Economizer Control Sequence: Setpoints vs. Limits in VAV Air Handlers
- Resolute Team
- Jun 16
- 9 min read
Updated: Jun 17
When control logic accumulates over time, every individual decision can look correct, while the system as a whole quietly stops doing its job.
Engineering Field Insight · Ed Pfannes · Senior Engineer, Resolute Building Intelligence
Executive Summary
Many commercial air handlers are capable of economizing, but control logic often prevents them from using outside air effectively. This article explains why mixed-air temperature should usually act as a safety limit, not a competing control setpoint, and how better economizer logic can reduce mechanical cooling load while protecting coils and humidity control.
Common Problems In An Economizer Control Sequence
ASHRAE 90.1 is clear on this: buildings of a certain size and climate zone should be able to economize. That means when outdoor conditions are favorable, the air handler brings in more outside air and uses it for free cooling, no compressors, no chilled water, just the outside air doing the work. It's one of the most straightforward energy-saving strategies in the industry.
And yet, in building after building, economizers are either running incorrectly or not running at all, not because something is broken, but because the control sequence is fighting itself.
The Cutoff Problem: Stopping Too Early
Here's a common setup. An air handler is designed to deliver 55°F discharge air temperature at peak conditions. The controls are programmed to allow economizer operation when outside air is 55°F or below, and to disable it the moment outside air climbs above that.
The logic behind this is understandable: if it's warmer than the target outside, the economizer can't fully do the job, so why run it? But that reasoning misses something important. Even when outside air can't fully satisfy the discharge-air setpoint by itself, it's still reducing the cooling load. You still need less from the chiller. You're still getting something for free.
“There should be what I'd call a hybrid mode, where you're economizing and mechanically cooling together. If the economizer is running and not quite meeting discharge air temperature setpoint, you're still reducing the load on the chillers. A lot of air handlers are set up not to do that.” - Ed Pfannes
In climate zone 5A, which covers much of the Midwest, dry-bulb cutoff should be 70°F — not 50 or 55. Some systems doing enthalpy-based control can push that to 75°F . Cutting off economizer operation at 55°F in this region means leaving free cooling on the table for a significant portion of the year.
The Mixed-air Temperature Setpoint Problem
Even when an economizer is allowed to run, a second problem often limits it before it should be limited: the mixed-air temperature setpoint.
A VAV air handler has one job: maintain discharge-air temperature setpoint to satisfy the downstream zones, the VAV boxes. That's the outcome. Everything else in the sequence exists to support that goal. So why do so many air handlers also have a mixed-air temperature setpoint?
Here's what typically happens. The discharge-air setpoint is 55°F. The mixed-air temperature setpoint is also 55°F. At first glance, that seems logical, if you want 55°F out, aim for 55°F in the middle too, right?
But the fan motors add heat between mixed air and discharge air temperatures. And that heat isn't fixed. A VAV system varies in airflow, which varies in fan speed, which varies in energy consumption and heat output. At any given moment, the delta T across the fan section might be 3°F, or it might be 7°F. It's never consistent.
So if mixed air temperature is controlled to 55°F and the fan adds even 4°F, discharge air temperature comes out at 59°F. The discharge-air temperature control loop says it's not at setpoint. It calls for more cooling. The chilled water valve opens, even though the economizer is already running. Now you've got a partially open economizer and an active chilled water control valve, when the AHU should be in full economizer mode before the chilled water control valve ever opens.
“You've got three things in a small area all doing exactly what they were told to do — and they're not working together. It never satisfies. It just keeps fighting.” - Ed Pfannes
There's another version of this problem: sometimes when this happens, there isn't even chilled water available. It's 50°F outside, the chilled water plant has been disabled, and the system is calling for mechanical cooling that doesn't exist. The air handler chases a temperature it can't hit, for reasons that trace back to a mixed-air temperature setpoint that shouldn't be there.
SetPoints vs. Limits: The Distinction That Matters
Here's the core idea, and it's worth being precise about it.
Setpoint Control toward this The system actively modulates outputs to hit this value. Discharge-air temperature is a setpoint, it's what the air handler is always working to achieve. | Limit Do not cross this The system operates freely until this boundary is approached. Mixed-air temperature belongs here, a low-limit of around 45°F protects the coil from freezing without competing with the discharge-air temperature setpoint goal. |
Mixed-air temperature absolutely needs to be monitored. If outside air is cold enough, you can drive mixed air temperature dangerously low, coils freeze, and when coils freeze, they burst. That's a real and expensive problem. But the right response to that risk is a low limit, not a full control target. "Don't let mixed air temperature fall below 45°F" is a guardrail. "Control mixed air temperature to 55°F" is a competing goal that creates the fight described above.
The same logic applies to other setpoints you sometimes see stacked into air handler sequences, leaving preheat coil temperature setpoints, leaving cooling coil temperature setpoints. What do any of these actually serve? The air handler has one temperature outcome that matters: discharge air temperature. If additional temperatures sensors need to be utilized, use them for monitoring purposes or for troubleshooting. Don't make them targets.
Why These Setpoints Exist In The First Place
Control contractors aren't adding mixed-air set points to cause problems. They're adding them because they don't want to be responsible for a frozen coil. They've always had a mixed-air set point on every unit they've commissioned. Removing it feels like removing a safety net.
The result, are systems where logic layers on top of logic. When you have setpoints in series you create conflict. Control programmers will try to make adjustments to resetting the other setpoint in order to match the discharge air temperature setpoint, but usually do not meet the needs of what the AHU discharge air temperature setpoint is set to at the time.
Keep it simple, stupid. When you add setpoints thinking you're being conservative, you're actually making it more complicated — and the system stops working toward what it actually needs to do. - Ed Pfannes
The Sensor Problem Underneath The Control Problem
There's another layer here that's easy to overlook. Even if the control sequence is right, it can fail if the mixed-air temperature sensing is wrong, and mixed-air temperature sensing is often wrong.
A single temperature probe placed somewhere in the mixed-air plenum is measuring one point in the airstream. Outside air and return air don't mix evenly. They stratify. Warm return air may float to the top while cold outside air hugs the bottom, and they travel in those separate layers all the way to the coil. If the probe happens to be sitting in the middle reading an average, it might show 52°F while part of the coil face is seeing air well below freezing.
Field example A Resolute engineer visiting a healthcare site found exactly this situation. The averaging bulb was installed in the wrong orientation — measuring left-to-right across the plenum when the stratification was actually running top-to-bottom. Once the bulb orientation was corrected, the system was finally reading what was actually the ave. mixed air temp airstream. Before that fix, the control system thought it was fine. Part of the coil was not fine. |
An averaging bulb, a serpentine sensor that runs across the full face of the mixed-air plane, gives a more accurate picture. But it only works if it's installed perpendicular to the direction of stratification. If outside air is coming in from the side and return air from the top, the stratification layers run vertically, and the bulb needs to run horizontally. Installing it vertically because that's how the last unit was done is a mistake that the data won't make obvious until something goes wrong.
And if the mixing plenum isn't actually mixing, which is common, because low-velocity plenums create laminar flow rather than the turbulence needed to blend airstreams, even a correctly installed averaging bulb may be reading an accurate average of a poorly mixed plenum. The sensor can be correct while the physical situation is dangerous.
The Humidity Objection, Addressed Directly
The most common pushback against extended economizer operation is humidity. If you bring in outside air on a foggy, 65°F day at 100% relative humidity, aren't you going to make the building cold and clammy? Won't you get mold?
The concern is understandable, but it's often aimed at the wrong cause. If the chilled water coil is driving discharge air temperature down to 55°F, that moisture is condensing on the coil and the discharge air relative humidity when at 55 deg. F whether coming from the outdoors or from mechanical cooling is usually around 100%, but space relative humidity remains about 50% or less because of the sensible heat it picks up it the space.
The situation where humidity actually becomes a problem is different: it happens when chilled water supply temperature gets reset upward to save energy on mild days. You're still maintaining building dry-bulb temperatures, but you've reduced latent cooling, the system's ability to remove moisture from the air. Temperature looks right. Relative humidity climbs anyway.
Field example A school had mold growing on ceiling tiles. The building felt cold and clammy. The facility director confirmed they had recently implemented a chilled water supply temperature reset strategy to save energy. With a high-occupancy building introducing significant volumes of outside air and picking up latent heat from the occupants as well, the system needed to dehumidify even when it didn't need to cool as much. Raising the chilled water temperature removed that capability. The fix wasn't to disable the economizer, it was to monitor return air humidity and hold chilled water temperature low enough to maintain dehumidification when needed. |
If humidity is a genuine concern, the right answer is monitoring, not limiting the economizer. Watch return air humidity. If it climbs above 50%, you can respond: drive down discharge-air temperature to increase latent cooling, or, as a last resort, override the economizer. But make that decision based on actual humidity data, not on a fear that hasn't materialized.
Enthalpy vs. Dry-bulb: A Note On Sensor Reliability
There are a few different strategies for economizer control logic. Dry-bulb temperature is the simplest, just outside air temperature compared to a cutoff. Comparative enthalpy is more sophisticated: compare the enthalpy of outside air to return air enthalpy, and use outside air when its enthalpy is lower.
Enthalpy-based control is more precise because it accounts for both temperature and moisture content. But it depends on humidity sensors, and humidity sensors drift. When a humidity sensor goes out of calibration, the enthalpy calculation goes with it, and you may not notice for a long time. Dry-bulb sensors are simpler and more consistently reliable.
More advanced control logic isn't automatically better. It's only better if the sensors supporting it are accurate. If they're not being verified and recalibrated, enthalpy control can make decisions based on numbers that stopped being true months ago.
What This Looks Like When It's Working
An air handler running a well-designed sequence for a VAV system looks like this: it has a discharge-air temperature setpoint, reset based on what the downstream zones are actually requesting. The economizer runs whenever outdoor conditions allow, all the way up to the appropriate dry-bulb cutoff, not a conservative 55°F, but the climate-appropriate limit. When outside air alone can't meet the discharge-air setpoint, mechanical cooling supplements, and the economizer stays open to reduce the mechanical cooling load as much as it can. Mixed-air temperature is monitored with a properly installed averaging bulb, with a low limit in place to protect the steam and hydronic coils.
No mixed-air temperature setpoint. No leaving coil temperature setpoint. No stacked control goals. One target, with appropriate limits as guardrails.
Want to know whether your economizers are actually reducing cooling load? Resolute helps facility teams turn building automation data into clear, prioritized action.
What is an economizer in HVAC?
An economizer uses favorable outside air conditions to reduce the need for mechanical cooling. When outdoor air is cool enough, the air handler can bring in more outside air and reduce chiller or compressor load.
Why should mixed-air temperature be a limit instead of a setpoint?
Mixed-air temperature should usually act as a safety guardrail to prevent coil freezing. When it becomes a control target, it can compete with discharge-air temperature control and cause the system to fight itself.
What is the difference between a setpoint and a limit in HVAC controls?
A setpoint is a value the system actively controls toward. A limit is a boundary the system should not cross. Discharge-air temperature is typically the setpoint, while mixed-air temperature should often be used as a low-limit safeguard.
Can an economizer and mechanical cooling run at the same time?
Yes. In many systems, a hybrid mode is useful. If outside air cannot fully meet the discharge-air temperature setpoint, the economizer can still reduce the cooling load while mechanical cooling supplements the remaining need.
Why do economizers stop working correctly?
Economizers often underperform because of conservative cutoff temperatures, stacked control setpoints, incorrect mixed-air sensing, poor sensor placement, or control sequences that disable free cooling too early.
How can building analytics help identify economizer problems?
Building analytics can compare outdoor air temperature, damper position, mixed-air temperature, discharge-air temperature, cooling valve position, and fan behavior to reveal when the economizer is not reducing load as intended.
What does ASHRAE 90.1 say about economizers?
ASHRAE 90.1 includes requirements for when certain buildings should be able to use economizer operation. In practice, that means the air handler should take advantage of favorable outdoor air conditions to reduce mechanical cooling load. The issue is that some systems are programmed with conservative cutoff points or competing control setpoints, which can prevent the economizer from doing its job even when outdoor air could still provide useful free cooling.
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