Why “every X years” is the wrong question
Two identical office floors can foul at completely different rates — one above a construction site with doors to a lobby, one sealed and pressurized. A fixed calendar either cleans too often (wasted budget) or too late (an audit finding). NADCA’s ACR standard therefore ties cleaning to inspection results: inspect on a schedule, clean on evidence.
Working baselines by facility type
Use these as starting intervals for the inspection, and let findings set the cleaning:
| Facility type | Inspection interval | Notes |
|---|---|---|
| Corporate offices | 12–24 months | Shorter after fit-outs or if filtration is poor |
| Hotels | 12 months | Plus quarterly kitchen exhaust and 6–12 month laundry duct cycles |
| Hospitals & healthcare | 6–12 months | Driven by infection-control requirements |
| Industrial / process | 6–12 months | Depends heavily on the process contaminants |
| Post-construction or fit-out | Immediately | Construction dust loads systems faster than years of normal use |
Exhaust systems are the exception to inspection-driven scheduling: kitchen grease ducts and laundry lint ducts accumulate combustible material on a predictable curve, so they run on fixed cycles (typically quarterly and 6–12 monthly respectively) regardless of appearance.
The triggers that override any schedule
Inspect promptly, whatever the calendar says, if you see:
- Visible dust discharge from supply diffusers
- Mold, or a musty smell that returns after cleaning
- An audit or EHS finding naming duct hygiene
- Water ingress into any part of the air path
- Unexplained rise in fan energy consumption — fouling makes fans work harder, which is why cleaning fouled systems cut fan energy 41–60% in NADCA’s energy research project
The practical answer for audit-answerable facilities
Put inspection on contract, not on memory. A duct-hygiene maintenance program fixes the inspection calendar, cleans only where findings justify it, and keeps every report in one standing audit file — so the answer to “when were these ducts last assessed?” is always a dated document, never a guess.