What is an air handler actually doing when it "economizes"?

Opening the outside-air dampers past minimum so cool outdoor air carries the cooling load, with mechanical cooling held back.. Economizing is free cooling: when outdoor air is a better deal than return air, the damper assembly modulates past its ventilation minimum and lets outside air do the coil's job, saving the compressor or chilled-water plant for weather that needs them. Everything else on this page — the changeover gate, the staging, the failure modes — hangs off that one move.

During economizer operation the outside-air, return-air, and relief dampers move together as one linked assembly. Why?

They answer one question — what fraction of supply air should be outdoor air — so as OA opens, RA closes to match and relief opens so the extra air can leave.. The three dampers are three faces of a single decision. More outdoor air in means less return air reused — and that displaced air has to exit the building, which is the relief damper's job. One signal drives the set so the recipe changes while total airflow through the unit stays roughly constant. What happens when the relief half of that bargain fails is the building-pressure story, next in the chapter.

A mild morning: 55 °F (12.8 °C) outside, return air at 75 °F (23.9 °C), and the economizer loop wants 60 °F (15.6 °C) mixed air. What outside-air percentage do the dampers need? Enter the answer in %.

75 %. The modulation is the mixing equation run backwards: % OA = (MA − RA) ÷ (OA − RA) × 100 = (60 − 75) ÷ (55 − 75) × 100 = 75 % (in °C: (15.6 − 23.9) ÷ (12.8 − 23.9) × 100 ≈ 75 %). Three-quarters outdoor air, and the cooling coil never opens. The Economizer Ratio Helper runs this calculation, including the cases where the answer comes back impossible.

While an economizer is free-cooling, what is its damper loop actually controlling?

The mixed-air temperature — the dampers find whatever blend lands on setpoint.. Free cooling is a plain temperature loop: modulate the blend until mixed air hits setpoint — typically the same 55 °F-ish (12.8 °C) number the cooling coil would otherwise be asked to make. OA-T is an input to the decision, not the controlled variable; the loop watches MA-T. That's also why MA-T is the sensor that exposes economizer failures: it's the one the loop lives and dies by.

A humid summer morning after overnight rain. The space is warm and sticky, and the occupants are complaining. The BMS shows the readings below. What's the story? OA-T 68.2 °F (20.1 °C) RA-T 74.9 °F (23.8 °C) MA-T 68.5 °F (20.3 °C) OA DAMPER CMD 100 % — economizing CLG COIL VALVE 100 % SA-T 58.9 °F (14.9 °C) SPACE 75.8 °F (24.3 °C) and rising

The dry-bulb changeover is admitting humid air that's cooler on the thermometer but carries more total heat than the return — the economizer is importing load.. Every reading is consistent — and this time the witnesses prove it, not just the commands: MA-T sitting on top of OA-T shows the damper really is open, and an SA-T of 58.9 °F (14.9 °C) — a valve at 100 % still missing a 55 °F (12.8 °C)-class leaving-air target — shows the coil genuinely working and genuinely losing. The problem is the decision itself. Near-saturated air at 68.2 °F (20.1 °C) carries more total heat than 74.9 °F (23.8 °C) return air at indoor humidity — a dry-bulb sensor can't see the moisture, so the unit hauls in latent load all morning and pays the coil to remove it. The fix is the changeover, not the coil: an enthalpy high limit, or a worst-case dry-bulb limit if the building has no humidity sensor.

An enthalpy changeover approves free cooling when h_OA < h_RA. What is it actually comparing?

Total heat per pound of air — sensible temperature and the latent heat riding in the moisture, counted together.. Enthalpy is the property that counts both loads at once: the sensible heat a thermometer sees plus the latent heat in the water vapor, which the cooling coil must also remove. Comparing h_OA against h_RA asks the only question that matters — will this air, taken as a whole, reduce the load or add to it? That's why enthalpy changeover wins in humid climates, at the price of humidity sensors that need to be kept honest.

A building has no outdoor humidity sensor — dry-bulb is all the changeover will ever have. What's the defensible way to set it in a humid climate?

Set the limit at the worst case: the dry-bulb where even saturated outside air carries less total heat than the return.. Do the psychrometrics once, on paper: find the dry-bulb temperature where even 100 %-RH outside air matches the return's total heat — 62.4 °F (16.9 °C) for a 75 °F (23.9 °C) / 50 % return — and set the changeover there. Below that line, no amount of humidity can make outside air a bad deal. You give up some warm-but-dry free-cooling hours, but the economizer can never be tricked into importing load it claims to be removing.

On an "integrated" economizer, what happens when 100 % outside air still can't meet the cooling load?

Mechanical cooling stages on while the dampers stay wide open — outdoor air keeps carrying its share and the compressor makes up the difference.. Integration is the difference between an economizer that helps all day and one that quits at the first compressor start. Non-integrated units force an either/or: the moment mechanical cooling starts, the dampers return to minimum — throwing away free cooling exactly when the building wants cooling most. Integrated sequencing keeps the outdoor air working and staged mechanical cooling only tops up the remainder.

When the changeover high limit trips mid-morning — the day heats up, the air goes muggy — the economizer drives its outside-air dampers fully closed.

False. False — the dampers ride back to minimum position, not closed. The ventilation floor is not negotiable while the building is occupied: the code-required fresh-air dose still has to come in, even when that air costs cooling energy. The high limit ends free cooling; it doesn't end ventilation. Fully closed is a fault state (or an unoccupied mode), never a changeover response.

You suspect an economizer's dampers are seized. It's a cool day. Which trend evidence proves the blades never actually moved when commanded to 100 %?

MA-T stays sitting on top of RA-T after the command — the mix is still almost all return air.. The command is a wish, and even actuator feedback only proves the motor turned — a stripped coupling or broken linkage leaves the blades behind. MA-T is the witness that can't be argued with: on a cool day, 100 % outside air must drag the mixed-air temperature toward OA-T. If MA-T keeps reading like return air, the blend never changed, so the blades never moved. Same arithmetic as the mixing box, doing forensic duty.

Practice · Economizers

Economizers Quiz Air Systems

Q: An enthalpy changeover approves free cooling when h_OA < h_RA. What is it actually comparing?

Total heat per pound of air — sensible temperature and the latent heat riding in the moisture, counted together.. Enthalpy is the property that counts both loads at once: the sensible heat a thermometer sees plus the latent heat in the water vapor, which the cooling coil must also remove. Comparing h_OA against h_RA asks the only question that matters — will this air, taken as a whole, reduce the load or add to it? That's why enthalpy changeover wins in humid climates, at the price of humidity sensors that need to be kept honest.

Q: A building has no outdoor humidity sensor — dry-bulb is all the changeover will ever have. What's the defensible way to set it in a humid climate?

Set the limit at the worst case: the dry-bulb where even saturated outside air carries less total heat than the return.. Do the psychrometrics once, on paper: find the dry-bulb temperature where even 100 %-RH outside air matches the return's total heat — 62.4 °F (16.9 °C) for a 75 °F (23.9 °C) / 50 % return — and set the changeover there. Below that line, no amount of humidity can make outside air a bad deal. You give up some warm-but-dry free-cooling hours, but the economizer can never be tricked into importing load it claims to be removing.

Q: On an "integrated" economizer, what happens when 100 % outside air still can't meet the cooling load?

Mechanical cooling stages on while the dampers stay wide open — outdoor air keeps carrying its share and the compressor makes up the difference.. Integration is the difference between an economizer that helps all day and one that quits at the first compressor start. Non-integrated units force an either/or: the moment mechanical cooling starts, the dampers return to minimum — throwing away free cooling exactly when the building wants cooling most. Integrated sequencing keeps the outdoor air working and staged mechanical cooling only tops up the remainder.

Q: When the changeover high limit trips mid-morning — the day heats up, the air goes muggy — the economizer drives its outside-air dampers fully closed.

False. False — the dampers ride back to minimum position, not closed. The ventilation floor is not negotiable while the building is occupied: the code-required fresh-air dose still has to come in, even when that air costs cooling energy. The high limit ends free cooling; it doesn't end ventilation. Fully closed is a fault state (or an unoccupied mode), never a changeover response.

Q: You suspect an economizer's dampers are seized. It's a cool day. Which trend evidence proves the blades never actually moved when commanded to 100 %?

MA-T stays sitting on top of RA-T after the command — the mix is still almost all return air.. The command is a wish, and even actuator feedback only proves the motor turned — a stripped coupling or broken linkage leaves the blades behind. MA-T is the witness that can't be argued with: on a cool day, 100 % outside air must drag the mixed-air temperature toward OA-T. If MA-T keeps reading like return air, the blend never changed, so the blades never moved. Same arithmetic as the mixing box, doing forensic duty.

Read the Economizers lesson first, then make the free-cooling decision from memory — ten questions down the lesson walk: what economizing actually is, why the three dampers move as one set, the mixing math run backwards, the deceptive air a dry-bulb changeover admits, the no-humidity-sensor fallback, integrated staging, and the trend evidence that convicts a stuck damper. Each reveal links back to the section it came from.

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