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An economizer's basic idea is simple enough, when outdoor air is cool enough to help cool a building, use it instead of running mechanical cooling. Deciding exactly when outdoor air actually qualifies as cool enough is where the real complexity lives, and ASHRAE Standard 90.1 recognizes six genuinely distinct ways of making that determination, each measuring something slightly different, each with real strengths and real limitations.
Why Six Different Strategies Exist At All
The core challenge an economizer control strategy has to solve is comparing outdoor air conditions against return air conditions and deciding whether bringing in more outdoor air genuinely reduces the cooling load, or whether it would actually increase it. The complicating factor is that cooling load depends on more than just temperature, humidity carries real energy too, as covered in the distinction between sensible and latent heat, which means a control strategy based purely on temperature can be fooled by humid air that feels, by the numbers, cool enough to use, while actually carrying a substantial latent load a temperature only measurement completely misses.
Fixed Dry Bulb Temperature Control
This is the simplest and least expensive strategy, comparing outdoor air temperature directly against a fixed setpoint, commonly in the range of fifty five to sixty five degrees Fahrenheit. If outdoor temperature is below that setpoint, the economizer is permitted to operate. It requires only a single temperature sensor, which is exactly why it remains the most common, lowest cost option in less humid climates, but it has a genuine, serious blind spot, it has no way of distinguishing dry, genuinely useful cool air from humid air that happens to share the same temperature but carries considerably more total heat content once humidity is accounted for.
Differential Dry Bulb Temperature Control
Rather than comparing outdoor temperature to a single fixed number, this strategy compares outdoor temperature directly against return air temperature, allowing economizer operation whenever outdoor air is genuinely cooler than the air already inside the building. This adapts more naturally to a building's actual current indoor conditions than a fixed setpoint does, but it shares the same fundamental blind spot as fixed dry bulb control, no humidity consideration at all, meaning it can still be fooled by humid outdoor air that happens to be cooler in temperature but isn't actually a genuine energy improvement once latent load is factored in.
Fixed Enthalpy Control
This strategy moves beyond temperature alone, using a single enthalpy sensor to measure outdoor air's total heat content, both sensible and latent combined, and comparing that measured value against a fixed enthalpy limit, commonly cited in a range around twenty eight Btu per pound. This directly addresses the humidity blind spot that both dry bulb strategies share, since enthalpy inherently accounts for moisture content, not just temperature.
Electronic Enthalpy Control
A variation using a different sensor technology to estimate enthalpy, generally considered functionally similar to fixed enthalpy control in terms of what it's actually trying to achieve, comparing a single measured outdoor air enthalpy value against a fixed threshold, distinguished mainly by its specific sensor implementation rather than a fundamentally different control logic.
Differential Enthalpy Control
This strategy compares outdoor air enthalpy directly against return air enthalpy, rather than against a single fixed threshold, allowing economizer operation whenever outdoor air genuinely carries less total heat than the air already inside the building. This is generally considered the most accurate and most adaptive of the enthalpy based strategies, since it responds to the building's actual current return air condition rather than a fixed assumption, though it requires two enthalpy sensors, one for outdoor air and one for return air, adding real cost and real maintenance burden compared to the single sensor strategies.
Dew Point and Dry Bulb Combination Control
The sixth recognized strategy combines a dry bulb temperature limit with a separate dew point based humidity limit, using a defined relationship between the two to determine permitted economizer operation. This approach aims to capture much of the accuracy benefit that comes from considering humidity directly, similar in spirit to the enthalpy based strategies, while using dew point specifically as the humidity related variable rather than a full enthalpy calculation.
Why This Variety Genuinely Matters, Not Just Technically
Each of these six strategies exists because no single approach is universally the correct choice. Dry bulb only strategies are the cheapest and simplest but carry real risk in humid climates. Enthalpy based strategies solve that humidity blind spot but cost more, require more sensors, and introduce their own maintenance burden, since a failed or poorly calibrated humidity sensor can actually make an enthalpy based strategy less reliable than a simpler dry bulb approach would have been in that specific failure condition.
ASHRAE Standard 90.1 doesn't simply recommend the most sophisticated option universally either, it restricts certain control types in certain climate zones specifically because some strategies genuinely perform poorly or unreliably in certain climate conditions, a distinction genuinely important enough to warrant its own separate, focused discussion.
The Practical Takeaway
An economizer's high limit control strategy, the actual logic determining when outdoor air qualifies as a genuine energy saving opportunity versus when it would actually increase cooling load, isn't a single standardized approach but a genuine choice among six recognized methods, ranging from simple, low cost dry bulb comparisons to more sophisticated, more accurate enthalpy based strategies. Understanding what each strategy actually measures, temperature alone, temperature against a building's own return air, or total heat content including humidity, is the foundation for understanding why climate, cost, and reliability all genuinely factor into which specific strategy makes sense for a given building and location.
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