What BIM Coordinators Actually Need From an AHU Export (DXF vs IFC, LOD, Clash)

An air handling unit is often the single largest piece of MEP equipment in a plant room, and it arrives in the federated model as a hand-off from selection software. From the coordinator's chair, the question is not "is this a nice drawing" but "can I place it, connect it, clash it, and trust it not to change under me." Those are four different requirements, and DXF, IFC, LOD and clash tolerance each answer one of them.

DXF vs IFC: they are not interchangeable

A DXF is 2D or 2.5D geometry — lines, arcs, layers. It is perfect for a fabrication outline, a plant-room plan, or dropping a footprint onto a coordination sheet. What it does not carry is typed MEP data: a DXF polyline does not know it is an AHU, and a rectangle at the end of it does not know it is a 500 mm supply duct connection.

IFC (IFC4, IfcUnitaryEquipment for the AHU body) carries the semantics. The unit is an object with properties; the duct spigots are IfcDistributionPort objects with a flow direction, a system classification and a nominal size. That is what lets Navisworks, Solibri or Revit auto-route a duct to the right face and run a rule-based clash rather than a dumb solid-on-solid check.

Need	DXF	IFC
Plant-room 2D outline / fabrication	✅	⚠️ overkill
Object with typed properties (airflow, weight)	❌	✅
Connection points as routable ports	❌	✅
Rule-based / soft clash	❌	✅
Universally openable, no MEP setup	✅	⚠️ tooling-dependent

The practical answer: ship both. DXF for anyone who just needs the footprint, IFC for the federated model.

LOD is a contract, not a slider

Level of Development (BIMForum's LOD spec, mirrored by RIBA stages) tells the coordinator how much to trust the geometry:

LOD 200 — approximate size and shape. Fine for early space reservation; do not dimension off it.
LOD 300 — accurate size, shape and location. This is the coordination baseline: real casing length, height, panel thickness, and connection positions.
LOD 350 — adds interfaces between systems: connection ports with correct face, size and orientation so ducts and pipes actually meet the unit. This is where AHU exports either succeed or generate RFIs.
LOD 400 — fabrication-level detail (fixings, access panel swing). Usually the manufacturer's shop model, not the selection export.

Most AHU coordination lives at LOD 300–350. The failure mode is an export that claims 350 but places the supply spigot at a schematic centre-line instead of the real physical penetration — the duct meets air, and the clash report is clean while the site is not.

Connection points and maintenance clearance

Two zones must be right or the clash check is worthless:

Connection faces. The supply, extract, fresh and exhaust apertures have to sit at their true coordinates on the true face. In cross-flow plate or rotary layouts the internal routing swaps connections across the bridge diagonal, so a naive "supply is always on the left" assumption puts the port on the wrong side. The export has to resolve airflow-to-aperture from the actual internal chain — see duct connection design and where quotes lose 50 Pa for why that mapping is subtle.
Access / pull space. Coil withdrawal, filter-change and fan-removal clearances are real reserved volumes. If they are not in the model (as a clearance solid or at least documented), the coordinator will pack a pipe rack into the space a 2.4 m coil needs to slide out. Soft-clash rules catch this only if the clearance geometry exists.

The requirement behind all of it: no stale geometry

Every one of these — DXF outline, IFC port, LOD 350 dimension — is only as good as the number it was built from. If the export re-reads a cached casing length that was set three revisions ago, the coordinator receives a confident, well-structured, wrong model. The export has to recompute geometry from the current component chain on every regeneration, so a swapped coil or an added damper module updates the DXF, the IFC and the data sheet together. That single-source discipline is the whole subject of what a clean DXF/IFC hand-off looks like.

A coordinator's acceptance checklist

Check	Pass criterion
Format	DXF footprint and IFC4 object both supplied
LOD	Dimensions and ports at LOD 300–350, not 200
Ports	Each duct connection an `IfcDistributionPort` at the real face
Cross-flow mapping	Supply/extract on physically correct sides
Clearance	Access/withdrawal zones present or documented
Freshness	Export regenerated after the final component change

AirSelect3D derives every export from the same live geometry that drives its 3D viewer — one calculation feeding the DXF, the IFC and the technical data sheet, with connection ports resolved from the real internal routing rather than a side-based guess. That is what keeps the model the coordinator receives identical to the unit that ships.

Build a unit and watch its export stay in sync with the 3D model →