Facilities & indoor air quality
Shops, offices and flats sharing one system each want different conditions. Air balancing shares the airflow out so every zone gets what it needs, and keeping it balanced is a job that never quite finishes.
A mixed-use building asks one ventilation system, or several sharing risers and plant, to keep very different spaces comfortable at once. Shops, offices and flats each want their own conditions, and the air has to be shared out so that every zone gets what it needs without stealing from its neighbour. Getting that share right is the job of air balancing, and keeping it right is a job that never quite finishes.
Balancing a ventilation system means regulating the flow of air so that each terminal, each branch and each zone receives the volume it was designed for. A fan produces a total quantity of air; balancing decides how that total is divided between the outlets. Without it, air follows the path of least resistance, so the rooms nearest the fan are over-supplied and the far ends of the system are starved. In a single-use building that is an inconvenience. In a mixed-use building, where a restaurant, a shop floor and a residential corridor may all draw on the same infrastructure, it is the difference between a system that works and one that generates complaints from the day it is switched on.
Where it starts
Balancing is first carried out at commissioning, when a new or modified system is set to work. In UK practice this follows established procedure, principally BSRIA's commissioning guidance for air systems, updated in 2024, working to CIBSE Commissioning Code A. The process has two halves: setting the fans to work at the right total flow, and then regulating the distribution of that flow around the system. Commissioning engineers work methodically, proportionally balancing the branches against one another before fine-tuning individual terminals, so that adjusting one outlet does not throw all the others out. It is iterative by nature, and it is recorded: measured flows for every terminal go into commissioning certificates and the building's operating manual.
Amendments to Parts F and L of the Building Regulations require newly installed ventilation to be properly commissioned, both so that occupants get adequate air and so that the system does not waste fuel and power moving it. A typical target is to bring each terminal within around ten per cent of its design flow rate. That tolerance sounds loose, but across a large mixed-use system it represents a great deal of careful adjustment at dampers, diffusers and fan settings, and it is the foundation everything else rests on.
Underlying all of this is the pressure regime. Designers set some spaces to run at slightly positive pressure and others at slightly negative, so that air moves in the intended direction: away from clean spaces and towards ones where odours, moisture or contaminants are generated. A kitchen or a changing room is held negative so its air does not drift into a lobby; a reception may be held slightly positive to keep draughts out. Balancing is what makes that pressure cascade real rather than theoretical.
Why mixed-use is harder
A mixed-use building multiplies the difficulty because the zones genuinely want different things. Retail units run long hours with heavy footfall and high heat gain; offices follow occupancy that swings through the day; residential areas need quiet, steady background ventilation. When these share supply and extract infrastructure, an adjustment that suits one can disadvantage another, and the pressure relationships between them have to be deliberately designed and then held. This is the same discipline that decides how a building sets up the air handling units at the heart of the system, because the plant and the distribution have to be commissioned as one. Zoning, damper settings and controls all have to reflect the reality that no single condition suits the whole building.
Keeping it balanced
A system balanced perfectly on handover does not stay that way. Filters load up and change the resistance the fan sees; tenants fit out their units and alter the airflow paths; walls move, dampers are nudged, and equipment ages. Each change tugs the system away from its commissioned state, and the symptoms, stuffy rooms, draughts, uneven temperatures and rising energy use, creep in slowly enough to be blamed on everything but the balance. Periodic re-checking and re-regulation, tied into the same schedule that governs how often the ventilation is inspected and cleaned, is what keeps a mixed-use building performing as designed. Balance is not a one-off; understanding how often office ventilation should be cleaned and inspected is part of protecting it.
Questions
It means regulating airflow so each terminal, branch and zone receives the volume it was designed for. A fan produces a total quantity of air, and balancing divides that total correctly between the outlets instead of letting most of it escape through the nearest ones.
First at commissioning, when a new or altered system is set to work. UK practice follows BSRIA commissioning guidance and CIBSE Commissioning Code A: fans are set to the right total flow, then the distribution is proportionally regulated branch by branch and terminal by terminal, and the measured flows are recorded.
Because retail, office and residential zones want genuinely different conditions and often share supply and extract infrastructure. An adjustment that suits one zone can disadvantage another, and the pressure relationships between them have to be designed deliberately and then held.
It is the deliberate setting of some spaces to slight positive pressure and others to slight negative, so air moves in the intended direction, away from clean spaces and towards ones generating odours or moisture. Balancing makes that pressure cascade real.
Filters load up and change the fan's working resistance, tenants alter airflow paths during fit-outs, and dampers get nudged over time. Each change pulls the system away from its commissioned state, so periodic re-checking and re-regulation is needed to keep it performing as designed.
Phoenix Duct Clean · by the numbers
Clean, clear ductwork is what lets a balanced system hold its design flows. Talk to Phoenix about keeping your ventilation infrastructure in the condition balancing depends on.