Diagnostics & problems
Tapping a new hood into an existing extraction system feels like a small job. It is not. A fan has a fixed capacity, and every new branch you hang off it takes airflow away from every hood that was already there.
The short answer
Adding a workstation and tapping its hood into the existing ductwork looks like a minor extension. In airflow terms it is a change to the whole system. A fan has a fixed capacity: it was sized to overcome the resistance of the original set of hoods and ducts while holding the right capture velocity at each one. Hang another branch off it and that fixed capacity now has to serve more openings, so the airflow and capture at every existing hood drop, not just at the new one.
The detail
The physics is unforgiving. The fan has to overcome the total resistance of everything connected to it, the sum of losses through every hood, duct run, bend and filter. Splice in a new branch and you both add resistance and open another path for the fan's limited airflow to divide between. The result is that the new hood often fails to capture properly and it robs the others while doing so. Because the loss is spread across every hood at once, nothing looks obviously broken: there is no single dead hood to point at, just a system that is quietly down everywhere.
There is a knock-on effect in the shared ductwork too. Spreading the same fan across more branches lowers the air velocity in the trunk, and if it drops below the transport velocity needed to keep particulate airborne, dust begins to settle inside the duct, narrowing it and dropping flow further. A proper addition is not a matter of finding a convenient duct to cut into; it means recalculating the system, sizing or upgrading the fan for the new total demand, and balancing the branches with dampers so each hood gets the airflow it needs.
What it means for you
When a workstation has been added without any of that design work, the fix starts with measurement. A thorough examination checks the system against its design and quickly shows that it no longer matches: the readings at the established hoods have fallen, the trunk velocity is down, and the new hood underperforms. From there the remedy is a proper balancing exercise, a fan upgrade, or in some cases a separate system for the added work, rather than leaving an overstretched fan to shortchange every hood.
The practical rule is simple: treat any new hood or branch as a change that has to be designed and recommissioned, not bolted on. Ask what it does to the fan's capacity and the trunk velocity before cutting into the duct. A system that was comfortably in control can be pulled below the line by one unplanned addition, and the people paying for it are the operators at the hoods that were working perfectly well before.
The service behind the guide
We measure every hood against the design, so an unplanned addition shows up as fallen readings across the system. You get a clear picture of what the fan is really delivering and what it takes to put it right.
Questions
Only with design work. A fan has a fixed capacity sized for the original hoods and ducts. Adding a branch spreads that capacity across more openings, so airflow and capture drop at every existing hood as well as underperforming at the new one.
Because the fan must serve all the hoods from one fixed capacity and overcome the combined resistance of everything connected to it. A new branch both adds resistance and divides the airflow further, so the whole system loses out at once.
Spreading the fan across more branches lowers the air velocity in the trunk. If it falls below the transport velocity needed to keep dust airborne, particulate settles inside the duct, narrowing it and cutting flow further.
By recalculating the system, sizing or upgrading the fan for the new total demand, and balancing the branches with dampers so each hood gets the airflow it needs, then recommissioning. It is a design job, not a splice.
A thorough examination compares readings to the design and shows the system no longer matches: the established hoods read low, trunk velocity is down and the new hood underperforms, pointing to balancing or a fan upgrade.
Phoenix Duct Clean · by the numbers
Adding a workstation to an existing fan can quietly weaken every hood. Call before or after the change and we will measure what the whole system is actually delivering.