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Why External Static Pressure (ESP) Always Considers Dirty Filters

September 2, 2025

When we talk about minimum external static pressure (ESP) in HVAC/BMS design, it’s not just about ducts and fittings.

👉 It’s about real-world operating conditions.

đź’ˇ Key Insight:
ESP is always calculated with dirty filters taken into account.

 

âś… Why?
Filters are clean only on Day 1.
As they trap dust and particles, resistance increases. If the fan was sized only for clean filters, airflow would drop drastically once filters get dirty.

🔧 Practical Example
Clean filter drop: 0.3 in.w.c.
Dirty filter drop (final): 0.6 in.w.c.
Ductwork + accessories: 0.7 in.w.c.

➡️ Total ESP = 1.3 in.w.c.
The fan must overcome 1.3 in.w.c., not just 1.0. That’s how we ensure comfort and efficiency, even at end-of-life filter condition.

🌍 How BMS Helps?
Pressure sensors monitor filter health.
Alarms & trends notify facility teams before airflow suffers.
Smart control sequences adjust fan speed (VFD) to maintain airflow until filter replacement.

đź’Ž Takeaway:
Always design with dirty filters in mind — and let your BMS controls protect efficiency, comfort, and indoor air quality.

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