Cooling doesn't always mean running a compressor — sometimes the cheapest ton of cooling is the one you never mechanically make at all.
A mechanical cooling coil doesn't care what's happening outside. Whether the outdoor air is 100°F or 50°F, a compressor-driven refrigeration cycle has to run to pull heat out of the supply airstream, and every hour it runs costs real compressor energy. But an air handling unit has a second, much cheaper way to cool a building: when outdoor air is already at or below the condition the space needs, simply bringing in more of it does the job — no refrigeration cycle required. That's an air-side economizer, and getting the switch between the two modes right (or wrong) is one of the biggest levers on an HVAC system's energy bill.
Mechanical coolinguses a refrigeration cycle — compressor, condenser, evaporator (cooling coil) — to actively pump heat out of the supply airstream and reject it outdoors. It works regardless of outdoor conditions, which is exactly why it's the fallback: it's always available, but it always costs compressor energy to run.
An air-side economizeris a control strategy, not a piece of equipment — it modulates the AHU's outdoor air, return air, and exhaust air dampers so that when outdoor air conditions are favorable, the unit brings in far more outdoor air than the minimum ventilation requirement, up to 100%, instead of recirculating and mechanically cooling return air. If that outdoor air is already at or cooler/drier than the supply condition the space needs, moving it through the building does the cooling job directly — the only real energy spent is fan power to move the extra air, not compressor power to make the cooling. This is often called "free cooling," and it can substantially reduce or entirely eliminate compressor runtime whenever the weather cooperates.
The economizer switch has to be governed by a changeover control, and there are two common ways to implement it. A dry-bulb economizer controlcompares outdoor air's dry-bulb temperature to a fixed setpoint (commonly somewhere in the mid-60s °F) — simple, cheap, and common, but it looks at temperature alone and says nothing about how much moisture that air is carrying.
An enthalpy economizer control instead compares outdoor air's total heat content — enthalpy, which bundles dry-bulb temperature and humidity together — against return air's enthalpy, and only enables economizer mode when outdoor air is actually the lower-enthalpy (lower total heat) option. That distinction matters because cool-but-very-humid outdoor air can carry more total heat than return air even while its dry-bulb temperature looks perfectly inviting — the extra latent heat locked up in all that water vapor more than makes up the difference. A dry-bulb-only control has no way to see that and can enable economizer mode anyway, while an enthalpy control correctly keeps the dampers at minimum and lets the compressor handle it.
Total heat content in moist air is made up of sensible heat (tied to dry-bulb temperature) plus latent heat (tied to the moisture the air is carrying, its humidity ratio). A dry-bulb comparison only checks the sensible half. Air that's 10°F cooler but carrying dramatically more moisture can easily have more total heat once the latent component is added back in — which is exactly the enthalpy chart above showing outdoor air at a lower temperature than return air but on a higher isenthalp. An enthalpy economizer control (or, more precisely, a differential enthalpy control comparing both outdoor and return air enthalpy directly) reads both halves at once, which is why it correctly avoids economizer mode in humid-but-cool weather that a dry-bulb-only control would misread as an opportunity to save energy.
False, and it's the exact reason enthalpy-based economizer control exists. A dry-bulb temperature comparison alone ignores humidity entirely. Outdoor air that's cool but very humid can carry more total heat — enthalpy, factoring in the latent heat locked up in all that water vapor — than the building's return air, even though its temperature alone looks favorable. Bringing that air in doesn't save energy; it can actually increase the total cooling load, specifically the latent/dehumidification portion, forcing the cooling coil to work harder removing moisture than it would have removed heat from warmer, drier return air. This humidity blind spot is precisely why enthalpy-based (rather than simple dry-bulb-only) economizer control logic is required by energy codes in many humid climates — dry-bulb-only economizers are commonly restricted or disallowed there for exactly this reason.
Explains why an air handling unit sometimes cools a building by simply bringing in more outdoor air through its dampers (an air-side economizer, or 'free cooling') instead of running its compressor-driven mechanical cooling coil — and why the changeover control deciding when that's actually a good idea has to account for humidity, not just temperature, to avoid backfiring.
It's intuitive to assume that any time outdoor air is cooler than the building's desired indoor temperature, bringing more of it in must help. That intuition only accounts for sensible heat (dry-bulb temperature) and ignores latent heat (moisture content) entirely. Total heat content — enthalpy — is the sum of both, and cool-but-humid outdoor air can have a higher enthalpy than warmer, drier return air. A control strategy that only compares dry-bulb temperatures has no way to detect that case, and can enable economizer mode at exactly the wrong time.
Mechanical cooling uses a vapor-compression refrigeration cycle (compressor, condenser, evaporator/cooling coil) to actively pump heat out of the supply airstream and reject it outdoors — it works at any outdoor condition but always consumes compressor energy while running. An air-side economizer instead modulates the outdoor air, return air, and exhaust air dampers so a much larger fraction of outdoor air (up to 100%) is brought in directly, displacing recirculated return air; if that outdoor air is already at or below the desired supply condition, no mechanical refrigeration is needed to reach it, and the only added energy cost is fan power to move the larger volume of air.
A dry-bulb economizer control switches to economizer mode based on outdoor dry-bulb temperature alone versus a fixed setpoint — simple and inexpensive, but blind to humidity. An enthalpy economizer control instead compares outdoor air's total enthalpy against return air's enthalpy (or against a fixed enthalpy setpoint), correctly enabling economizer mode only when outdoor air actually represents less total heat to remove, including its latent (moisture) component.
This distinction is a standard energy-code requirement (ASHRAE 90.1 and adopted equivalents like IECC) in many climates, particularly humid ones, where dry-bulb-only economizer control is restricted or disallowed and differential enthalpy or fixed-enthalpy control is required instead. It's also a frequent root cause of unexplained humidity or cooling-load complaints in buildings with dry-bulb-only economizers — the system looks like it's 'saving energy' by running more outdoor air on a cool, humid day, while actually driving up the latent cooling load the coil has to remove.
An air-side economizer is a control strategy where an air handling unit's outdoor air, return air, and exhaust air dampers modulate to bring in more outdoor air than the minimum ventilation requirement — up to 100% — when outdoor conditions are favorable enough to help cool the building directly, reducing or eliminating the need to run mechanical (compressor-driven) cooling.
A dry-bulb control compares only outdoor air's dry-bulb temperature against a fixed setpoint to decide whether to enable economizer mode — simple, but blind to humidity. An enthalpy control compares total heat content (enthalpy, which combines temperature and humidity) of outdoor air against return air (or a fixed enthalpy setpoint), correctly identifying cases where cool-but-humid outdoor air actually carries more total heat than return air and should not be used for economizer cooling.
Yes. If the outdoor air is cool but very humid, its total enthalpy can exceed that of the building's return air even though its dry-bulb temperature is lower. Bringing in more of that air increases the latent (dehumidification) portion of the cooling coil's load, even as the sensible (temperature) portion looks favorable — a net increase in total cooling load despite the cooler-feeling outdoor temperature.
In humid climates, dry-bulb-only economizer control frequently misidentifies cool-but-humid outdoor air as favorable, increasing latent cooling load and energy use rather than reducing it. Energy codes such as ASHRAE 90.1 and adopted building energy codes address this by requiring differential enthalpy or fixed-enthalpy economizer control (or restricting dry-bulb-only control) specifically in climate zones where this humidity blind spot is common enough to cause real energy penalties.
Not always entirely, but often substantially. In full economizer mode with outdoor air conditions well below the supply air setpoint, the compressor can be fully unloaded or shut off, with the cooling coil doing little or no work. In partial economizer mode, the outdoor air fraction increases and reduces — but doesn't fully eliminate — the mechanical cooling the compressor still has to provide to reach the final supply air condition.
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