A fume cupboard is a machine for moving air. Everything it does for the person standing in front of it depends on a thin, steady inward flow across the sash opening, and a doorway is the most reliable way in a laboratory to disturb that flow. Two things arrive through a door: a pressure pulse every time it swings, and a person walking. Both are larger than the air velocity the cupboard is working with.
The face velocity a containment test looks for is in the region of 0.5 m/s. A person crossing a room at an ordinary pace moves at roughly 1.0 to 1.4 m/s and drags a wake behind them. Put those two numbers side by side and a separation distance stops being a matter of taste.
The number LabPlanner checks
The shipped threshold is FUME_MIN_DOOR_DISTANCE_MM = 1500 mm, measured from the cupboard to every doorway in the room. Its stated source is EN 14175-2 installation guidance together with general laboratory planning practice, and its status in the rule set is placeholder. EN 14175 is a paywalled standard; until a purchased copy has been read clause by clause, 1500 mm is an engineering figure drawn from practice rather than a sentence quoted verbatim. The rules panel says so, and the ruleset version is printed on every exported sheet, because a number presented as a clause when it is not is exactly the detail that later destroys a specification.
- Floor 900 mm. Below the width of a single door leaf the cupboard is effectively in the doorway, where every opening pushes a draught straight across the sash.
- Default 1500 mm. The value the rule ships with, and the figure a project inherits unless it says otherwise.
- Ceiling 4000 mm. Past four metres the requirement stops being satisfiable in the room sizes this kind of work happens in, and a rule nobody can satisfy is a rule that gets switched off.
The band is not decoration. A project can move the threshold to the figure its own authority uses; the clamp lives in the rule engine rather than in the interface, so editing a saved project file by hand cannot get round it; and every relaxation is printed on the exported sheet with both numbers and the reason. A dial that is silently ignored would be worse than no dial at all.
Why it is a warning and not an error
Door distance produces a warning. The generator already prefers the extract-capable wall farthest from the doors, so a generated layout normally clears the distance by a wide margin without being told to. The check earns its place on hand edits, and in rooms where the physics and the plumbing disagree.
There is an honest caveat behind the wall preference too. Across a reference corpus of 26 issued manufacturer layouts covering ten real projects, the farthest-wall habit held most of the time but not always: hospital projects repeatedly cluster fume cupboards on an interior serviced wall, because that is where the exhaust riser is. Ducting is a building constraint, not a layout preference, and no plan-view rule can see it. So the wall choice is preferred and never enforced beyond services and this distance.
The same corpus is why 1500 mm should be read as a floor. The tightest issued fume-to-door distance in it is about 2200 mm, and larger laboratories routinely run six metres and more. A layout sitting exactly on the threshold is a signal to look again, not a result to be pleased with.
How the distance is measured
This is the part that decides whether the rule does anything at all. Every door-proximity rule in the set — fume cupboards, microbiological safety cabinets, and the tall-cabinet egress buffer — measures to the doorway SPAN, jamb to jamb, and not to the midpoint of the opening.
Measuring to the centre understates the gap by half the door width, so the wider the opening the weaker the rule becomes. Against an 1800 mm double door, a unit standing flush against one jamb reads as 900 mm from the door centre: it clears a 900 mm buffer outright while physically touching the opening. Double doors are precisely where the draught is worst. A convention that fails hardest on the worst case is not a convention, and it was changed.
The clearance in front, which nobody should trade away
Distance from the door is not the only air-related figure around a sash. The operator zone in front of the cupboard, FUME_FRONT_CLEARANCE_MM, defaults to 1000 mm and must stay clear of equipment, wall returns and door swings. Its band floor is 700 mm, which is roughly the front-to-back depth a standing operator occupies. Below that, the person working the sash is standing in the aisle rather than in front of the cupboard, and the aisle rules take over the argument.
Two sashes must never face each other across that zone. Two cupboards drawing on the same volume of air, with a person working between them, is a containment problem for both of them at once.
What this check does not do
- It does not model the duct. Extract capacity, riser position and the air balance of the room sit outside a plan-view model.
- It does not test containment. That is a measured test on an installed cupboard in a finished room, not a geometric property of a drawing.
- It does not know the door. Whether an opening is held open, self-closing, or an airlock into a corridor changes the draught completely; the model sees a gap in a wall.