Economizer
A damper section on a packaged rooftop unit that admits outdoor air for cooling instead of running the mechanical cooling system, modulating from a minimum ventilation position toward 100 percent outdoor air when a dry-bulb or enthalpy control strategy determines conditions allow it.
Also called air-side economizer, free cooling damper
What is an economizer on a rooftop HVAC unit?
An economizer is a damper section on a rooftop unit that admits outdoor air for cooling instead of running mechanical cooling, modulating toward 100 percent outdoor air whenever a stated dry-bulb or enthalpy threshold says outdoor conditions allow it.
Economizer: how it works
At minimum position, the economizer damper opens only enough to admit the calculated ventilation rate. When the control sequence's changeover strategy determines outdoor conditions favour it, the damper modulates toward full open, and the unit uses that outdoor air for cooling instead of running the compressor, provided the damper and outdoor air path are physically sized to carry the unit's full supply airflow at that position.
Where you meet the term
- On a packaged rooftop unit, as the damper section between the outdoor air intake and the mixed air plenum.
- In a sequence of operations, where the changeover strategy names the exact dry-bulb or enthalpy threshold that commands full position.
- In an energy audit, where an undersized damper opening caps achievable outdoor air fraction well below what the control sequence commands.
How to check it
- Confirm the damper is sized for full supply airflow, not only minimum ventilation. A damper sized adequately for the smaller minimum ventilation flow can still be a meaningful restriction once asked to carry the unit's full supply airflow during a free-cooling call. Technician
- Check the relief or return air path capacity. Air pulled in through the economizer has to leave the building somewhere. An undersized relief path limits achievable outdoor air fraction regardless of how generous the intake damper is. Technician
Economizer: what it is not
- An economizer's control strategy and its physical sizing are two separate checks. A correctly programmed changeover sequence still cannot deliver more outdoor air than the damper and duct path can physically carry.
- Enthalpy changeover is not automatically better than dry-bulb changeover. It depends on a working, correctly calibrated humidity sensor, and a poorly maintained one can perform worse than a simple dry-bulb strategy.
Economizer questions
What is the difference between dry-bulb and enthalpy changeover?
Dry-bulb changeover triggers free cooling from outdoor temperature alone. Enthalpy changeover also accounts for outdoor humidity, avoiding free cooling on a cool but humid day where outdoor air would add latent load.
Does an economizer replace the mechanical cooling system?
No. It supplements it during hours outdoor conditions allow, and the mechanical cooling system still operates the rest of the time, or alongside the economizer when outdoor air alone cannot meet the full load.
Why might an economizer look programmed correctly but still underperform?
The sequence of operations may command full position correctly while the damper's physical opening, or the relief air path, caps how much outdoor air actually reaches the space, which is a sizing issue the control logic alone cannot fix.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- ASHRAE Handbook, FundamentalsASHRAE reference volume carrying the psychrometric relationships, the climatic design condition tables and their annual percentile basis, duct and pipe fitting loss coefficients, and the bin and degree-day methods used for seasonal energy estimation.
- Standard 62.2, Ventilation and Acceptable Indoor Air Quality in Residential BuildingsASHRAE residential ventilation standard setting a whole-building mechanical ventilation rate from floor area and occupancy, defining local exhaust requirements for kitchens and bathrooms, and describing how measured envelope leakage is credited against the whole-building rate.