ProcedureOfficial FreeMaint

Air handling unit (AHU) filter replacement

Replace the filters of an air handling unit: record the pressure drop, isolate the fan, fit new filters of the same class in the right direction, check for bypass gaps, restart, record the new pressure drop and dispose of old filters.

10 stepsHVACVersion 1Updated

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  1. Record the filter pressure drop

    With the fan running, read the differential pressure across each filter bank and note it with the final pressure drop given for the filter. This confirms the change is due and shows how fast the filters load.

    Measurement5 min

    Tools: Differential pressure gauge

  2. Stop and isolate the fan

    Stop the unit from its controls, then isolate and lock out the fan supply following your lockout / tagout procedure. Wait for the fan to stop before opening the access doors.

    Action10 min

    Tools: Lockout devices

    Risks: A fan can freewheel in the air stream; access doors can be pulled by suction.

  3. Remove the old filters

    Note the airflow direction arrow and the filter class and size, then remove the filters and bag them at once to avoid releasing the dust they hold.

    Action15 min

    Tools: Gloves, dust mask, waste bags

    Risks: Dust, mould and biological contaminants; wear gloves and a respiratory mask.

  4. Inspect frames, seals and the filter section

    Check that holding frames, clamps and gaskets are intact, and clean the filter section. Note any dirt on the coil or drain pan downstream, a sign that air bypassed the filters.

    Inspection10 min

    Tools: Vacuum cleaner, cloth, torch

  5. Fit the new filters

    Fit filters of the same class and size as specified for the unit, with the arrow in the airflow direction and pockets vertical. Never downgrade the filter class to save money or pressure drop.

    Action15 min

    Tools: Replacement filters

  6. Check for bypass gaps

    Check there is no gap between filters or around the frame and that clamps press the gaskets evenly. Air that bypasses a filter is not filtered.

    Verification5 min

  7. Close the unit and restart

    Close and latch the access doors, remove the locks following your lockout procedure and restart the unit.

    Action5 min

  8. Record the new pressure drop

    With the fan at normal speed, read and record the initial pressure drop of the clean filters, and update the filter change label on the unit.

    Measurement5 min

    Tools: Differential pressure gauge

  9. Dispose of the old filters

    Dispose of the bagged filters following your site’s waste rules. Filters from healthcare or contaminated areas may have to be handled as hazardous waste.

    Action10 min

  10. Record the replacement

    Record the date, the filter class and quantity used, and the pressure drop before and after.

    Action5 min

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Licence

Official FreeMaint template, free to use and adapt inside your FreeMaint workspace. An adaptation may be shared in the library under CC BY 4.0; its page then credits this template.

Templates are general guidance. They do not replace the manufacturer’s instructions, your site procedures or applicable regulations.

Equipment: HVAC

Industries: Facilities, Healthcare, Education, Hospitality

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