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ASHRAE 90.1 Economizer High-Limit Shutoff Selection

Reference Compliance Updated Jun 19, 2026 ยท HVAC

Why this matters

The economizer high-limit setpoint determines when the outside-air damper shuts back to minimum and the unit returns to compressor cooling. Set it wrong and you either burn unnecessary compressor kWh in shoulder seasons (limit too low) or import hot, humid air into a building running cold-coil compressor cooling (limit too high). ASHRAE 90.1-2022 Table 6.5.1.1.3 sets climate-zone-specific limits with prescriptive values, and inspectors will pull RTU controllers during energy-code review on commercial new construction.

High-limit control types

Five control types are recognized by the standard. Fixed dry-bulb: economizer disables when OAT exceeds a fixed setpoint. Differential dry-bulb: economizer disables when OAT exceeds return-air dry-bulb (always at least the RAT). Fixed enthalpy: economizer disables when OA enthalpy exceeds a fixed value, typically 28 Btu/lb. Differential enthalpy: economizer disables when OA enthalpy exceeds RA enthalpy. Electronic enthalpy: a curve in OAT/RH space that approximates differential enthalpy without a return sensor. Each has a climate-zone-specific maximum allowed setpoint in Table 6.5.1.1.3.

Climate-zone limits per Table 6.5.1.1.3

  • Zones 1B, 2B, 3B, 3C, 4B, 4C, 5B, 5C, 6B, 7, 8: fixed dry-bulb allowed at 75 F maximum.
  • Zones 1A, 2A, 3A, 4A, 5A, 6A: fixed dry-bulb is NOT allowed; differential dry-bulb is permitted with no fixed limit, fixed enthalpy at 28 Btu/lb, or differential enthalpy.
  • All zones: fixed enthalpy at 28 Btu/lb is permitted as a fallback.

The reasoning: humid east-coast zones (the A series) routinely see 80 F at 75 percent RH where dry-bulb suggests free cooling but enthalpy says no. A fixed dry-bulb economizer in 4A pulls in 75 F air at 30 grains, then a 50 F supply set spends serious latent capacity drying it. The standard prohibits this combination.

Selecting the right type for the job

For climate zones 1A through 6A (humid), specify fixed enthalpy at 28 Btu/lb if the building has a single sensor or differential enthalpy if the economizer controller supports two sensor inputs. For dry zones (1B/2B/3B/4B/5B/5C/6B/7/8), fixed dry-bulb at 70 to 75 F is fine and avoids the calibration drift inherent in RH sensors. For mixed climates 3C and 4C (Pacific coast), fixed dry-bulb works because the dry-bulb peak rarely coincides with high humidity.

Setpoint values worth holding

For fixed dry-bulb in zones that allow it, 70 F is conservative and aligns with most RTU OEM defaults; the standard caps at 75 F. For differential dry-bulb, no fixed setpoint; the controller compares OAT to RAT continuously. For fixed enthalpy, 28 Btu/lb is the code maximum; some designers pull to 26 Btu/lb to add margin in coastal zones. For electronic enthalpy A/B/C/D curves (Honeywell W7220 lineage), curve A is roughly equivalent to fixed enthalpy at 28 Btu/lb; curve D is the lowest cutoff and most conservative.

Sensor placement and calibration

Outside-air sensors must be in shade, out of supply-air mixing, and at least 12 inches off any exhaust louver. Mount on a north-facing wall or under a stand-off shield; a sensor in afternoon sun reads 8 to 15 F high and the economizer cuts out hours before it should. RH sensors drift by 1 to 3 percent per year; ASHRAE 90.1 Section 6.4.3.4.7 requires recalibration or replacement on a periodic basis tied to the equipment's commissioning interval. Return-air sensors for differential control mount in the mixed return, not in a duct branch where supply leakage can hit them.

Commissioning verification

For new installations, run an OA damper command override at outdoor conditions just below and just above the limit. Verify damper strokes to minimum at the cutoff and reopens on falling OA below the cutoff plus a 2 to 3 F deadband. Log OAT, RAT, mixed-air temp, supply-air temp, and damper position for at least 48 hours through a shoulder-season swing. The Title 24 acceptance test (NA7.5.4 for California adopters) is the most rigorous template; even non-California jobs benefit from following it.

References

  • ASHRAE Standard 90.1-2022, Section 6.5.1 Economizers, Table 6.5.1.1.3 High-Limit Shutoff Control Settings
  • ASHRAE Standard 169-2021, Climatic Data for Building Design Standards
  • 2022 California Title 24 Part 6 NA7.5.4 Air Economizer Controls Acceptance Test
  • Honeywell W7220 Economizer Module Application Guide
  • Trane Engineers Newsletter ENEWS-43-3 Air-Side Economizer High-Limit Controls

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