TL;DR
Reduced airflow with high fan static pressure is almost always a clogged filter or a fouled coil. Replace filters first. If static pressure is still high, the cooling or heating coil needs cleaning.
What you might see
- discharge air temperature not reaching cooling or heating setpoint
- filter differential pressure above the replacement threshold
- coil entering air temperature rising from restricted airflow
- fan operating at high static pressure with low delivery velocity
Likely causes
Air filters loaded with dust beyond their rated resistance, restricting airflow through the unit
Cooling coil fouled with dust, lint, or biological growth reducing fin openings
Condensate drain pan overflow wetting the downstream media, increasing resistance
Fan wheel accumulation of dust reducing impeller blade depth and effective airflow
Required tools
- Filter differential pressure gauge
- Coil cleaner (non-foaming, pH-neutral)
- Low-pressure water spray or coil cleaning wand
- LOTO kit
- Chemical-resistant gloves and splash goggles
Safety first
- Lock out the AHU before entering the fan or coil section. Fans can freewheel from building pressure even with power off.
- Coil cleaning chemicals are corrosive irritants. Wear eye protection and gloves. Ventilate the space.
- Rooftop or elevated AHU access requires fall protection compliance.
Procedure
- 1
Read the filter differential pressure gauge. If above the replacement threshold (typically 1.0-1.5 inch WC for most bag and rigid media filters), replace all filters.[1]
- 2
After replacing filters, re-measure the fan static pressure. If still above normal for the system design, the coil is fouled.
Warning: Lock out the AHU disconnect before opening any access door to the coil or fan section. AHU fans can freewheel from system pressure even with power off. - 3
Open the coil access door and inspect the coil face with a flashlight. A visibly dark or matted coil face confirms fouling.
- 4
Clean the coil with a non-foaming, pH-neutral coil cleaner and low-pressure water spray from the leaving air side toward the entering air side. Do not exceed 200 psig water pressure.
Warning: Coil cleaning chemicals are irritants. Wear chemical-resistant gloves, splash goggles, and work in a ventilated space or wear respiratory protection. - 5
Inspect and clean the condensate drain pan. Remove debris, check the drain line for blockage, and verify the float switch operates.
- 6
If the fan wheel is dusty, clean with a brush while the fan is locked out. An unbalanced wheel from debris buildup causes vibration.
- 7
Restore power and measure supply air volume at the diffusers. Verify the flow is back to the design CFM.
Sources
Trane Trane Performance Climate Changer Air Handling Unit (AHU) general technical documentation, Trane
AHU coil maintenance, filter replacement, and coil cleaning procedures, general HVAC references (general)
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