AHU Economizer Control Sequences

AHU Economizer Control Sequences

Air-side economizers provide free cooling by using outdoor air when conditions are favorable, reducing mechanical cooling energy.

Economizer Fundamentals

What is an Economizer?

An economizer modulates outdoor, return, and relief dampers to use outdoor air for cooling when it can offset mechanical cooling.

Basic Operation

When OA conditions favorable:
   1. Increase outdoor air beyond minimum
   2. Reduce return air proportionally
   3. Relieve excess air through exhaust
   4. Reduce/eliminate mechanical cooling

Economizer Types

Dry-Bulb Economizer

Uses outdoor air temperature only:

Control Point	Action
OAT < High Limit	Economizer enabled
OAT > High Limit	Minimum OA only

Typical high limit setpoints:

Climate Zone	High Limit
1A-2A (Hot humid)	65-70°F
2B-4B (Hot dry)	75°F
3C-5C (Marine)	75°F
4A-6A (Mixed/Cold)	70-75°F
7-8 (Cold)	75°F

Enthalpy Economizer

Uses outdoor air enthalpy (temperature + humidity):

If OA_Enthalpy < High_Limit_Enthalpy:
   Enable economizer
Else:
   Minimum OA only

High limit: 28-30 BTU/lb typical

Differential Economizer

Compares outdoor vs return air:

Differential Dry-Bulb:

If OAT < RAT - Offset:
   Enable economizer
   (Offset typically 1-2°F)

Differential Enthalpy:

If OA_Enthalpy < RA_Enthalpy:
   Enable economizer

ASHRAE 90.1 Requirements

When Economizers Required

Climate Zone	Required?	Type
1A, 1B	No	-
2A, 2B	Varies by size	Fixed or diff
3-8	Yes	See code

High Limit Settings (90.1 Table 6.5.1.1.3)

Device Type	Climate 1-3	Climate 4-8
Fixed dry-bulb	75°F	70°F
Fixed enthalpy	28 BTU/lb	28 BTU/lb
Differential dry-bulb	OAT < RAT	OAT < RAT
Differential enthalpy	OA h < RA h	OA h < RA h

Control Sequences

Modulating Economizer Sequence

Integrated economizer (with mechanical cooling):

Stage 1: Economizer Only
If cooling required AND OA conditions favorable:
   1. Modulate OA damper: Min → 100%
   2. Modulate RA damper: Max → 0%
   3. Mechanical cooling: Off
   4. Target: DAT setpoint

Stage 2: Economizer + Mechanical
If economizer at 100% AND still need cooling:
   1. OA damper: 100%
   2. Enable mechanical cooling
   3. Modulate cooling to maintain DAT

Stage 3: Mechanical Only
If OA conditions unfavorable:
   1. OA damper: Minimum position
   2. Mechanical cooling: Modulates

Damper Control Logic

Cooling Mode - Favorable OA:
   OA_Damper = PID(DAT, DAT_Setpoint)
   RA_Damper = 100% - OA_Damper
   Relief_Damper = Function of building pressure

Cooling Mode - Unfavorable OA:
   OA_Damper = Minimum_Position
   RA_Damper = 100% - Minimum_Position
   Relief_Damper = Closed (or building pressure control)

High Limit Lockout

Implementation

Economizer_Enable = FALSE

If Control_Type = "Fixed Dry-Bulb":
   If OAT < High_Limit_Temp:
      Economizer_Enable = TRUE

If Control_Type = "Fixed Enthalpy":
   If OA_Enthalpy < High_Limit_Enthalpy:
      Economizer_Enable = TRUE

If Control_Type = "Differential Dry-Bulb":
   If OAT < (RAT - Deadband):
      Economizer_Enable = TRUE

If Control_Type = "Differential Enthalpy":
   If OA_Enthalpy < RA_Enthalpy:
      Economizer_Enable = TRUE

Sensor Requirements

Control Type	Sensors Required
Fixed dry-bulb	OAT
Differential dry-bulb	OAT, RAT
Fixed enthalpy	OAT, OA RH (or enthalpy)
Differential enthalpy	OAT, OA RH, RAT, RA RH

ASHRAE Guideline 36 Sequences

Advanced Economizer Control

Freeze protection override:

If MAT < 38°F OR OAT < 38°F:
   Close OA damper to minimum
   Enable heating

Minimum OA control:

During occupied mode:
   Maintain minimum OA for ventilation
   (Per ASHRAE 62.1 calculations)

During unoccupied mode:
   OA damper may close to 0%
   (Building pressurization excepted)

Supply Air Temperature Reset

When economizer active:
   SAT_Setpoint may reset higher
   (e.g., 55°F → 60°F)
   Takes advantage of free cooling capacity

Commissioning and Testing

Functional Testing Points

Test	Expected Result
OAT below high limit	Economizer enables
OAT above high limit	Economizer disables
Call for cooling + favorable OA	OA damper modulates
100% OA insufficient	Mechanical cooling stages
Freeze condition	OA damper closes

Damper Stroke Verification

1. Command OA damper 0% → verify closed
2. Command OA damper 100% → verify open
3. Verify RA damper opposes OA damper
4. Verify relief damper operation

Sensor Calibration

Sensor	Tolerance
OAT	±1°F
RAT	±1°F
MAT	±1°F
Humidity	±3% RH

Common Problems

Economizer Not Enabling

1. Check OAT sensor location (in sun? near exhaust?)
2. Verify high limit setpoint
3. Check damper actuator operation
4. Verify enable signal from BAS

Economizer Hunting

1. Widen deadband
2. Slow down damper response
3. Check for air current affecting OAT sensor
4. Tune PID loop

Comfort Complaints During Economizer

1. Verify SAT setpoint appropriate
2. Check for direct sun on OAT sensor
3. Ensure adequate dehumidification when needed
4. Review changeover delay timing

Energy Savings Estimation

Variables Affecting Savings

• Climate zone (more hours = more savings)
• Internal loads (higher = more cooling hours)
• Occupancy schedule
• Equipment efficiency

Typical Savings

Climate	Annual Savings
Hot-humid	5-15%
Hot-dry	10-25%
Mixed	15-30%
Cold	10-20%

References

• ASHRAE Standard 90.1 - Energy Standard, Section 6.5.1
• ASHRAE Guideline 36-2021 - High-Performance Sequences
• ASHRAE Handbook - HVAC Systems - Economizer chapter