Picture rough-in on a twelve-month job. The rooftop units are set, ductwork's up, and an electrician is standing at a disconnect with a print in one hand and a phone in the other — the spec says "VFD furnished by mechanical" on one page and never says who wires it three pages later. Nobody's wrong yet. Nobody's right either. That's a change order forming in real time, and it started months earlier when someone skimmed the equipment schedule instead of reading it.
Capacity, electrical characteristics, accessories: every unit gets priced row by row, because the schedule row is the bid item and everyone knows it. The bid falls apart at the boundaries instead, around Divisions 21, 22 and 23 — wiring splits that read differently unit to unit, a temporary-heat duty that's formally someone else's but lands here anyway, and pads and openings marked "confirmed by others" until nobody has.
Three places the fight starts
- Controls and BMS. Building automation sits inside mechanical on some tenders and goes straight to a controls vendor on others, and the tender rarely says which up front. Extract three separate things, not one: who supplies the controls hardware, who wires it, and what protocol the building automation system demands. A BACnet integration clause buried in a general note can mean commissioning hours nobody priced anywhere near the equipment schedule.
- Control and interlock wiring. Line-voltage power to equipment is Division 26 work almost everywhere, but the low-voltage signal wiring that starts and stops a unit, trips it on a fire alarm interlock, or ties it to a duct detector splits between Divisions 23 and 26 inconsistently — sometimes within the same schedule. "Control wiring by Division 26" is a real sentence in real specs, worth real money either way it lands. Pull it verbatim, unit by unit; don't let the first one set the pattern for the rest.
- Temporary heat. Providing temporary heat during construction, or running permanent equipment early to condition a space before substantial completion, is formally a Division 01 duty. In practice it lands on mechanical often enough that "by others" needs confirming, not assuming. Two costs worth separating: the temporary system itself, and the warranty exposure of running permanent equipment early. Start a rooftop compressor's warranty clock in month two of a twelve-month schedule, and that's a conversation to have before bid day.
The systems piece is rarely the fight
- HVAC systems, complete — equipment, ductwork, hydronic and refrigerant piping, insulation, and testing/adjusting/balancing per Divisions 21–23 as assigned.
- Plumbing systems, complete — fixtures, domestic water, sanitary, storm, gas and specialties per Division 22. The fixture schedule and riser scope drive the count more than the drawings alone.
- Fire suppression, where assigned — sprinkler and standpipe systems per Division 21, often carried as a licensed specialty sub under the mechanical umbrella. Extract the assignment; don't presume it either way.
- Equipment supply-and-set per the schedules — capacity, electrical characteristics (voltage, phase, MCA, MOP) and accessories, unit by unit. Missing one unit means missing a line, not a detail.
What's genuinely not yours is shorter: line-voltage power wiring to equipment is Division 26's, full stop — except starters, VFDs and disconnects, which split unpredictably enough to earn their own entry below. Concrete housekeeping pads are usually the concrete trade's to pour, but extract the pad schedule anyway; this estimator locates and sizes them, and concrete just builds what mechanical tells it to.
Five places it hides beyond the schedule
- Starter, VFD and disconnect ownership. Furnished under Division 23, wired under Division 26, or the reverse — the classic Div 23/26 gap, and it recurs once per piece of equipment, not once per tender. Sweep every schedule note and spec line that assigns them; checking the first three units and calling it done misses the rest.
- Electrical characteristics mismatches. Voltage, phase, MCA and MOP on the mechanical schedule versus what the electrical drawings actually show. Flag every unit for coordination — one mismatch found at rough-in is a change order, not a rounding error.
- Commissioning and TAB scope. Cx participation hours, seasonal testing, and whether testing/adjusting/balancing runs independent or in-house is real labour, usually stated in Division 01 or 23 08 — easy to wave off as boilerplate rather than price as a line.
- Roof openings, curbs and supports. Curb supply versus curb installation, structural support steel, and roofing coordination meet at every rooftop unit — three trades converging on one opening, and any of them can assume it's someone else's.
- Owner-furnished equipment. OFCI units that mechanical sets and connects still need a supply-versus-install line extracted per item. "Owner-furnished" says who buys it, not who's on the hook to rig, set and pipe it in.
Who you're sharing the mechanical room with
| Trade | What to reconcile before you price |
|---|---|
| Electrical | Starters, VFDs, disconnects, control power and fire alarm interlocks — the Division 23/26 boundary needs every assignment extracted, not assumed. |
| Structural & steel | Equipment loads, roof and floor openings, support steel and hanger attachment; a late equipment pick can force a structural change nobody priced. |
| Concrete | Housekeeping pads, trenches and sleeves through slabs — located by mechanical, built by concrete, so the pad schedule needs to exist and be right. |
| Fire alarm contractor | Duct detectors, fan shutdown and damper interlocks split supply, install and wiring across three trades at once. |
| General contractor | Temporary heat responsibility, and the warranty consequences of running permanent systems early. |
Read every unit on the schedule, not just the first few that set a pattern — the starter/VFD split and the electrical-characteristics check both repeat per unit. Then read Division 01 for temporary heat and commissioning language before handing either one to somebody else by default. That's what keeps rough-in from turning into the change order this article opened with.