The part of a fix nobody sees in the 3D view
A clash fix looks finished the moment the two elements stop overlapping. It is not. Two extra elbows on a supply duct change its pressure drop; a dip in a sprinkler branch creates a low point that needs draining; a heating pipe that climbs over a beam gets a high point where air collects. None of that is visible in the model, and most of it is discovered late, by the engineer who owns the system, when the coordinator has moved on.
ClashFree cannot do the engineer's calculation, and does not pretend to. What it can do is make sure the engineer hears about every change that touches their numbers, at the moment the fix is proposed, in a form they can check in a minute.
The bill
Each option in the list, and each proposal you see in the pane, carries the same itemised bill, counted against the route before the fix:
- Fittings added and removed, and the bends in degrees.
- Net length: positive when the fix adds material, negative when it saves some.
- How far the run moves, and which parts are moved, replaced or deleted.
- Tees moved along their main and terminals carried with the run.
- New high or low points.
- Wall crossings, with their notes.
The bill is also how options compare at a glance. An offset with fewer fittings but more length, a drag that adds nothing but carries two diffusers, a reroute that saves a metre but crosses a partition: the score weighs them, and the bill shows what was weighed.
Impacts: one line per check that applies
The impacts come from the sensitivity tags of the run's system class. Only the tags that apply to this change appear, and each says what to re-check, never that the result is fine:
| Tag or change | Systems | What the line asks for |
|---|---|---|
pressure_loss | air ducts, smoke extract, kitchen exhaust | Airflow and static pressure re-check, or "the pressure drop falls" when the fix saves fittings. |
hydraulic_calc | sprinkler | The hydraulic calculation; a new low point needs an auxiliary drain (NFPA 13). |
head_loss | domestic water, hydronic, other pipe | Pump head or supply pressure, if the run is on the index circuit. |
| high or low point | any pressurised pipe that goes up or down | An air vent at a new high point, a drain valve at a new low point, whatever the class. |
fall | gravity drainage, condensate, steam | The gradient end to end (these fixes are sideways only). |
rated | smoke extract, kitchen exhaust, medical gas | The fire engineer's approval. |
grease | kitchen exhaust | No low point where grease collects (NFPA 96). |
bends | conduit | No more than 360° of bends between pull points. |
bend_radius | cable tray | The bend radius the cables allow. |
| terminals carried | ducts with diffusers or grilles | The ceiling layout. |
| wall crossing | any | A wall opening for the architect; a damper (duct) or firestop (pipe, conduit, tray) when the wall is fire-rated. |
Where you see it
The bill and its impacts are on every option ClashFree lists (the AI reads them before it chooses), on the proposal card in the pane, and on the Keep. The AI's explanation may repeat a tag ("pressure_loss: +2 elbows on supply duct SA-12; the HVAC engineer should re-check the airflow"), but it is not allowed to claim an engineering result. It cannot say the pressure drop is acceptable, because nothing calculated it.
When you press Keep or Reject, the local decision log records the chosen option's bill next to its runners-up, in your project folder, so a coordination lead can later see what was changed and why.
Counts every change a fix makes against the route before it, and lists one re-check per tag of the run's system class that the change triggers, plus wall notes for the architect and the fire engineer. It does not simulate airflow, hydraulics or cable pulls; every listed change shows, nothing is filtered.
Why this makes fixes better, not just documented
Pricing the impacts is what lets the scorer prefer the right fix. The fitting factor of an air duct is 2 because of pressure_loss; a sprinkler's is 2.5 because of hydraulic_calc; a new low or high point on a sprinkler costs twice the base, three times on kitchen exhaust. So between two fixes that clear the clash, the one that triggers fewer and smaller re-checks scores better, and the one that improves the system, like the duct drag above, can win outright.
It also changes who moves. Because a change on a duct costs more than the same change on a small pipe, the pipe gives way; see why the pipe moves, not the duct.
How to read a bill in thirty seconds
Coordinators who use ClashFree for a while tend to read the bill in the same order, and it is a good order to start with:
- Impacts first. If there is a line in orange, someone other than you has to look at this fix. Decide now whether to send it, or to ask for a different fix in the chat ("keep the sprinkler level, go sideways").
- Fittings, net. Added minus removed. A drag that removes a redundant jog can be net negative, which is better than the original design.
- Net length. A few hundred millimetres is normal for an offset; metres usually mean a reroute, and a reroute should have a reason.
- High and low points. On any pressurised pipe, each one is a vent or a drain someone has to add. On kitchen exhaust, a low point is where grease collects.
- What moved. Parts moved, replaced or deleted, tees slid along their main, terminals carried. Carried diffusers mean the ceiling layout changes.
If all five look ordinary, the fix probably is. If one does not, the explanation above the bill should say why that option still won; if it does not convince you, Reject costs nothing and the model is exactly as it was.
A note for the engineers
If you own a system, the impacts are written for you. They are deliberately short, they name the run and the change, and they appear only when your numbers are touched. A coordinator who keeps a fix with a hydraulic_calc line has a reason to send it to you that day, not after the model is issued. That is the whole point: no surprise at the end of the job.
Read next: the limits no score can cross, in Hard limits the AI cannot break, or see the bills on your own model in the free beta.