HVAC
Air-side economizer fault detection field guide for HVAC
Find the broken economizer that runs mechanical cooling unseen: stuck dampers, dead actuators, bad sensors, the wrong changeover, the FDD logic, and the functional test that proves it.
Direct answer
Economizer FDD is the logic that compares an economizer's expected state to its actual state and flags the fault when they disagree. A broken economizer runs mechanical cooling instead of free outside air and rarely throws a comfort complaint, so the waste goes unseen. Adopted energy codes now require FDD on many units; the equipment and the code edition control.
Key takeaways
- Economizer FDD is logic that compares an economizer's expected state to its actual measured state and flags the fault when they disagree.
- Field studies find roughly 60 to 80 percent of rooftop economizers malfunctioning; a broken one hits setpoint via the compressor and throws no complaint.
- A stuck damper raises a unit's cooling energy by about a third; a stuck-closed damper also starves ventilation and is the most common economizer fault.
- California Title 24 requires economizer FDD on nonresidential packaged and split systems above roughly 54,000 Btu/h (about 4.5 tons) with an air-side economizer.
- Test by commanding the damper full open and closed and watching the blades stroke, spoofing the OA sensor across changeover, then restore overrides and record results.
Economizer fault detection, and the waste nobody sees
Economizer fault detection and diagnostics, FDD, is the logic that compares what an economizer should be doing against what it is actually doing, and flags the gap. The economizer is the damper assembly that brings cool outside air into a packaged unit or air handler for free cooling, so the compressors stay off when the weather will do the work. When it breaks, the unit just runs mechanical cooling and pays for cooling it could have gotten for free. FDD is what catches that, because the building never will.
Here is the problem that makes this its own guide. A broken economizer does not throw a comfort complaint. The space still hits setpoint, because the compressor picks up the load the free cooling was supposed to carry. Nobody calls. The only symptom is on the utility bill, spread across twelve months, where no single number looks wrong. A stuck damper, a dead actuator, a drifted sensor, the unit looks fine from the ground and runs wrong for years.
For how an economizer and demand-control ventilation are supposed to operate and get commissioned, see the economizer and DCV guide. For the controllers, sensors, and sequence that the FDD logic reads, see the BAS and DDC fundamentals guide. This guide is about one thing: finding the fault that is already there.
How should an air-side economizer work?
An air-side economizer should open its outside-air damper for free cooling whenever the outdoor air is cool enough, and keep the compressors off or staged back while that free cooling carries the load. When there is a call for cooling and the outside air is below the changeover point, the damper modulates open, the return-air damper closes in step, and the mixed air holds the supply-air setpoint without mechanical cooling. When the space needs more cooling than the outside air can give, the compressors stage on to finish the job while the damper stays open.
A high-limit control sits over the top of all of it. When the outside air gets too warm, or too humid on an enthalpy setup, the high limit locks the economizer back to its minimum position and hands cooling to the compressors, because bringing in hot or wet air would cost more to cool than recirculating the return air. That lockout point is the changeover, and it decides how many hours a year the free cooling actually runs.
Underneath the free cooling there is always a minimum outside-air position the damper holds for ventilation, even when the economizer is locked out. Free cooling, high-limit lockout, and a ventilation floor: three jobs, one damper. Every fault in this guide is one of those three jobs going wrong without anyone noticing.
Why do economizers fail so often?
Economizers fail silently and they fail in large numbers. Field studies of rooftop units have found a striking share of economizers not working as intended. A New Buildings Institute review of more than 500 rooftop units found roughly two out of three with a malfunctioning economizer, and other field surveys put the figure in the 60 to 80 percent range. Treat any economizer you did not personally commission as suspect until you have tested it.
The reason the failure rate stays that high is the same reason it is hard to fix: there is no obvious symptom. A leaking valve floods a floor. A dead compressor stops cooling and the phone rings. A broken economizer does neither. The compressor quietly covers for it, the space stays comfortable, and the only evidence is a cooling bill that runs higher than it should across the whole season. No complaint means no work order means no one looks.
On top of that, the economizer lives on the roof in weather, full of moving parts that nobody touches between service calls. The damper, the linkage, the actuator, and the sensors all sit out in sun, rain, and temperature swings. They drift, seize, corrode, and disconnect on their own schedule. A stuck damper can raise a unit's cooling energy by a third or more, and a failed sensor can drive peak loads well above a unit with no economizer at all. The waste is real even though it is invisible.
The common economizer faults
Most economizer faults fall into a short list, and a tech who carries that list in their head can usually name the fault before the cover is off. The damper sticks, open or closed, from a seized linkage or a bound blade. The actuator dies or loses its drive, so the damper no longer follows the command. A sensor, outside-air, mixed-air, or return-air, drifts or fails, so the economizer decides on bad data. The linkage between the actuator and the damper disconnects, so the actuator strokes and nothing moves.
The control side fails just as often as the mechanical side. The high-limit changeover is set to the wrong type or the wrong setpoint, so the free cooling either never runs or lets in air it should have rejected. The minimum outside-air position is set wrong, so the unit over- or under-ventilates. The economizer sequence is disabled in the controller and nobody knows. Each of these has its own symptom and its own check, which is what FDD is built to sort out.
The table below is the field shorthand. It is the same set of faults the code-required FDD logic is written to catch, and the same set you confirm by hand on a functional test.
| Fault | What it does |
|---|---|
| Damper stuck closed | No free cooling, compressors run, under-ventilation |
| Damper stuck open | Over-ventilation, freeze risk, coil load in heat and humidity |
| Dead or unplugged actuator | Damper no longer follows the command |
| Disconnected linkage | Actuator strokes, damper does not move |
| Drifted or failed sensor | Economizer decides on wrong temperature or enthalpy |
| Wrong changeover type or setpoint | Free cooling locked out, or wet air let in |
| Wrong minimum position | Too little ventilation, or too much energy |
| Economizer disabled in controller | No economizing at all, looks intentional |
What is a stuck economizer damper?
A stuck economizer damper is an outside-air damper that no longer moves to its commanded position, and it is the single most common economizer fault. It sticks one of two ways, and the two failures cost you in opposite directions.
Stuck closed is the quiet money loser. The damper sits at minimum, the controller calls for free cooling, and nothing opens, so the compressors run through every cool hour the building could have ridden for free. There is no comfort complaint, because the mechanical cooling covers the load. The same stuck-closed damper also starves the building of ventilation air, so an indoor-air-quality problem rides along with the energy waste, and neither one announces itself.
Stuck open is louder but it gets blamed on other things. The damper hangs open when it should be at minimum, so the unit pulls a flood of outside air the whole time. In cold weather that is a freeze risk on the coil and a heating bill that makes no sense. In heat and humidity it is raw coil load, the compressors fighting hot wet air they should never have seen. A stuck-open damper in winter is a frequent cause of a frozen or nuisance-tripping unit that gets chased as a refrigerant problem when the fault is a seized linkage on the roof. Find it by commanding the damper and watching whether it actually goes where you sent it.
Sensor faults: deciding on bad data
An economizer is only as good as the sensors it decides on. The outside-air, mixed-air, and return-air temperature sensors feed the changeover and the staging logic, and when one drifts or fails, the economizer makes the wrong call with full confidence. A drifted outside-air sensor reading high will lock out free cooling that should be running. Reading low, it brings in warm air the high limit should have rejected. Either way the logic is sound and the data is wrong, which is the hardest kind of fault to spot from the front-end.
Mixed-air and return-air sensors matter just as much, because FDD and the staging logic compare them. A mixed-air sensor placed in a stratified airstream reads a temperature that does not represent the real mix, and that bad placement looks exactly like a drifted sensor on a trend. Check sensor placement before you condemn the sensor.
Enthalpy sensors are the worst offenders. A dry-bulb temperature sensor is cheap, stable, and easy to verify against a known reference. A humidity or enthalpy sensor drifts faster, fails more often, and is far harder to confirm in the field, which is why many veterans favor a dry-bulb or differential dry-bulb changeover over enthalpy on equipment that will not get faithful sensor maintenance. For how the controller reads, scales, and trends these sensors, see the BAS and DDC fundamentals guide.
What is economizer changeover temperature?
The changeover, or high-limit setpoint, is the condition at which the economizer stops bringing in outside air and hands cooling back to the compressors. Above it, mechanical cooling costs less than cooling the outside air, so the damper drops to minimum. Set this wrong and you either throw away free-cooling hours or drag in air that costs more to cool than the return air you gave up. The wrong changeover is a control fault that hides in plain sight, because the unit still cools.
There are a few changeover strategies, and they are not interchangeable. Fixed dry-bulb locks out the economizer above a set outdoor temperature, commonly somewhere in the 60s to low 70s Fahrenheit depending on climate and the design. Differential dry-bulb compares outside-air temperature to return-air temperature and economizes whenever outside is cooler. Fixed and differential enthalpy do the same comparison on total heat content, to account for humidity. The right choice depends on the climate and on whether the humidity sensors will actually be maintained.
Two faults dominate here. The setpoint is left at a factory default that does not fit the climate, or the changeover type is wrong for the building, often an enthalpy setup running on a drifted humidity sensor. In a dry climate a high-quality dry-bulb changeover often beats a poorly maintained enthalpy one, which is a long-running debate in the trade with good evidence behind the dry-bulb side. The specific setpoint and the allowed changeover type are set by the adopted energy code and the equipment listing, so confirm both against the code edition the jurisdiction enforces and the controller's documentation before you change a number.
How does economizer FDD find the fault?
FDD finds the fault by comparing the expected state of the economizer against its actual measured state, point by point, and flagging the disagreement. The logic knows what mode the unit should be in from the outside-air, mixed-air, and return-air temperatures, the cooling call, and the damper command. Then it checks whether the measured air temperatures and the damper feedback agree with that expected mode. When they do not, it raises a fault and, on better systems, names which one.
The two headline faults are symmetrical. Economizing when it should not, the damper is open and pulling outside air while the high limit says it should be locked out, which wastes energy and loads the coil. Not economizing when it should, the conditions are right for free cooling but the damper is sitting at minimum and the compressors are running, which is the stuck-closed damper or the disabled sequence. A third class is the sensor sanity check, where the temperatures themselves do not make physical sense, mixed air colder than both outside and return, for instance, which points at a failed or misplaced sensor.
Damper-position faults get caught by comparing the commanded position to the feedback. A common rule flags a fault when the damper feedback differs from the command by more than about 10 percent for several continuous minutes while the unit is in minimum-ventilation mode. The exact thresholds and timers belong to the controller and the code, but the principle is constant: expected versus actual, and the gap is the fault.
Built-in FDD and what the code now requires
Economizer FDD is no longer optional on a lot of new equipment, because the energy codes now require it. In California, Title 24 requires economizer fault detection and diagnostics on nonresidential packaged and split air systems above a size threshold, commonly cited around 54,000 Btu/h, roughly 4.5 tons, that carry an air-side economizer. ASHRAE 90.1 and the IECC drive economizers onto units above similar thresholds in many climate zones, and the FDD requirement has been moving in step. Confirm the exact threshold, the qualifying climate zones, and the FDD requirement against the code edition the jurisdiction has actually adopted, because these numbers shift between cycles and carry local amendments.
The FDD itself shows up two ways. Many packaged rooftop units now ship with an economizer controller that has FDD built in, the controller watching its own sensors and damper and reporting faults on board. The other path is FDD written into a building's direct digital control system as part of the sequence of operations, the BAS running the fault rules across the points it already reads. Code generally accepts either, standalone or integrated, as long as the analytics meet the requirement and are certified where the code calls for it.
The point for the field tech is that a code-compliant unit is supposed to tell you when its economizer is broken. That only helps if the FDD was set up, the faults are visible to someone, and the alarms are not buried. A built-in FDD reporting to a screen nobody watches is the same broken economizer with a light on.
BAS trends and analytics across the fleet
When the economizers live on a building automation system, the most powerful diagnostic is the trend. Trend the outside-air, mixed-air, and return-air temperatures, the damper command, the damper feedback, and the mechanical cooling stages together, and the fault draws itself. A damper command swinging while the feedback stays flat is a dead actuator or a disconnected linkage. Compressors running through cool outside-air hours with the damper at minimum is a stuck-closed economizer or a bad changeover. You are reading the same expected-versus-actual story the FDD logic reads, just by eye.
Analytics scale that up. A fault-detection platform sitting on top of the BAS runs the same rules continuously across every unit in the building or across a whole portfolio, and surfaces the broken economizers in a ranked list instead of waiting for someone to open a trend. That is how an operator finds the dozen bad economizers among a hundred rooftop units without climbing onto a single roof first.
The analytics narrow the search; they do not close the fault. A rule that flags non-economizing tells you which unit to visit, not whether the cause is the actuator, the linkage, or the sensor. You still confirm it at the equipment with a functional test. For the trending and point structure that makes this possible, see the BAS and DDC fundamentals guide.
How do you test an economizer?
You test an economizer by driving it through its states and watching what actually happens, not by reading the front-end and assuming. This functional test is the same one used at commissioning acceptance, and it is the only way to know a fault is real and a fix actually worked. Test it. Do not assume it.
Command the damper full open and full closed from the controller, and confirm it strokes the whole way both directions. Stand where you can see the blades or the actuator, because a command that the controller reports as satisfied means nothing if the linkage is disconnected and the blades never moved. Then force the economizer into and out of free cooling. Spoof or override the outside-air sensor across the changeover point, or set the changeover temporarily to provoke the transition, and confirm the damper opens for free cooling below the setpoint and locks to minimum above it. Verify the compressors stage the way the sequence says while you do it.
Last, verify the minimum position. With the economizer locked out, confirm the damper holds the minimum outside-air setting that the ventilation design calls for, measured, not assumed from the command. Put back every override and setpoint you touched before you leave, and write down what the unit did. A functional test you ran but did not record is a test the next tech has to run again.
- Command the damper full open and full closed; confirm the blades actually stroke both ways.
- Watch the actuator and linkage at the unit, not just the feedback on the screen.
- Spoof or override the outside-air sensor across the changeover and confirm the mode flips.
- Confirm the compressors stage as the sequence specifies during the transition.
- Verify the minimum outside-air position against the ventilation design, measured.
- Restore every override, setpoint, and forced point before leaving.
- Record what the unit did at each step, pass or fail.
Minimum position and the ventilation floor
The minimum outside-air position is the damper opening the unit holds for ventilation even when free cooling is locked out, and it is wrong on a surprising number of units. Set it too low and the building does not get the ventilation air it needs, which is an indoor-air-quality problem the occupants feel as stuffiness long before anyone measures it. Set it too high and the unit conditions more outside air than the ventilation design requires, every hour the unit runs, which is straight energy waste hiding behind a damper that looks like it is doing its job.
The minimum position is easy to get wrong because it is usually a single number set once and never checked, and because a damper rarely flows what its position percentage implies. A damper commanded to a 20 percent position does not pass 20 percent of full flow; the relationship between blade angle and airflow is not linear. The real check is the measured outside-air fraction at the minimum setting, not the position readout.
On units with demand-control ventilation, the minimum position becomes a moving floor tied to occupancy, which is a related setting worth getting right alongside this one. For how minimum outside air and CO2-based demand-control ventilation are set and commissioned, see the economizer and DCV guide.
Inspecting the damper, linkage, and actuator
When FDD or a trend points at a damper fault, the confirmation is hands-on at the unit. Start with the damper blades. Look for blades that are bent, jammed on debris, or fouled, and check that they close to a real seal instead of hanging a half inch open across the whole bank. A damper that cannot fully close leaks outside air continuously, which shows up as a minimum position that runs higher than anyone set.
Then the linkage and the actuator. A disconnected or slipped linkage is the classic fault where the actuator strokes its full range and the blades barely move, so the feedback can even look reasonable while nothing happens to the air. Grab the linkage and confirm it is tight from the actuator to the blade shaft. Stroke the actuator and watch the blades follow through the whole range. A dead actuator gives no motion, a weak or failing one stalls partway or cannot drive against the spring.
Check the seals and the blade edges last. Economizer dampers on the roof corrode, the seals harden and tear, and a unit that tested fine three years ago can leak badly now. The leak is the part that erases the energy savings even when the actuator and the controller are doing everything right.
Seasonal drift and why it needs a recheck
An economizer that passed last year is not an economizer that passes this year. The sensors drift out of calibration, the linkage loosens, the seals harden, and the changeover that was right for one season's setup gets left wrong for the next. Economizers drift, and the drift is invisible until someone rechecks. A commissioning pass is a snapshot, not a guarantee.
The practical answer is to put the economizer on a recurring check instead of trusting it between service calls. Re-verify the sensors against a known reference, re-run the functional test, and re-confirm the changeover and minimum position on a seasonal cadence, at the least heading into the cooling season when the free cooling has the most to give. Catching a drifted sensor in spring saves the whole cooling season; catching it in fall saved nothing.
Fold the economizer check into the unit's preventive maintenance so it is not a separate thing anyone can skip. For where economizer verification fits in the broader rooftop and air-handler PM routine, tie it to the maintenance program, because an economizer is exactly the kind of system that quietly fails between the visits nobody scheduled for it.
What a working economizer actually saves
A working economizer earns most of its money in the shoulder seasons and in cool climates, where there are long stretches of hours when the outside air is cold enough to cool the building while the compressors sit idle. In those hours the cooling is nearly free, paid for only by the fan energy to move the air. Across a cooling season that adds up to a real fraction of a unit's cooling bill, which is exactly the number a broken economizer is throwing away without leaving a fingerprint.
Run it the other way and the cost of a fault is concrete. A stuck damper has been measured raising a unit's cooling energy by roughly a third, and a failed sensor that drives the wrong mode can push peak demand well above what the same unit would draw with no economizer at all. Multiply that across a building or a portfolio of rooftop units, most of which field studies say are broken, and the economizer is one of the largest pools of recoverable waste in commercial HVAC.
The reason the fix pays back fast is that the repair is usually cheap relative to the savings. An actuator, a sensor, or a linkage repair is a small parts-and-labor number against a cooling-season energy bill it was quietly inflating. The economizer is the rare repair where the diagnosis is the hard part and the fix is the easy one.
Fix the parts or retrofit the controller
Once the fault is confirmed, the decision is repair the failed component or replace the economizer control altogether. Most of the time it is a repair. A dead actuator, a drifted sensor, a disconnected linkage, or a torn damper seal is a parts swap and a recalibration, and the unit goes back to economizing for the cost of the part. If the damper, blades, and bearings are sound and only the control side failed, repair is the answer.
The case for a controller retrofit is the old economizer control that has no FDD, a clumsy changeover, and a history of the same fault coming back. Replacing an aging economizer controller with a current FDD-capable one buys built-in diagnostics, a cleaner changeover, and a controller that reports its own faults instead of hiding them. On a unit the code now requires to have FDD, the retrofit can also be what brings it into compliance during a major repair or replacement.
Weigh it on the age and condition of the whole unit. Pouring a new controller onto a rooftop unit at the end of its life rarely pays; the economizer retrofit goes onto units with years left in them. The damper and actuator have to be mechanically sound for either path to hold, so confirm the mechanics before you spend on controls.
The fault, the symptom, and the check that finds it
This is the field reference, the same logic the FDD runs and the same checks you confirm by hand. Match the symptom to the fault, then run the check to prove it before you spend a part on it.
The pattern under every row is the same: expected versus actual. The symptom is the gap showing up in the bill, the trend, or the air, and the check is how you confirm which fault opened the gap.
| Fault | Symptom | Check |
|---|---|---|
| Damper stuck closed | Compressors run in cool weather, high bill, stuffy space | Command full open, watch the blades stroke |
| Damper stuck open | Freeze trips in winter, high coil load, heating waste | Command to minimum, confirm blades close and seal |
| Dead actuator | Damper command moves, feedback flat, no air change | Stroke the actuator at the unit, watch for motion |
| Disconnected linkage | Actuator strokes full range, blades barely move | Grab the linkage, confirm it drives the shaft |
| Drifted OA/MA/RA sensor | Wrong mode for the weather, illogical trend temps | Compare each sensor to a known reference |
| Wrong changeover type/setpoint | Few free-cooling hours, or wet air pulled in | Spoof OA across setpoint, confirm the mode flips |
| Wrong minimum position | Stuffy space or high steady energy | Measure outside-air fraction at minimum |
| Economizer disabled | Never economizes, no fault shown | Read the sequence and the controller config |
Finding the broken economizers across an RTU fleet
A building or a campus rarely has one rooftop unit; it has dozens, and the broken economizers are scattered through them with no pattern you can see from the ground. This is where FDD earns its place on the operations side. Run the fault rules across every unit at once, whether through the onboard controllers, the BAS trends, or an analytics layer, and the dozen bad economizers sort themselves out of the hundred good ones without a tech setting foot on a roof.
The order of work falls out of that. Triage from the fault list, send the tech to the flagged units first, confirm each one with the functional test at the equipment, and fix in priority order by the size of the waste and the severity of the fault. A stuck-open damper risking a winter freeze jumps the line ahead of a slightly off changeover. Without the fleet view, a crew checks units at random and most of the broken ones never get visited at all.
The recurring offenders are the real prize. A unit that keeps failing the same way is telling you something the one-off repair will not fix, a chronic actuator, a bad sensor location, a changeover that does not suit the climate. You only see the pattern if the faults and the fixes are tracked across time, which is the next section.
What to document and track
An economizer fault that nobody logged is a fault the building gets to pay for twice. Record the unit, the fault you found, the symptom that pointed at it, the check that confirmed it, the parts and setpoints you changed, and the functional-test result after the fix. That record is what tells the next tech whether the economizer was ever right, and it is what turns a pile of one-off repairs into a pattern you can act on.
The tracking matters as much as the single record. When the faults and fixes live in one place across a fleet and across seasons, the recurring failures surface, the actuator model that keeps dying, the units that drift out every spring, the changeover that never suited the climate. A field tool like FieldOS that captures the unit, the fault, the photo of the linkage, and the test result on site keeps that history attached to the equipment instead of scattered across paper tickets that never get read. The chronic fault you can name is the one you can finally fix for good.
| Field to record | Why it matters |
|---|---|
| Unit ID and location | Ties the fault to the right equipment for trending |
| Fault and confirming check | Separates a real fault from a guess |
| Sensor calibration readings | Shows drift over time across seasons |
| Changeover type and setpoint | Catches the wrong strategy for the climate |
| Minimum position, measured | Proves the ventilation floor, not just the command |
| Parts changed and date | Surfaces the recurring component failure |
| Functional-test result after fix | Proves the economizer actually works again |
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Common mistakes
- Letting a broken economizer run mechanical cooling unseen because no one ever tested it.
- Trusting a drifted sensor, so the economizer decides on wrong temperature or enthalpy.
- Leaving the wrong changeover type or a factory-default setpoint for the climate.
- Setting the minimum position by the command percentage instead of the measured outside-air fraction.
- Skipping the functional test at commissioning and assuming the front-end reflects reality.
- Reading the damper feedback on the screen instead of watching the blades at the unit.
- Commissioning once and never rechecking, so seasonal drift goes uncaught.
- Treating an enthalpy changeover as fit-and-forget when its humidity sensor never gets maintained.
Standards and references
The energy codes are where the economizer and its FDD requirement live. ASHRAE 90.1 and the IECC require air-side economizers on equipment above a capacity threshold in many climate zones, commonly cited around 54,000 Btu/h, and California's Title 24, Part 6 adds an economizer fault detection and diagnostics requirement for nonresidential packaged and split systems above that size with an economizer. The qualifying size, the climate zones, and the FDD scope move between code cycles and carry local amendments, so confirm every one of them against the edition the jurisdiction has actually adopted before you cite it.
ASHRAE Guideline 36 gives standardized high-performance sequences of operation for HVAC systems, including the economizer logic and the fault-detection rules written to run in real time on a DDC system. It is the reference for what a correct economizer sequence and its FDD should contain, and it is increasingly what specifications point to. Pair it with the controlling sequence of operations for the actual project.
Below the code sits the equipment. The rooftop unit or air-handler manufacturer and the economizer controller documentation set the actual changeover options, the minimum-position method, the FDD thresholds and timers, and the acceptance test for that specific unit. Where a built-in FDD is used to satisfy code, the analytics may need to be certified to the code's requirement. Cite the standard that governs the point, hedge the FDD scope, the changeover type, and the setpoints to the adopted code edition and the equipment listing, and let the project specification and the manufacturer's instructions control the number.
Units and terms
Economizer work crosses a few terms and unit systems, so the same idea reads differently across a controls drawing, a code section, and a manufacturer's sheet.
The economizer is sometimes called the free-cooling or air-side economizer. The changeover is also called the high-limit or high-limit shutoff. Outside air is OA, mixed air is MA, return air is RA, and the ventilation floor is the minimum outside-air position or minimum OA. Capacity thresholds in the code are given in Btu/h or in tons, where one ton of cooling is 12,000 Btu/h, so a 54,000 Btu/h threshold is about 4.5 tons. Temperatures are degrees Fahrenheit in most US equipment and code, degrees Celsius in metric documents. Enthalpy is total heat content, in Btu per pound of dry air, which is why it accounts for humidity where dry-bulb temperature alone does not.
- Economizer FDD
- Logic that compares an economizer's expected state to its actual state and flags the fault
- Changeover / high limit
- The condition at which free cooling locks out and mechanical cooling takes over
- Dry-bulb vs enthalpy
- Changeover on temperature alone, or on total heat content to include humidity
- OA / MA / RA
- Outside air, mixed air, and return air, the temperatures the economizer decides on
- Minimum position
- The damper opening held for ventilation when free cooling is locked out
- Outside-air fraction
- The measured share of supply air that is outside air, the real check on a damper position
FAQ
What is economizer fault detection?
Economizer fault detection and diagnostics, FDD, is logic that compares an economizer's expected state to its measured state and flags the gap. It watches the outside-air, mixed-air, and return-air temperatures, the damper command, and the feedback, then reports faults like a stuck damper, dead actuator, or bad sensor that the building never complains about.
Why do economizers fail so often?
Economizers fail silently, so nobody fixes them. Field studies of rooftop units have found roughly 60 to 80 percent with a malfunctioning economizer. A broken one still hits setpoint because the compressor covers the load, so there is no comfort complaint and no work order, while the cooling bill quietly runs high all season.
How do you test an economizer?
Drive it through its states and watch what happens. Command the damper full open and closed and confirm the blades stroke, spoof the outside-air sensor across the changeover to confirm the mode flips, verify the compressors stage correctly, and measure the minimum position against the ventilation design. Restore every override, then record the result.
What is economizer changeover temperature?
The changeover, or high-limit setpoint, is the condition where the economizer stops using outside air and hands cooling to the compressors. Strategies include fixed dry-bulb, differential dry-bulb, and fixed or differential enthalpy. The right type and setpoint depend on climate and the adopted energy code, so confirm both against the code edition and the equipment.
What is a stuck economizer damper?
A stuck damper no longer moves to its commanded position. Stuck closed kills free cooling and ventilation while the compressors run, a quiet money loser with no complaint. Stuck open floods the unit with outside air, risking a winter coil freeze and heavy summer coil load. It is the most common economizer fault. Confirm it by commanding the damper open.
Does code require economizer FDD?
On many units, yes. California Title 24 requires economizer FDD on nonresidential packaged and split systems above roughly 54,000 Btu/h with an air-side economizer, and ASHRAE 90.1 and the IECC drive economizers onto units above similar thresholds. The exact size, climate zones, and FDD scope shift by code cycle, so confirm the adopted edition and local amendments.
Why is my economizer running the compressor in cool weather?
Cool weather plus mechanical cooling usually means the economizer is not opening when it should. The likely faults are a damper stuck closed, a dead actuator, a disconnected linkage, a drifted outside-air sensor reading high, or the economizer sequence disabled in the controller. Command the damper open and watch the blades to confirm which one.
Dry-bulb or enthalpy changeover: which is better?
Dry-bulb is simpler and its sensor is stable and easy to verify. Enthalpy accounts for humidity but its sensor drifts faster and is harder to confirm in the field. In dry climates and on equipment that will not get faithful sensor maintenance, a quality dry-bulb or differential dry-bulb changeover often beats a neglected enthalpy one.
How much energy does a broken economizer waste?
More than the lack of complaints suggests. A stuck damper has been measured raising a unit's cooling energy by about a third, and a failed sensor driving the wrong mode can push peak demand above a unit with no economizer at all. Across a fleet where most economizers test broken, it is a large pool of recoverable waste.
How often should you check an economizer?
Recheck it at least seasonally, especially heading into the cooling season when free cooling has the most to give. Economizers drift: sensors lose calibration, linkages loosen, seals harden. A commissioning pass is a snapshot, not a guarantee. Fold the sensor check, the functional test, and the changeover and minimum-position verification into the unit's preventive maintenance.
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