What AHU size triggers FDD requirements under Title 24 2025?

Title 24 2025 Section 120.2 requires fault detection and diagnostics for air-handling units (AHUs) serving single zones in buildings classified as nonresidential with AHU cooling capacity at or above 110,000 Btu/hr (approximately 9 tons), and for constant-volume AHUs serving multiple zones at or above 54,000 Btu/hr (4.5 tons). VAV systems with central AHUs are subject to FDD at the AHU level regardless of size when they serve buildings over 10,000 sq ft. The thresholds have specific occupancy-type modifiers - healthcare and laboratory buildings have lower triggers.

What faults must California Title 24 FDD detect?

Title 24 Section 120.2 requires FDD to detect and alert on the following fault categories: economizer faults (economizer not opening when it should; economizer not closing when it should; stuck economizer damper); supply fan faults (fan not running when commanded; fan running when not commanded); cooling fault (cooling not meeting setpoint when outdoor conditions permit it); heating fault (heating not meeting setpoint); and simultaneous heating and cooling (a common control logic error that wastes energy). Each fault must generate an alert visible to building operations staff.

Who certifies FDD compliance under California Title 24?

FDD compliance is verified by an Acceptance Test Technician (ATT) - a CEC-certified third-party technician who performs the NRCA-MCH acceptance tests. The ATT verifies that the FDD system generates the required fault alerts under simulated fault conditions. The ATT completes the NRCA-MCH-04 acceptance test form and the results are submitted as part of the permit closeout documentation. The ATT is distinct from the CxA for full commissioning - ATT testing is a narrower functional verification.

How does Title 24 FDD interact with an existing BAS system?

Title 24 FDD can be satisfied by a properly configured BAS (building automation system) if the BAS includes the required fault detection algorithms and alert generation. Major BAS platforms (Siemens Desigo, Honeywell Niagara, Schneider EcoStruxure, Distech) support FDD alarm configuration that can be programmed to meet Section 120.2 requirements. If the existing BAS does not support FDD, a dedicated FDD software module (such as Siemens Navigator, Facilio, or Clockworks Analytics) can be added as a cloud-based overlay. The FDD system must be integrated with the AHU controls - a stand-alone monitoring system not connected to the AHU controls does not satisfy Title 24.

What does Title 24 FDD cost to install on a California commercial building?

Installed FDD cost for a California commercial building depends on whether the requirement is met through BAS programming or a dedicated FDD overlay. BAS-integrated FDD (programming existing BAS sensors and alarms to meet Section 120.2 requirements): $2,000–$8,000 per AHU in programming and configuration labor, assuming adequate sensors are already installed. Dedicated FDD software overlay (cloud-based, new sensor installation required): $5,000–$15,000 per AHU including sensor installation, software licensing, and integration. Hardware-based FDD module (embedded in newer AHU controllers): $1,500–$5,000 per AHU for controller upgrade.

Energy Code · Controls · Published 2026-06-03

Fault detection and diagnostics (FDD) for California commercial HVAC: Title 24 2025 requirements and what building owners must do

California's 2025 Title 24 energy code added expanded fault detection and diagnostics (FDD) requirements for commercial HVAC systems. For GCs, property managers, and facility directors with active projects, FDD is now a permit-required compliance item - not an optional upgrade. This guide covers the trigger thresholds, what the system must detect and alarm, who certifies compliance, and how to integrate FDD with existing BAS infrastructure.

Commercial HVAC air handling unit requiring Title 24 FDD fault detection and diagnostics — Commercial AHUs above the Title 24 Section 120.2 threshold must include fault detection and diagnostics - monitoring economizer operation, supply fan status, and heating/cooling performance.

What FDD is and why California mandated it

Fault detection and diagnostics (FDD) is a control-system capability that continuously monitors HVAC system operation against defined performance benchmarks, identifies deviations indicating faults or degraded performance, and generates alerts to building operations staff. The concept is straightforward: an air-side economizer that is stuck open when outdoor conditions are hot, or a cooling coil that is not meeting its supply-air-temperature setpoint, wastes significant energy and causes comfort problems - but these faults can go undetected for weeks or months without automated monitoring.

California's FDD mandate in Title 24 2025 Section 120.2 is driven by CEC analysis showing that economizer faults alone account for 5–10% of HVAC energy waste in commercial buildings, and that many faults remain undetected between annual maintenance visits because they don't produce immediate comfort complaints - just elevated energy bills. FDD closes that gap by automating fault detection.

Section 120.2 FDD trigger thresholds

Not every commercial HVAC system requires FDD. The triggers in Title 24 Section 120.2 are based on AHU cooling capacity and building type:

Single-zone AHUs at or above 110,000 Btu/hr (approximately 9 tons) in nonresidential buildings: FDD required.
Multi-zone constant-volume AHUs at or above 54,000 Btu/hr (4.5 tons): FDD required.
VAV systems with a central AHU serving buildings over 10,000 sq ft: FDD required at the AHU level.
Healthcare and laboratory occupancies: Lower trigger thresholds apply - FDD is required on AHUs 5 tons and larger in these occupancy types.

Small RTUs below 9 tons serving single zones in standard commercial occupancies generally do not trigger Section 120.2. This excludes many small tenant spaces and QSR buildings from the FDD requirement. However, if a TI project replaces or significantly modifies an AHU above the trigger threshold, FDD must be provided for the new or modified system.

What FDD must monitor under Section 120.2

Title 24 Section 120.2 defines specific fault categories that the FDD system must detect and alert on. These are minimum requirements - a well-configured BAS can detect many more faults, but these are the ones that must be verifiable by the ATT during acceptance testing:

Economizer faults

Economizer not opening when it should (failure to economize): Outdoor conditions are suitable for economizer operation (below the high-limit setpoint), but the OA damper is not opening to maximum. This is typically a stuck actuator, failed sensor, or incorrect control setpoint.
Economizer not closing when it should (failure to close): Outdoor conditions are above the high-limit setpoint, but the OA damper is not returning to minimum position. This fault causes refrigeration plant to fight against hot outdoor air being delivered to the supply fan.
Stuck economizer damper: The OA damper does not modulate in response to control signals. Detected by comparing commanded position to feedback position (requires position feedback sensor, which must be installed).

Supply fan faults

Fan not running when commanded: Control system commands fan on, but current or airflow sensor indicates no flow. Typically detected by current transducer or airflow switch.
Fan running when not commanded: Fan status shows running when control system has commanded off. Can indicate a stuck relay or VFD fault.

Cooling and heating faults

Cooling not meeting setpoint: Supply air temperature is above cooling setpoint by more than the fault threshold (typically 5°F) for more than 30 minutes during a cooling call. Indicates refrigeration system degradation, low charge, or coil fouling.
Heating not meeting setpoint: Supply air temperature is below heating setpoint during a heating call. Indicates heating system degradation.
Simultaneous heating and cooling: Both heating and cooling are active simultaneously, indicating a control logic error or stuck actuator on a reheat coil. This fault can occur in VAV systems where a zone with a stuck open reheat coil calls for cooling while the AHU is simultaneously in cooling mode.

Who certifies FDD compliance: the ATT process

FDD compliance under Title 24 is verified by an Acceptance Test Technician (ATT) - a CEC-certified technician who performs the NRCA-MCH acceptance tests. The ATT is a third-party technician, separate from the mechanical contractor and the controls contractor. The process:

After controls installation and startup, the ATT schedules an acceptance test session.
The ATT simulates fault conditions for each required fault category by temporarily overriding sensors or actuators, or by inducing fault conditions in the control logic.
The ATT verifies that the FDD system generates the appropriate alert (alarm, notification, or dashboard flag) within the required response time.
Results are documented on NRCA-MCH-04 and submitted as part of the permit closeout package.

The controls contractor must ensure the FDD fault logic is programmed and tested before the ATT visit - the ATT is not debugging the controls system. A common schedule problem: the ATT is scheduled immediately after startup, before the controls contractor has confirmed FDD logic programming. This results in a failed acceptance test and a re-test cycle that can delay CO by 2–3 weeks.

FDD acceptance testing interacts with the broader Title 24 commissioning process - on projects requiring both, the ATT verification is typically done as part of the CxA-led commissioning process, but the ATT must be separately credentialed.

BAS integration: making existing systems FDD-compliant

For building owners upgrading existing commercial HVAC systems to comply with Title 24 2025 FDD requirements, the integration approach depends on the existing BAS infrastructure:

Modern BAS (Niagara N4, Siemens Desigo CC, Schneider EcoStruxure, Distech Controls): These platforms support FDD alarm configuration natively or through FDD modules. The approach is to program the fault detection algorithms in the BAS graphics layer, configure alert notifications to building operations staff (email, SMS, or on-screen alarm), and ensure adequate sensor coverage. Most modern BAS retrofits for FDD run $2,000–$8,000 per AHU in programming labor.
Older or proprietary BAS: If the existing BAS cannot be programmed to meet Section 120.2 requirements, a cloud-based FDD overlay is the most practical approach. Products such as Clockworks Analytics, Facilio, GridPoint, and Siemens Navigator connect to the BAS via API or gateway, run FDD algorithms in the cloud, and generate alerts without requiring BAS reprogramming. These platforms typically run $1,500–$4,000/year per building in software subscription costs.
No BAS (standalone controls): Buildings with standalone thermostats and no central controls cannot meet Section 120.2 without upgrading to a networked BAS. For buildings triggering FDD, a BAS upgrade is required. The minimum BAS for FDD compliance includes: networked AHU controllers with OA damper position feedback, supply-air temperature sensors, supply-fan current monitoring, and a centralized alert system. Installed cost for a minimal FDD-compliant BAS upgrade on a 5-zone commercial HVAC system typically runs $15,000–$35,000.

For duct leakage compliance that is also part of the Title 24 2025 acceptance test package, see our companion article. For full commissioning scope that encompasses FDD verification, see our Title 24 commissioning guide. To review ongoing FDD monitoring as part of a preventive maintenance program, see our service agreement options.

Documentation for plan check and permit closeout

The FDD requirement must appear in the permit set - not just at startup. For plan check, include in the mechanical drawings:

A note on the AHU schedule identifying each unit that triggers Section 120.2 FDD requirements
A controls specification note identifying how FDD will be implemented (BAS-integrated, cloud-overlay, or hardware module)
Identification of the sensors required for FDD (OA damper position feedback, supply-air temp sensor, supply-fan current transducer)
Reference to NRCA-MCH-04 as the required acceptance test form

At permit closeout: the completed and signed NRCA-MCH-04 from the ATT, along with confirmation of fault-alert delivery mechanism (email log or BAS alarm history). Some AHJs also require a screenshot or printout showing active FDD alarms are enabled in the BAS. For project-specific FDD documentation guidance, contact our bid desk.

Working with Sierra Mechanical on FDD-compliant projects

Our controls coordination team manages FDD programming specification with BAS contractors on all projects triggering Title 24 Section 120.2. We identify FDD-triggering AHUs at the design-assist stage, specify adequate sensor packages, and coordinate ATT testing scheduling so it does not fall on the critical path to CO. We maintain working relationships with CEC-certified ATTs in Northern and Southern California.

For active projects: request a bid with your building address and HVAC scope. Bid desk: (916) 638-8605.

References: California Energy Code Title 24 Part 6 (2025) § 120.2; CEC Nonresidential Compliance Manual (2026); ASHRAE Guideline 36 (High-Performance Sequences of Operation for HVAC Systems). Information current as of 2026-06-03.

This article is general guidance. Consult your local AHJ, a licensed mechanical engineer, and a CEC-certified ATT for project-specific FDD requirements.