter sits at its initial resistance; as dust loads, the reading climbs until it reaches the change setpoint, at which point airflow would otherwise start to fall.
An AHU filter is specified not just by what it catches but by how much resistance it adds, because that resistance is paid for every hour by the fan. Reading the filter schedule means reading both the efficiency class and the pressure drop at which it is considered spent.
Filters are graded by the fraction of test dust they hold, expressed in ISO ePM or legacy MERV terms, and a finer grade protects the coil and the room better but starts with higher resistance. The grade is chosen against the air quality the space needs and the coil the filter must protect from loading and fouling.
The initial resistance is the clean-filter value the fan sees on day one, and the final resistance is the alarm point at which the filter is replaced. The fan must meet design airflow at the final, higher resistance, so the selection point is taken dirty, not clean, and the motor carries that margin through the service interval.
Monitoring is the discipline that turns the numbers into action. A differential-pressure switch or gauge across the filter gives a live reading, and a log of the rise over time predicts the next change so the coil never runs starved of air. Letting the filter run past final resistance silently loses airflow and loads the fan.
The company's 80,000 m² production site builds air handling units under ISO 9001:2015 with CCC certification and supplies more than 50 countries, so filter grade, initial and final resistance, and a monitoring recommendation are documented per AHU and its engineers respond to selection enquiries within 24 hours.
The filter is only as good as its seal and its housing: a bag that bypasses through a poorly seated frame or a dented cell passes unfiltered air around the media, so the housing is specified for a positive seal and the install is checked at the frame. A panel or bag type is chosen with the housing in mind, and a small pressure port on the box makes the monitoring reading trustworthy rather than an estimate across a leaky case.
Filter classes set expectations: a coarse panel filter may start near 30-50 Pa, a bag filter near 100-150 Pa, and a final HEPA can begin around 100-200 Pa and be allowed to rise toward 250-400 Pa before change, reflecting their different media area.
Running past the setpoint wastes fan energy, because every extra pascal is paid for in electricity, and risks forcing dust through or collapsing the media under the higher load, which then contaminates the downstream coil and the served space.
A magnehelic or electronic transmitter gives a live value; pairing it with the building system turns the reading into an alarm and a work order rather than a manual inspection round that is easy to skip when the team is busy.
Setting the Change Point
A common rule replaces the filter at about twice its initial resistance, or at the manufacturer's final resistance, whichever the schedule specifies, so capacity is not lost mid-interval and the fan is not driven to an inefficient operating point.
Dusty sites reach the setpoint faster, so the interval should be based on the measured rate of rise, not a generic quarterly assumption that may be far too long or short for the actual environment the unit serves.
Staged filtration protects the expensive final stage; a pre-filter that is cheap to change keeps the fine filter clean longer and lowers lifetime cost, and it also protects the coil from fouling that would otherwise erode heat-transfer performance.
Records of each change, with the pressure at change, build a real duty profile that lets the next interval be set from data rather than guesswork, and they support energy audits that attribute fan power to filter condition over time.
| Item | Specification | Note |
|---|---|---|
| Coarse filter | 30-50 Pa | Initial |
| Bag filter | 100-150 Pa | Initial |
| HEPA | 100-200 Pa | To 250-400 final |
| Change rule | ~2x initial | Or maker final |
| Monitoring | DP gauge | Live reading |
| Staging | pre-filter | Protects final |
Selection Checklist
- Install a differential-pressure gauge across each filter stage.
- Replace at roughly twice initial resistance or the maker's final value.
- Use the measured rate of rise to set the interval, not a fixed date.
- Stage with a pre-filter to extend the fine filter life.
- Log pressure at each change to build a duty profile.
- Alarm on high ΔP so energy and capacity are not lost.
About the Manufacturer
Keruite Air Conditioning Group, headquartered in Dezhou, Shandong, has more than 30 years of HVAC manufacturing experience and operates an 80,000 m² production site with its own performance testing laboratory. Products are built under ISO 9001:2015 quality management and carry CCC certification, and are supplied to customers in over 50 countries.
Our engineers respond to selection enquiries within 24 hours and can provide project-specific sizing, performance curves, and documentation to support specification and procurement.