Duct cleaning & ventilation hygiene
Two words get used interchangeably, and they should not be. One proves a system works as designed at a single moment; the other keeps it working as the years pass. Both are necessary.
Two words get used almost interchangeably around ventilation systems, and they should not be. Commissioning and maintaining are different jobs, done at different times, answering different questions. One proves a system works as designed at a single moment; the other keeps it working as the years pass. Confuse them, or do one and skip the other, and a system that looked perfect on handover slowly stops performing without anyone noticing.
Commissioning asks: does this system, as built, actually deliver what it was designed to deliver? It is a structured, one-off verification carried out when a system is new or has been significantly altered. Maintenance asks a different question, and asks it again and again: is this system still delivering what it should? It is recurring work that preserves performance over the operating life of the system. The first is a gate you pass through once; the second is a discipline you sustain. Both are necessary, and neither substitutes for the other.
What commissioning is
Commissioning is the process of testing and verifying that a ventilation system works as designed before it is handed over and relied upon. In UK practice it follows established procedure, principally the BSRIA commissioning guidance for air systems, updated in 2024, working to CIBSE Commissioning Code A. The work involves setting the fans to their design duty, regulating and balancing the flows to every terminal, verifying controls and interlocks, and documenting measured results against the design figures. It is thorough precisely because it happens once: this is the moment the paper design is proven, or disproven, against physical reality, and the commissioning record becomes the baseline every future check is measured against.
That baseline matters more than it sounds, because a surprising number of systems never actually meet it. Field studies have repeatedly found that a large proportion of ventilation systems, by some assessments half or more, do not perform as designed, often because commissioning was rushed, skipped or never properly documented. A system that was never correctly commissioned starts its life already adrift from its design intent, and no amount of later maintenance can restore performance the system never had. Commissioning is where performance is established; if it is skipped, there is nothing solid to maintain.
Maintenance, by contrast, is the ongoing management of the system's condition once it is in service. It comes in two broad forms: planned maintenance, carried out on a calendar or running-hours interval regardless of measured condition, and condition-based maintenance, triggered when inspection or monitoring shows performance drifting. Cleaning ductwork, changing filters, checking fans and dampers, re-checking flows: these preserve the performance that commissioning established, and they are what stop the slow slide from a system that works to one that merely runs.
Why performance slips
Even a perfectly commissioned system does not stay commissioned. Filters load up and shift the resistance the fan works against; ductwork accumulates dust and grease that narrow it and change its flows; dampers drift, belts wear, and building alterations reroute the air. Every one of these nudges the system away from the baseline it was signed off at, and because the changes are gradual and hidden, the decline is easy to miss until comfort complaints or energy bills force attention. This is why performance is understood to degrade over time without active maintenance and periodic re-checking, and why the sensible response is a planned regime rather than waiting for failure, the same thinking that explains how poor ventilation drives up cleaning frequency.
Two halves of one job
The cleanest way to think about it is that commissioning and maintenance are two halves of caring for the same asset across its life. Commissioning gets the system to its design performance and proves it; maintenance holds it there. Skip the first and there is no reliable baseline; skip the second and the baseline is slowly lost. For most buildings the practical implication is straightforward: insist on proper commissioning and documentation when a system is installed or altered, then fold the ongoing checks, cleaning and re-balancing into a maintenance schedule that keeps it honest, exactly the discipline behind deciding how often office ventilation should be cleaned. Do both, and the system keeps the promise its design made.
Questions
Commissioning is a one-off, structured verification that a system works as designed, done when it is new or significantly altered. Maintenance is recurring work that preserves performance over the system's life. One is a gate passed once; the other a discipline sustained. Neither substitutes for the other.
Setting fans to their design duty, regulating and balancing flows to every terminal, verifying controls and interlocks, and documenting measured results against design figures. In the UK it follows BSRIA commissioning guidance and CIBSE Commissioning Code A. The record becomes the baseline for every future check.
Field studies have repeatedly found that a large proportion of ventilation systems, by some assessments half or more, do not perform as designed, often because commissioning was rushed, skipped or poorly documented. A system never correctly commissioned starts adrift from its design intent.
Because performance degrades over time. Filters load up, ductwork accumulates dust and grease, dampers drift and building alterations reroute air. Each change nudges the system away from its commissioned baseline, and because the decline is gradual and hidden, it is easy to miss until complaints or energy costs force attention.
Planned maintenance is carried out on a calendar or running-hours interval regardless of measured condition. Condition-based maintenance is triggered when inspection or monitoring shows performance drifting. Cleaning ductwork, changing filters and re-checking flows preserve the performance commissioning established.
Phoenix Duct Clean · by the numbers
Maintenance is what holds a system to the baseline commissioning set. Talk to Phoenix about the cleaning and inspection that preserve ventilation performance over time.