Demand Control Ventilation Requirements Under Title 24 in California

Title 24 of the California Code of Regulations (CCR) contains the California Building Standards Code. In particular, demand controlled ventilation is covered under Title 24, Part 6: Energy Efficiency Standards for Residential and Nonresidential Buildings. This set of regulations is also known as the California Energy Code.

Ashrae standards are the main design reference for ventilation systems under Title 24. Low-rise residential projects are subject to Ashrae standard 62.2, while high-rise residential and other building types are subject to Ashrae standard 62.1.

A common requirement for all building types under the California Energy Code is preventing air leakage, due to its negative effect on HVAC efficiency and temperature control. The following are both presented as mandatory requirements:

All potential air leaks in the building envelope must be sealed with methods such as caulking, gasketing and weatherstripping.
Furnaces, air conditioners, heat pumps and other air-handling equipment must qualify as low leakage air-handling units, through a testing procedure specified in the code.
The maximum leakage allowed for duct systems is 6% of the air handler capacity.

The goal of this article is informing about the general design requirements for DCV according to Title 24, Part 6. However, the code provides exceptions and special considerations in many cases. This article is not intended to be used instead of the actual code, since covering all the special cases would require a very extensive guide.

The California Energy Code follows the Ventilation Rate Procedure (VRP) from ASHRAE for ventilating air requirements. The Indoor Air Quality Procedure (IAQP) has not been approved yet under Title 24, but this could change in future versions of the code.

Title 24 Requirements for DCV in Commercial and High-Rise Residential Structures

The California Energy Code provides a detailed list of design requirements for DCV, but most are related with three main aspects: Minimum outdoor air (OA), rated airflow of the airing system, and control methods

The minimum OA flow rate is based on floor area and occupancy type. Title 24 provides minimum rates in CFM per square foot, and the DCV system cannot reduce the oa airflow below these values while indoor spaces are occupied. In other words, the minimum OA flow rate the lower operating limit for the DCV system, and the code only allows a 10% margin of error between the actual airflow and the calculated value.

Auto repair workshops: 1.50 cfm/sq.ft.
Barber shops and beauty shops: 0.40 cfm/sq.ft.
Bars, cocktails, lounges, casinos and retail stores: 0.20 cfm/sq.ft.
Coin-operated dry clean: 0.30 cfm/sq.ft.
Commercial dry clean: 0.45 cfm/sq.ft.
High-rise residential: In this case, the calculation procedure is provided by the California Building Code. The required airflow is 0.06 cfm/sq.ft. in general, plus 5 cfm per occupant for areas found inside dwelling units.
Hotel rooms below 500 sq.ft.: 30 cfm/room
All other areas, including offices: 0.15 cfm/sq.ft.

The capacity of the airing system should be enough to provide 15 cfm/person at full occupancy. However, the minimum OA flow rate should determine capacity if this calculation yields a smaller value. Using transfer air from other building zones is acceptable, as long as two requirements are met:

The transfer air cannot be from zones with air pollution sources.
The outdoor air supply must be enough to meet the individual needs of all zones.

Variable air volume (VAV) systems, which are necessary for DCV, are subject to a fan power limit if their total capacity is over 25 hp. Under Title 24, Part 6, the fan power index cannot exceed 1.25 watts per cfm. The code also provides design guidelines for static pressure sensors, where the goal is to reduce fan consumption and achieve energy savings.

Sensors should have a location that allows a control set point below one-third of the design static pressure of the fan.
When sensors are found in parallel duct branches, the fan should increase static pressure until all of them reach the set point.
Ventilating air systems with digital control of individual zones can use lower setpoints, based on their control algorithm.

The California Energy Code also provides requirements for the control methods used in DCV. First of all, the code requires the minimum OA flow rate at any time when indoor spaces are occupied. During the one-hour period before occupancy is expected, indoor spaces must be supplied with the minimum OA supply or three complete air changes (ACH), whichever is smaller.

Title 24 establishes the following requirements for the sensors/detectors and controls used in DCV:

Installing at least one CO2 occupancy sensor for every 10,000 sq.ft.
CO2 detectors should be between 3 and 6 ft. above the floor, or at the same level as the heads of occupants.
Indoor CO2 must be maintained below the outdoor concentration, which can be assumed at 400 ppm or measured directly within 4 feet of the air intake, plus 600 ppm.

Title 24 only specifies CO2 detectors as valid control inputs for DCV. Occupancy sensors are only allowed as shut-off controls for space conditioning systems.

To enhance energy savings, an economizer is mandatory for any air handler that manages over 54,000 BTU per hour of cooling capacity. Both airside and waterside economizers are acceptable, and they must be capable of supplying 100% percent of the cooling load with outdoor air when allowed by weather conditions.

Conclusion

The California Energy Code (Title 24, Part 6) has stringent performance requirements for DCV, which emphasize both HVAC energy savings and indoor air quality. This can be expected, considering that California has assumed a global leadership role in the implementation of emerging technologies and green structures.

Title 24 gives a high importance to keeping a minimum OA supply under all operating conditions; DCV HVAC system without this feature can save more energy, as long as they account for air pollutant concentrations. ASHRAE introduced the Indoor Air Quality Procedure (IAQP), which allows an OA supply below the minimum rate as long as pollutants are kept under control.

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