An economizer uses cool outdoor air, or cool water from a cooling tower or dry cooler, to cool a building instead of running mechanical refrigeration. In most Canadian cities, outdoor conditions are cool enough for a large share of the year that economizers can provide substantial free cooling, particularly in interior zones that need cooling even in winter.
Economizers are also among the most failure-prone parts of an HVAC system. Stuck dampers, failed sensors and bad control settings are very common, and many go unnoticed because the building stays comfortable while the energy is wasted. This guide covers how economizers work, how they are controlled and how they fail.
Airside economizers
An airside economizer modulates outdoor air, return air and relief dampers on an air handler or rooftop unit. When outdoor air is cool enough, the outdoor air damper opens beyond its minimum ventilation position, the return damper closes, and the relief damper or fan removes the excess air.
Operation typically follows this sequence:
- Economizer only: outdoor air dampers modulate to maintain the mixed or supply air temperature setpoint. Mechanical cooling is off.
- Integrated economizer: when outdoor air alone cannot meet the setpoint, mechanical cooling runs while outdoor dampers stay fully open.
- Minimum outdoor air: when outdoor air is warmer or more humid than return air, the economizer is locked out and dampers return to minimum ventilation position.
Integrated operation, where the economizer and mechanical cooling can run at the same time, gives significantly more savings than non-integrated designs, which lock out the economizer once compressors start. NECB and ASHRAE 90.1 generally require integrated economizers on systems that require economizers at all.
Waterside economizers
A waterside economizer uses a heat exchanger to cool chilled water with condenser water from a cooling tower or with glycol from a dry cooler, bypassing the chiller. It is common in buildings with chilled water plants, data centres and systems where airside economizers are impractical, such as fan coil or chilled beam systems. See waterside economizers and free cooling in Canada.
Energy code requirements
NECB 2020 and NECB 2017 require economizers on many air systems above a threshold capacity, with exceptions for certain system types and climate conditions. They also specify acceptable high-limit controls. Thresholds and exceptions differ between editions and between provinces that have adopted different editions, so confirm with the authority having jurisdiction. See NECB 2020 and HVAC compliance.
High-limit controls
The high-limit control decides when outdoor air is no longer beneficial and returns the dampers to minimum position.
| Control type | How it works | Notes |
|---|---|---|
| Fixed dry-bulb | Economizer enabled when outdoor temperature is below a fixed setpoint | Simple and reliable; setpoint depends on climate zone |
| Differential dry-bulb | Enabled when outdoor temperature is below return temperature | Adapts to building conditions; needs two accurate sensors |
| Fixed enthalpy | Enabled when outdoor enthalpy is below a setpoint, often combined with a dry-bulb limit | Accounts for humidity; relies on humidity sensors |
| Differential enthalpy | Enabled when outdoor enthalpy is below return enthalpy | Most theoretically accurate; most sensor-dependent |
Humidity sensors drift faster than temperature sensors, so enthalpy controls can perform worse than dry-bulb controls in practice if sensors are not maintained. In much of Canada, where summer humidity is moderate, dry-bulb controls are often a sound, reliable choice. In more humid regions such as southern Ontario, enthalpy-based controls can capture more savings or avoid bringing in humid air, provided the sensors are maintained. ASHRAE Guideline 36 sequences reflect these trade-offs.
Common failures
Field studies have repeatedly found that a large share of economizers do not work as intended. The usual failure modes are:
| Failure | Effect | How to detect |
|---|---|---|
| Outdoor damper stuck closed or linkage disconnected | No free cooling; compressors run in cool weather | Mixed air temperature equals return temperature when it should be cooler |
| Outdoor damper stuck open | Excess outdoor air; high heating cost, freeze risk | Mixed air temperature near outdoor temperature in cold weather |
| Failed or miscalibrated outdoor air sensor | Wrong economizer decisions | Compare with a reference sensor or weather data |
| Humidity sensor drift | Enthalpy control locks out wrongly | Compare readings against a calibrated instrument |
| High-limit setpoint wrong | Economizer disabled too early or too late | Review BAS settings against design and climate |
| Relief path inadequate | Building over-pressurized; outdoor damper cannot open fully | Doors hard to close, whistling, high building pressure |
| Actuator failed | Damper does not move | Command damper and observe |
| Minimum position wrong | Too much or too little ventilation | Measure outdoor airflow |
| Non-integrated sequence | Economizer locks out when compressors run | Review sequence and trends |
Finding economizer problems
The simplest check uses three temperatures: outdoor air (OAT), return air (RAT) and mixed air (MAT). The outdoor air fraction is approximately (MAT minus RAT) divided by (OAT minus RAT). Comparing this fraction with the damper command reveals problems quickly.
A practical field check:
- Record outdoor, return and mixed air temperatures and the damper command.
- On a cool day with the economizer active, command dampers to 100% outdoor air and confirm mixed air approaches outdoor temperature.
- Command dampers to minimum and confirm mixed air moves toward return temperature.
- Visually confirm all damper blades move and seal.
- Check sensor accuracy against a calibrated reference.
- Review the high-limit setting and the integrated sequence in the BAS.
Trend data makes this much easier. A few weeks of trends of these points, reviewed against the sequence, reveals most economizer faults. Fault detection and diagnostics software can flag economizer problems automatically.
Canadian winter considerations
Economizers in Canada must handle very cold outdoor air. Key concerns include:
- Freeze protection: poor mixing can send streams of sub-zero air across coils. See mixing boxes, stratification and freeze protection.
- Humidity: running large amounts of very dry winter outdoor air increases humidification load.
- Snow and ice: outdoor air intakes need snow-resistant louvers or hoods, and dampers can freeze shut.
- Low-limit control: mixed air low-limit controls prevent coils from seeing air below a safe temperature.
Frequently asked questions
Should economizers run in winter?
Yes, where the building has cooling loads in winter, such as interior zones. Using outdoor air is usually far cheaper than mechanical cooling, as long as freeze protection and humidity are managed.
How often should economizers be checked?
At least annually, and ideally continuously through BAS trends or FDD. A retro-commissioning study typically includes a full economizer check.
Key takeaways
- Economizers provide free cooling using outdoor air or cool water, valuable in Canada for much of the year.
- Integrated operation and the right high-limit control are essential for savings.
- Humidity sensor drift makes enthalpy controls less reliable than they appear on paper.
- Stuck dampers, failed sensors and wrong settings are common and often go unnoticed.
- Check economizers with outdoor, return and mixed air temperatures, and monitor them with trends or FDD.
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