Commercial Economizer Exceptions: A Practical Title 24 Guide
Each cooling air handler over 33,000 Btu/hr needs an economizer unless one of eight exceptions in Section 140.4(e)1 applies. The usual ways out are higher cooling efficiency under Table 140.4-F, hotel guestrooms, computer rooms, special filtration, or a small unit served by a heat-recovery outdoor air system.
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Check the exceptions in this order
- Capacity. Read each air handler's design cooling capacity from the schedule. At 33,000 Btu/hr or less, no economizer is required.
- Use. Systems serving hotel or motel guestrooms are excepted. So are fan systems primarily serving computer rooms, which follow the economizer rule in Section 140.9(a).
- Process conflicts. Special filtration or treatment for unusual contaminants, outdoor air cooling that would raise overall building long-term system cost by upsetting humidification, dehumidification or supermarket refrigeration, and carbon dioxide enrichment in horticulture spaces.
- System type. An air handler below 54,000 Btu/hr served by a dedicated outdoor air system with exhaust air heat recovery (Sections 140.4(p) and (q)) qualifies if the DOAS provides at least 0.3 cfm per square foot during economizer conditions. VRF heat pump and four-pipe fan coil systems with DOAS under Section 140.4(a)3 are excepted as well.
- Buy it out. Table 140.4-F, below.
The efficiency trade-off
Table 140.4-F sets the cooling efficiency improvement that replaces the economizer: 70 percent in Climate Zones 1, 5 and 16, 65 percent in Zones 2, 3 and 4, and 30 percent in Zones 6 to 15. A unit rated with a part-load metric raises only its applicable minimum cooling efficiency by that percentage, and one rated only on EER or COP raises that metric. Where the rating is energy input over work output, convert to COP, multiply, then convert back.
If the economizer stays
The Table 140.4-G fixed dry-bulb high limit is 75 degrees F in Zones 1, 3, 5 and 11 to 16, 73 in Zones 2, 4 and 10, 71 in Zones 6, 8 and 9, and 69 in Zone 7; differential dry-bulb is also listed. Dew point, electronic enthalpy and differential enthalpy controls need Executive Director approval. The economizer must not raise heating energy and must give partial cooling, and DDC sequences follow ASHRAE Guideline 36. One NRCA-MCH-05-A covers each system, as the economizer requirements page notes.
A replacement has one more route: the 54,000 Btu/h exception on the small commercial replacement page. See also rooftop unit questions and heat pump replacement. Commercial reports are quoted per project.
Common Title 24 Questions
- When does a commercial project need Title 24?
- Does an office tenant improvement need Title 24?
- Does a restaurant remodel need Title 24?
- Does a retail tenant improvement need Title 24?
- What plans are needed for a commercial Title 24 report?
- What is an NRCC certificate of compliance?
Frequently Asked Questions
Not under Section 140.4(e)1. Exception 4 removes the requirement for comfort cooling systems that meet or exceed the table percentage, so the equipment efficiency has to be shown on the schedule.
Choosing a heat pump gives none by itself. Section 140.4(e) applies to air handlers by cooling capacity, so the exceptions above are what matter.
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