Energy Code · Commissioning · Published 2026-06-03

Title 24 commissioning requirements for California commercial HVAC: what mechanical contractors and building owners need to know

California Title 24 Section 120.8 requires commissioning on most new and significantly altered commercial HVAC systems. It is one of the most-misunderstood compliance requirements in the state - and one of the most reliable causes of last-minute CO delays. This guide covers who must commission, who hires the Commissioning Authority, what functional performance tests actually cover, and how to schedule Cx so it stays off the critical path.

What Section 120.8 actually requires

Title 24 Part 6 Section 120.8 - the Commissioning Requirements section - mandates a commissioning process for commercial buildings and systems meeting specific thresholds. The triggers:

• New commercial buildings over 10,000 sq ft: Full commissioning of all HVAC systems covered by Title 24 Part 6.
• Additions over 10,000 sq ft: Commissioning of the new addition's HVAC systems.
• Alterations to existing buildings where HVAC scope exceeds 20% of the system: Commissioning of the altered systems. This is the trigger most often missed on TI projects - a large enough mechanical alteration brings the whole Section 120.8 process in.

Systems required to be commissioned under Section 120.8 include: HVAC equipment with economizers; demand-controlled ventilation systems; variable-air-volume (VAV) systems; supply-air temperature reset controls; supply fan variable-speed drives; and central plant equipment (chillers, boilers, cooling towers). Simple rooftop unit replacements on small buildings typically fall below the trigger threshold, but complex TI projects on large buildings almost always trigger commissioning.

The 2025 Title 24 update added FDD (fault detection and diagnostics) requirements that interact with commissioning - see our FDD article for details on that overlap.

The Commissioning Authority (CxA): independent, owner-hired

The single most important structural requirement of Section 120.8 is that the Commissioning Authority must be independent of the mechanical contractor, the engineer of record, and the GC. The CxA is hired directly by the building owner (or owner's representative), and the CxA's contractual loyalty runs to the owner's interest in a properly functioning building - not to the contractor's interest in passing a test quickly and moving on.

The CxA has three core responsibilities:

1. Design phase review: Review mechanical construction documents and specifications to confirm commissioning scope is adequately defined and that systems are designed to be testable.
2. Construction phase oversight: Develop the commissioning plan and functional test procedures; witness equipment startup; perform pre-functional inspections; and witness the functional performance tests.
3. Commissioning report: Document test results, deficiencies, resolution of deficiencies, and final confirmation of system performance. Submit the Cx report as part of the permit closeout documentation.

Common misconception: that the mechanical contractor can "self-commission" by completing the NRCA-MCH acceptance test forms. This is not equivalent to Section 120.8 commissioning. NRCA-MCH forms are the contractor's self-verification tool; a CxA-led commissioning process is a separate, independent verification. Both are required when Section 120.8 applies.

CxA vs. TAB: what each covers

Test and balance (TAB) and commissioning are frequently confused - and frequently contracted separately in ways that create gaps in scope. The difference:

• TAB (Testing, Adjusting, and Balancing): Measures actual airflows, water flows, and static pressures at every terminal device; adjusts dampers, valves, and fan speeds to match design values. TAB confirms the system delivers the right quantities of air and water. It is typically performed by a NEBB or AABC-certified TAB contractor, retained by the mechanical sub or GC.
• Commissioning (Cx): Verifies that the entire system - equipment, controls, and sequences of operation - responds correctly to operating conditions. Cx confirms the system does the right things at the right times. It is performed by the CxA hired by the owner.

On a Section 120.8 project, TAB must be complete before the CxA begins functional performance tests. An incomplete TAB is one of the most common causes of failed functional tests - the system cannot perform correctly at design conditions if airflows haven't been balanced to design values first.

Functional performance test scope for RTUs and VAV systems

Functional performance tests (FPTs) for the most common California commercial HVAC system types cover:

Packaged RTU with economizer

• Thermostat setpoint response - verify system transitions to cooling and heating mode correctly at setpoint offsets
• Economizer changeover - verify economizer opens and provides free cooling at appropriate outdoor conditions; verify high-limit shutoff at correct dry-bulb or enthalpy threshold
• Occupied/unoccupied mode transition - verify setback setpoints are engaged on schedule
• Morning warm-up/cool-down sequence - verify system reaches setpoint within specified time
• Supply air temperature control - verify actual supply air temperature matches control setpoint
• DCV response (if installed) - verify CO2 sensor modulates OA damper correctly

VAV system (AHU with VAV boxes)

• All RTU/AHU tests above, plus:
• VAV box minimum air volume - verify each box maintains minimum CFM in occupied mode
• VAV box maximum air volume - verify each box opens to design maximum at full cooling call
• Supply fan static pressure reset - verify duct static pressure resets down as VAV boxes close
• Supply air temperature reset - verify SAT resets up as cooling demand decreases
• Reheat coil control - verify reheat activates at box minimum position in heating mode

For duct leakage requirements that also feed into the commissioning closeout documentation, see our companion article.

Common Cx failures that delay CO

Based on our experience on California commercial projects, the most common functional performance test failures that delay certificate of occupancy are:

1. Economizer high-limit control not calibrated or not wired. The enthalpy sensor was installed but not commissioned; the system runs the economizer in all outdoor conditions, inflating energy use and potentially causing comfort complaints before CO. Resolution requires controls contractor re-programming and re-test.
2. DCV CO₂ sensors not communicating to BAS. Sensors were installed physically but not integrated into the controls system. The OA damper runs at design max rather than modulating. Resolution requires BAS contractor integration and re-test.
3. VAV box actuator failures on functional stroke test. Actuators that passed pre-functional inspection fail the full-stroke FPT due to mechanical binding or firmware issues. Resolution requires actuator replacement or adjustment.
4. HERS duct leakage test failure after ceiling close. Duct joints or penetrations left unsealed during rough-in fail the duct leakage test. Resolving this after ceilings are closed is expensive and time-consuming.
5. Supply air temperature not reaching setpoint. Undersized equipment or incorrect refrigerant charge results in actual cooling capacity below design. Resolution requires refrigerant charge verification or equipment replacement in severe cases.

Owner acceptance testing and O&M documentation

Section 120.8 also requires that the building owner receive a Cx report and - for buildings over 50,000 sq ft - that an operations and maintenance plan be prepared. The O&M plan must include recommended commissioning intervals (typically every 3–5 years for major systems), control system documentation, and equipment service requirements.

Many building owners are surprised to learn that California requires periodic recommissioning (sometimes called "monitoring-based commissioning" or retrocommissioning) for large buildings. While the frequency is not mandated at the same level as the initial Cx, the existing-building energy benchmarking requirements in some California cities (San Francisco, Oakland, Los Angeles) effectively create an ongoing Cx pressure through energy performance reporting requirements.

For ongoing mechanical system maintenance after Cx, our service agreements page covers how Sierra's preventive maintenance programs are structured to maintain the functional performance verified at commissioning. For active construction projects, contact our bid desk to discuss commissioning coordination and scheduling.

Working with Sierra Mechanical on commissioned HVAC projects

Our project managers coordinate directly with the owner's Commissioning Authority from pre-construction through startup. We build commissioning witness dates into our baseline schedule, complete TAB before CxA arrival, and maintain documentation packages that meet Section 120.8 report requirements. Our installation practices - accessible controls, labeled sensors, documented setpoints - are designed for CxA verification.

For projects where the CxA has not yet been engaged at the bid stage, we can provide a list of qualified California CxAs who have worked on similar commercial projects. Bid desk: (916) 638-8605. Request a bid online.

References: California Energy Code Title 24 Part 6 (2025) § 120.8; CEC Commissioning Requirements Reference (2026); ASHRAE Guideline 0 (Commissioning Process); NEBB Procedural Standards for Commissioning. Information current as of 2026-06-03.

This article is general guidance. Consult your local AHJ and a qualified CxA for project-specific requirements.