Diagnostics & problems
You can break a local exhaust ventilation system without laying a finger on it. Move a machine, put up a partition or seal a doorway and you change the airflow the system was designed around, often starving it of the very air it needs to work.
The short answer
A local exhaust ventilation system is commissioned to control one particular arrangement of machines, benches, doors and airflow. Change that arrangement and you can undo the control without anyone opening a duct or adjusting a fan. This is one of the most common reasons a system that once tested fine quietly stops working: the extraction did not fail, the workshop around it changed.
The detail
The most overlooked mechanism is replacement air. Every cubic metre an extraction system pulls out of a room has to be replaced by air coming in from somewhere. Seal up a doorway, board over a louvre, add a partition or tighten the building envelope, and you cut off the replacement-air path. Negative pressure then builds in the space, the system has to fight to pull its designed volume, and capture velocity at the hood falls, all with no visible sign: the fan still turns and air still moves, but the hood is no longer reaching the contaminant.
Rearranging the floor causes trouble in other ways too. Relocating a machine away from its fixed hood leaves the capture point too far from the source, and capture velocity falls off steeply with distance, so a hood that worked at the old spacing may barely reach at the new one. New partitions, roller doors, fans or air-conditioning units create cross-draughts that sweep across a hood and peel the contaminant plume away before it can be captured. Even a well-meant comfort fan aimed at an operator can defeat the extraction it sits next to.
What it means for you
The lesson is that any significant change to the layout or the building should be treated as a trigger to re-examine the extraction, not as a separate matter. A thorough examination checks that the system still matches the process and its surroundings, uses smoke to reveal cross-draughts and dead spots created by the new arrangement, and measures capture and airflow against the design to show whether the change has cost control.
This is also why good practice keeps the extraction in the conversation whenever a workshop is reorganised. Before a machine is moved onto an existing hood, a wall goes up or a vent is closed, the question worth asking is what it does to the airflow the system depends on. Catching that at the planning stage is far cheaper than discovering months later, through dust, smell or symptoms, that a tidy-looking reorganisation quietly switched off the protection.
The service behind the guide
After any layout or building change we check the extraction still matches how the workshop is now arranged, revealing cross-draughts and starved make-up air and measuring capture against the design.
Questions
Yes, without anyone touching the extraction. The system was commissioned for one arrangement of machines, doors and airflow. Moving machines, adding partitions or sealing vents changes the air the system depends on and can undo the control.
Every cubic metre extracted must be replaced by air entering the room. If you block the replacement-air path by sealing doors or vents, negative pressure builds, the system cannot pull its designed volume, and capture velocity falls with no visible warning.
Capture velocity falls off steeply with distance from the hood. A machine relocated away from its fixed hood leaves the capture point too far from the source, so a hood that worked at the old spacing may barely reach at the new one.
They create cross-draughts that sweep across a hood and peel the contaminant plume away before capture. Even a comfort fan aimed at an operator can defeat the extraction beside it.
Yes. Treat any significant layout or building change as a trigger to re-examine. A thorough examination confirms the system still matches the process, reveals new cross-draughts and measures capture against the design.
Phoenix Duct Clean · by the numbers
Moving machines, partitions or doors can undo a system that tested fine. Call to book an examination that confirms the extraction still matches how your workshop is now arranged.