Mechanical Scope of Work: What GCs Need to Include When Buying Out a Mechanical Sub

Mechanical is the most complex and highest-risk sub-scope on most commercial buildings. Between HVAC equipment, hydronic piping, ductwork, controls coordination, and commissioning, it touches every other trade on the project. The gaps in mechanical scopes show up as change orders at the worst possible time — after equipment is on order and walls are closed in. This guide covers what to include in a mechanical scope of work, organized by the sub-trade specific work, package items, and coordination requirements.

Heating Plant

• Boilers: Specify each boiler by tag (B-1, B-2, etc.), type, capacity (kW or MBH), efficiency rating (AFUE or combustion efficiency — specify minimum), fuel type, and venting configuration. High-efficiency condensing boilers require stainless steel double-wall venting systems — specify it. Require factory start-up and commissioning by the manufacturer's representative on all commercial boilers.
• Hydronic piping: Specify pipe material by system — Type L copper for runouts under 50mm; Schedule 40 black steel for mains 65mm and above. Require shop drawings showing all pipe routing, hanger spacing, and expansion loop locations before fabrication begins. Expansion loops sized by calculation, not by default.
• Pumps: Identify each pump by tag. Specify type (in-line centrifugal vs. base-mounted), design flow (L/s or GPM), design head (kPa or ft), motor voltage and phase, and efficiency. Require suction diffusers and triple-duty valves (or isolation/check combination) on all pumps. Pump curves and certified performance data required at commissioning.
• Expansion tanks and air separators: Specify quantity, location, pre-charge pressure, and acceptance volume. These are often left as "the sub's problem" — missing or undersized expansion tanks cause hydronic noise and system pressure instability. Specify air separator type (centrifugal, tangential) and microbubble requirements on sensitive systems.

Cooling Plant

• Chillers: Specify type (air-cooled vs. water-cooled), nominal capacity (tons or kW), full-load and part-load efficiency (kW/ton or COP at rated conditions), refrigerant type, and BMS integration requirements (BACnet/IP or Modbus — confirm compatibility with controls contractor).
• Refrigerant piping: ACR copper tube with brazed fittings. Pressure test to 1.5× system test pressure and leak test (electronic detection) before refrigerant charge. Specify refrigerant detection requirements in mechanical rooms — CSA B52 in Canada, ASHRAE 15 in US. This requirement is frequently omitted and can be flagged at occupancy inspection.
• Water treatment: Glycol systems — specify glycol type (propylene vs. ethylene), concentration percentage, and initial charge. Include glycol feed station with tank, pump, and relief valve. Open condenser water systems — specify biocide treatment program and water treatment contractor.

Air Distribution

• Ductwork: Specify construction standard (SMACNA), sheet metal gauge by pressure class, interior liner requirements, duct sealant class (Class A for all supply systems — not Class C), and seismic bracing requirements. This is where mechanical subs save the most money on underbid scopes. Get explicit on every item.
• Air terminal units: VAV boxes — specify each unit by tag, minimum and maximum airflow (L/s or CFM), reheat coil type and capacity (hot water vs. electric), actuator type, and controls interface. Fan coil units — specify tag, pipe configuration (2-pipe vs. 4-pipe), filter type, and whether an auxiliary drain pan is required above sensitive ceiling areas.
• Diffusers, registers, and grilles: Provide a complete schedule. Specify neck size, face size, throw and spread characteristics, and finish. Ceiling type drives diffuser frame selection — T-bar vs. drywall frames must be confirmed before equipment is ordered.
• Fire and smoke dampers: Required at all duct penetrations of fire-rated assemblies. Specify the sub's responsibility for access door procurement and installation to allow fusible link access and reset. ULC/UL listing and installation certification required. Confirm AHJ requirements for damper inspection records at closeout.

Testing, Adjusting, and Balancing (TAB)

• TAB responsibility: Specify whether TAB is by the mechanical sub or a third-party TAB firm. Recommend third-party TAB for projects where the commissioning authority is independent — it removes a conflict of interest. All air systems balanced to within ±10% of design airflow; hydronic systems to within ±10% of design flow.
• TAB report: Required as a closeout deliverable before occupancy is granted. Many GCs don't enforce this — and then can't get occupancy because the commissioning agent or AHJ flags it during final inspection.

Commissioning

• Commissioning support: Specify whether the project has a Commissioning Authority (CxA). The mechanical sub must support all functional performance testing, including providing technicians to operate equipment during testing. This is not optional — it must be in the subcontract.
• Owner training: Specify minimum hours of training on major mechanical equipment for owner's operating staff. Include training on BMS interface for HVAC systems.

• Controls wiring interface: Low-voltage interlock wiring between HVAC equipment and the BMS is a persistent gap between the mechanical sub and the controls sub. Specify which trade provides control wiring to field devices vs. which provides the BMS controller infrastructure. Actuators, dampers, and control valves are commonly supplied by the controls sub but installed by the mechanical sub — confirm the split in both subcontracts.
• Pipe and duct insulation: All hydronic piping, refrigerant piping, and ductwork insulation must be specified by system, pipe size, and insulation thickness per energy code. PVC jacketing in mechanical rooms; aluminum jacketing for all exterior rooftop piping. Frequently bid separately from the mechanical sub's piping scope — confirm what's included.
• Hoisting and rigging: Crane days for roof equipment sets (chillers, ERVs, RTUs, cooling towers) must be in the mechanical sub's scope. On projects where GC carries the crane, confirm this is explicitly stated and the mechanical sub provides a rigging plan. Crane logistics on urban sites drive significant schedule risk.
• Core drilling: All core drilling for pipe penetrations through concrete slabs and walls. Sub is responsible for firestopping restoration after coring through fire-rated assemblies. Specify that the sub must have a penetration log reviewed and approved before any core drilling proceeds.
• Permits: Mechanical and gas permits obtained and paid for by the sub. Gas piping pressure tests must be witnessed by the gas utility inspector — sub schedules and attends all inspections.
• Refrigerant detection: Required by code in mechanical rooms containing refrigerant systems. Specify sensor count, alarm setpoints (typically 25% of IDLH), and tie-in to BMS for remote monitoring. This item is omitted on nearly half of all mechanical scopes.
• Temporary heating during construction: If the GC schedule requires temporary heat, it must be an explicit line item in the mechanical subcontract. Temporary heat is not automatically included in the sub's scope.

• Electrical: Motor starters and disconnects — confirm whether mechanical or electrical sub supplies and installs. VFDs — specify supply, installation, and commissioning responsibility. Critical: VFDs must be ordered with BACnet communication cards compatible with the BMS — this cannot be added after the fact without significant cost. The electrical sub and mechanical sub must coordinate on this before VFDs are ordered.
• Structural: Operating weights of all roof-mounted equipment must be confirmed with the structural engineer before equipment is ordered. Roof dunnage (structural steel support frames) is typically by the structural or steel sub — confirm. Pipe hanger inserts for concrete slabs must be coordinated before pours.
• Plumbing: Condensate drain connection points from all HVAC equipment (chillers, ERVs, fan coil units, boiler condensate, humidifiers) to the nearest floor drain provided by the plumbing trade. Plumbing sub must know condensate drain locations before plumbing rough-in.
• Controls/BMS: Specify whether the mechanical sub or the controls sub provides and installs control valves and sensor wells. Controls sub typically furnishes valves; mechanical sub installs them — confirm this in both subcontracts. Damper actuators — same rule applies.
• Architectural: Ceiling types in all areas must be confirmed before diffuser frames are ordered. Louver size, quantity, and color must be confirmed with the architect before procurement — lead times on large louvers can be 8–12 weeks.
• Fire protection: Duct smoke detector locations must be coordinated with the fire alarm sub. Fire damper locations must be aligned with architectural fire-rated assembly drawings. Access door locations for dampers must be shown on coordination drawings before ductwork is installed.