Facilities & indoor air quality
There is no single date that fits every building. The right cleaning frequency for office ventilation is set by risk classification, system use and what a competent inspection measures inside the ductwork.
It is one of the most common questions a facilities manager asks about ventilation, and one of the hardest to answer with a single number. How often should the ducts serving an office be cleaned? The honest answer is that there is no fixed interval that fits every building. The right frequency is set by how the system is used, what it serves and what a competent inspection finds inside it.
Ventilation hygiene in the UK is governed by BESA's TR19 Air specification, published in April 2024, which sits alongside the older grease-focused guidance for kitchen extract. TR19 Air takes a risk-based approach: rather than prescribe a blanket cleaning date, it classifies each system, or each zone within a building, by the purpose of the space it serves, and lets that classification drive both how often the ductwork is inspected and the point at which cleaning becomes necessary. An office is not a hospital theatre, and neither is cleaned to the same trigger. That is the whole point of the standard.
How the classification works
In line with the European standard BS EN 15780, systems are placed into one of three cleanliness classes: low, medium or high. The class reflects the sensitivity of the space, not the age or size of the duct. A ventilation system serving a hospital operating theatre or a pharmaceutical laboratory is classed as high; a general office or an educational building typically falls into the medium category; and spaces such as plant rooms and storage areas are classed as low. A single building can hold several classifications at once, because different zones serve different purposes and carry different risk.
The class matters because it sets the standard the system is held to. For a high-cleanliness system, the post-clean surface must meet a measured deposit of less than 0.3 grams per square metre, assessed by a European vacuum test rather than a glance. That gives a verifiable pass or fail. A general office in the medium class is inspected and cleaned against its own threshold, and the decision to clean follows what the inspection actually measures, using a deposit thickness test on representative internal surfaces rather than a guess.
This is why a good contractor talks about inspection intervals before cleaning intervals. A common baseline is to inspect a general air system at least once every twelve months, more often where occupancy is high or the environment is dusty, and to clean only when the measured contamination breaches the threshold for that class. The system is monitored; the cleaning is triggered by evidence. That is both more defensible and, over time, more economical than cleaning to an arbitrary date whether the duct needs it or not.
What drives the interval up
Several things push an office towards more frequent inspection and cleaning: heavy occupancy, a dusty or high-footfall setting, a system that recirculates a large proportion of its air, and any zone serving a more sensitive use than open-plan desks. The internal surface area of a ventilation system is roughly equivalent to ten per cent of the floor area it serves, so there is a great deal of hidden surface for dust, debris and microbial matter to collect on, all of it out of sight. That is also why the standard leans on objective evidence: much like an air handling unit's filters, duct condition is best judged by measurement and inspection rather than by how the grilles look from the floor. If you want to understand what a neglected central plant does to the air downstream, the story of the air handling unit and why it needs cleaning is the same principle applied to the heart of the system.
Getting the record right
The practical route for any facilities manager is to commission a competent inspection, get the system classified, and let that classification and the measured findings set the schedule, with a full report and photographic evidence to prove the state of the system before and after any work. That record is not a tick-box exercise: insurers increasingly expect evidence of compliance, and it forms part of the building's wider maintenance and fire-safety story, in exactly the same way a kitchen extract system relies on a canopy cleaning fire risk assessment to set its own cleaning frequency. Clean when the evidence says clean, document it, and the question of how often stops being a guess.
Questions
No fixed interval is set in law. UK employers must supply adequate fresh air and maintain ventilation systems, and TR19 Air with BS EN 15780 sets a risk-based approach where the frequency follows the system's cleanliness class, its use and inspection findings rather than a mandated date.
A competent inspection places the system, or each zone, into a low, medium or high cleanliness class based on the sensitivity of the space it serves. General offices usually fall into the medium class; plant and storage areas are low; and highly sensitive spaces such as theatres or labs are high.
The decision follows measurement. An inspector assesses representative internal surfaces, often using a deposit thickness test, and compares the result against the threshold for the system's class. Cleaning is triggered when the measured contamination breaches that threshold, not by the calendar alone.
A common baseline is at least once every twelve months for a general air system, with more frequent inspection where occupancy is high, the environment is dusty, or the system serves a more sensitive use. Regular inspection is what makes a risk-based schedule work.
A full report with photographic evidence of the system before and after, the areas cleaned, the verification method used and a recommended interval to the next inspection. That documentation supports insurance and forms part of the building's maintenance record.
Phoenix Duct Clean · by the numbers
We inspect and classify your ductwork, then set a cleaning schedule based on evidence, not guesswork. Talk to Phoenix about a ventilation hygiene assessment.