Economizers Air Systems

The coil is not the only cooling an air handler owns. For a real slice of the year — mornings, shoulder seasons, whole mild weeks — the cheapest cooling in the building is sitting outside the intake hood, and the economizer is the sequence that goes and gets it: open the outside-air dampers past minimum, let cool outdoor air carry the load, and save the compressor or the chilled-water plant for weather that deserves them. Simple idea, famously easy to get wrong. One question, start to finish: when should an air handler cool with outside air instead of running the coil — and how do the dampers, the changeover check, and the minimum-OA floor make that decision safely?

One Signal, Three Dampers

Mechanically, an economizer is nothing new — it's the same three dampers from the mixing box, doing something more interesting. On a plain unit they sit at fixed positions; on an economizer they become one modulating assembly: as the outside-air damper drives open, the return-air damper closes to match, and the relief damper opens so the extra air has a way back out. One control signal moves all three, because they're three faces of a single decision — what fraction of the supply air should be outdoor air right now? The total airflow through the unit barely changes; what changes is the recipe.

And what drives that signal? A plain temperature loop. During free cooling the economizer modulates the blend to hold the mixed-air temperature at setpoint — typically the same 55 °F-ish number the cooling coil would otherwise be asked to make. Cool morning, mild load: the dampers find whatever blend of 55 °F outdoor air and 75 °F return air lands on setpoint, and the coil valve never opens. Two hard limits frame the modulation: the dampers never close below minimum position — the ventilation floor is not negotiable while the building is occupied — and at the top the "blend" is simply 100 % outdoor air. One promise to file for the next page: every extra cubic foot the OA damper admits has to leave the building somewhere, which is why the relief damper opens in step — and what happens when the relief path can't keep up is a page of its own, next in this chapter.

The Changeover Decision

Before any of that modulation is allowed to happen, a gate has to open: is outside air actually a better deal than return air right now? That's the changeover decision (you'll also hear high-limit lockout for its other half — the condition that slams the gate shut). The simplest version compares temperatures: economize when outdoor air is cooler than some limit — either a fixed number, or the return-air temperature itself (a differential dry-bulb changeover). One sensor the unit already has, no moving parts in the logic. For dry climates, honestly, it's fine.

Here's the trap: a thermometer only sees half the load. Air carries heat two ways — sensible heat you can measure with a dry-bulb sensor, and latent heat riding in its moisture, which the cooling coil must also remove before the space feels right. The property that counts both at once is enthalpy — total heat per pound of air (Psychrometrics Basics defines it properly). And humid air holds a shocking amount of heat in its moisture. Run the comparison on a muggy morning: outside air at 68 °F and 85 % RH against return air at 75 °F and 50 % RH. The outside air is 7 °F cooler — and carries about 1.8 Btu/lb more total heat. A dry-bulb changeover economizes on that air all morning. The dampers open, the "free cooling" pours in, the space gets muggier, the coil inherits a latent load it can barely keep up with, and the trends show cooling running flat out while the economizer insists it's helping.

An enthalpy changeover closes the trap: compare h_OA against h_RA (or a fixed enthalpy limit) and only economize when the outside air genuinely carries less total heat. In humid climates it's the difference between an economizer that saves energy and one that costs it. The price is a humidity sensor — two, for differential — and humidity sensors drift, which is the classic way a smart enthalpy economizer ends up underperforming a dumb dry-bulb one. Whichever flavor a unit runs, the shape is the same: the changeover is a gate above the loop. When it says no, the dampers ride at minimum no matter how hard the mixed-air loop wants to open them.

The shaded wedge is where dry-bulb lies to you — every state in it is cooler than the return on a thermometer and carries more total heat in fact. Notice what happens where the equal-heat line meets the saturation curve: left of that dry-bulb, no amount of humidity can make outside air a bad deal, because even saturated air carries less total heat than the return. That intersection is the escape hatch for buildings with no humidity sensor at all: do the psychrometrics once, on paper, and set the dry-bulb changeover limit at the worst case — for a 75 °F / 50 % return that lands near 62.4 °F. You give up the warm-but-dry mornings (the teal wedge), but the economizer can never be tricked into heating the building with "free cooling." Run your own streams through the enthalpy tab of the Economizer Ratio Helper, or walk them on the Psychrometric Chart.

Watch the Two Changeovers Disagree

Drag the outside air around and let both philosophies vote. Return air is pinned at 75 °F and 50 % RH; the enthalpy math is the same ASHRAE moist-air engine behind the psych tools. The interesting territory is where the two verdicts split — find it, then check what the worst-case limit line has been saying all along.

What the widget holds still: the return-air state (a real differential changeover compares against the live return, which moves with the space), sea-level pressure, and clean calibrated sensors — the exact assumption that fails most often in the field. The two verdicts are the two halves of the lesson: dry-bulb compares one property, enthalpy compares the whole load. When they disagree, the disagreement is the wedge from the sketch above.

First Stage of Cooling

Where does the economizer sit in the cooling sequence? First. When the unit needs cooling and the changeover gate is open, free cooling is stage one: the dampers modulate up from minimum, holding the mixed-air setpoint, and mechanical cooling stays off. Only when the dampers reach 100 % outdoor air and the load is still unmet does the compressor or chilled-water valve join in. You've already met the enable half of this logic as a wiresheet — the worked economizer-enable sheet in Function Blocks is the AND of "cooling call" and "changeover says yes." This page is what the enabled device actually does.

The chart shows the arrangement worth insisting on: integrated economizing, where mechanical cooling runs with the dampers wide open — the outdoor air keeps doing whatever share it can and the compressor only makes up the difference. Older and simpler units are non-integrated: the moment the first compressor stage starts, the dampers snap back to minimum, throwing away free cooling exactly when the building needs cooling most. If a unit you're commissioning does that, it's worth asking whether it's configuration or vintage. Staging mechanical cooling itself — lead/lag, deadbands, minimum run times — is the same discipline you met in Equipment Staging. And one more gate outranks everything here: if the high limit trips mid-sequence — the day heats up, the air goes muggy — the dampers ride back to minimum and mechanical cooling carries the whole load alone.

Where Economizers Fail in the Field

Economizers have a rough reputation in the field, and it's earned — study after study finds a huge share of them not economizing at all. The failure modes are worth knowing cold. Stuck dampers lead the list: a seized actuator, a broken linkage, a set screw that let go years ago. The tell is in the trends, and it's the same arithmetic as the mixing box: command the dampers and watch MA-T. Commanded to 100 % on a cool day with MA-T sitting on top of RA-T? The blades never moved. Commanded to minimum on a cold morning with MA-T tracking OA-T? Stuck open — and the heating coil is quietly paying for it all winter. The command is a wish; MA-T is the witness.

Hunting is the second act: dampers that swing instead of settling, usually a too-hot loop or a mixed-air sensor reading somewhere unrepresentative — the Air Handlers lesson's widget hides a war story about exactly that. And freeze protection is the floor under everything: free cooling has a limit, and on a frigid morning an economizer chasing a cool supply setpoint can drive the mixed air toward coil-bursting territory. The low-limit override — drive dampers toward minimum when MA-T drops too far, whatever the cooling logic wants — plus the freeze-stat hard trip are the "when safe" half of "use cool air when safe." A unit missing them isn't aggressive, it's uninsured.

That's the economizer, end to end: a linked damper assembly solving the mixing equation backwards, behind a changeover gate that had better count both kinds of heat, staged ahead of the mechanical cooling it exists to spare, and watched through MA-T because that sensor doesn't take sides. One thread left deliberately loose: all morning the relief damper has been quietly matching the outside-air damper, cubic foot for cubic foot. What happens to the building when that accounting slips — doors that won't shut, lobbies that whoosh, exhaust fans fighting the air handler — is the next page in this chapter: building pressure.