What it is

An economizer is a damper system on a rooftop unit that brings in outdoor air to cool the building when the outdoor air is cool and dry enough to do the job — instead of running the compressors. It's "free cooling": you're cooling with a fan and a damper instead of mechanical refrigeration. On the right day it can carry the whole cooling load with the compressors off, which is real money on a commercial electric bill. It's also, hands down, the most neglected and misunderstood component on an RTU — economizers fail constantly and quietly, and a stuck one wastes far more than it ever saved.

How it works

The economizer has two main dampers that move together in opposition: an outdoor-air damper and a return-air damper, driven by an actuator. When free cooling is available, the outdoor damper opens and the return damper closes proportionally, so the unit pulls in cool outdoor air and mixes it down to the right supply temperature. A barometric relief damper or power exhaust dumps an equal amount of building air so the building doesn't over-pressurize.

The brain is the economizer control, which decides whether outdoor air is "good enough" to use. That decision is the changeover:

  • Dry-bulb changeover: uses outdoor temperature only. If outdoor air is below a setpoint, free cooling is allowed. Simple and cheap, but it ignores humidity — it can let in cool-but-humid air that adds latent load.
  • Enthalpy changeover: uses outdoor total heat content (temperature + humidity), which is smarter, especially in humid climates, because it won't bring in muggy air just because it's cool.

When the controller says outdoor air is suitable and there's a cooling call, it modulates the dampers to hold a target mixed-air temperature (commonly around 55°F), adding mechanical cooling only if the economizer can't carry the load alone. When outdoor air isn't suitable, the dampers go to minimum position (just enough fresh air for ventilation) and the compressors do the cooling.

In the field

  • Minimum position: even when not free-cooling, the outdoor damper holds a small minimum opening to meet the building's ventilation/fresh-air requirement. That minimum is set during commissioning.
  • Free-cooling mode: dampers modulate to hold the mixed-air target; compressors stay off or supplement.
  • Mechanical-cooling mode: outdoor air not suitable → dampers to minimum, compressors run.
  • Checking it: drive the actuator through its range (many controllers have a checkout/test mode), confirm the dampers actually move full stroke and the linkage isn't broken, verify the sensors read sanely, and confirm the changeover behaves at the setpoint.

Normal values & targets

  • Mixed-air setpoint: commonly around 50–55°F target for the blended supply when economizing — verify the unit/design.
  • Dry-bulb changeover setpoint: often set somewhere around the high-50s to mid-60s °F outdoor, depending on the controller and climate — it's a setting, not a universal number.
  • Minimum outdoor-air position: set to the building's required ventilation rate at commissioning; too high wastes energy heating/cooling excess outdoor air, too low starves ventilation.
  • Enthalpy changeover is preferred in humid climates so cool-but-humid air doesn't get pulled in.

These are typical setpoints; always commission to the specific unit and the building's ventilation design.

Common faults & what they mean

  • Stuck open in cold weather: the #1 economizer failure. A seized actuator, broken linkage, or failed control leaves the outdoor damper open in winter, flooding the unit with cold air — freezes coils, overworks the heat, drives huge energy waste, and triggers "won't heat / freezing up" complaints. Always suspect the economizer on a winter RTU comfort call.
  • Stuck closed / minimum: unit never economizes, so you pay to run compressors on mild days when free cooling was available. Quiet money leak — nobody notices because it still cools, just expensively.
  • Failed actuator: dampers don't move or don't move full stroke. Common from age, weather, and roof exposure.
  • Bad or drifted sensor: outdoor/enthalpy or mixed-air sensor reads wrong, so the changeover decision is wrong — economizing when it shouldn't or vice versa.
  • Broken/disconnected linkage: actuator turns but dampers don't follow.
  • Barometric relief stuck or blocked: building over-pressurizes when economizing — doors hard to open, whistling.

Tech tips & gotchas

  • Check the economizer on EVERY RTU PM. It's the component most likely to be silently broken, and the failures cost the customer real money in both directions. Drive it full stroke and watch the dampers.
  • A winter no-heat or "freezing up" complaint on an RTU? Look at the economizer first. Stuck-open outdoor dampers in cold weather are a classic and the symptom mimics other problems.
  • Verify the sensors, not just the dampers. An economizer with perfect mechanics and a drifted sensor makes the wrong call all season.
  • Enthalpy beats dry-bulb in humid climates. If a dry-bulb economizer is bringing in muggy air and hurting comfort, that's the control type, not a malfunction — note it.
  • Don't disable a broken economizer and walk away. Some techs strap the damper shut to "fix" it — that kills the free cooling the customer paid for. Repair it.

Safety / code notes

  • Economizers tie into ventilation/fresh-air requirements (minimum outdoor air per the applicable mechanical code, e.g., IMC ventilation provisions) — disabling or wrongly setting minimum position can put the building out of compliance and starve occupants of fresh air.
  • A stuck-open economizer freezing a coil can lead to liquid floodback and compressor damage — address it, don't just clear the ice.
  • Verify building pressure relief works so the space doesn't over- or under-pressurize, which affects exhaust systems and combustion appliances elsewhere in the building.