Rooftop economizers: the part most likely to be quietly broken on your building

An economizer is the damper section on a rooftop unit that brings in cool outdoor air so the compressors can stay off. It is also the part most likely to be broken without anyone knowing. A review of West Coast field studies covering more than 500 rooftop units found economizer problems on 64 percent of them.

What an economizer is supposed to do

Physically, it is an outdoor-air damper, a return-air damper, an actuator that moves them, and a controller reading temperature sensors. When the air outside is cool enough, the controller opens the outdoor damper and lets that air carry the cooling load. When it gets too warm or too humid outside, a high-limit setting, usually called the changeover, sends the dampers back to a minimum position for ventilation and the compressors take over.

Fall and spring, when a building full of people and equipment still needs cooling but the outdoor air is cold enough to provide it, are when an economizer earns its keep. That makes right now the time to find out whether yours works.

Why nobody notices a broken one

When an economizer fails, the compressor picks up the load the outdoor air should have carried, and the building stays comfortable. The only evidence is run time and the power bill.

A glance from the roof hatch does not catch it either. In a field test by Eugene Water & Electric Board in Oregon, three of five economizers upgraded to a premium specification were not operating properly about two years after installation, even though each had passed a documented contractor checkout. Their conclusion: a visual inspection is not enough. The economizer has to be made to run.

That utility’s premium specification calls for locking the compressor out below 55 to 60 degrees outdoors, so a failed economizer shows up as a comfort complaint instead of hiding on the bill. If your compressors run on cool mornings, ask why.

How economizers fail

Pacific Northwest National Laboratory’s building re-tuning training lists the usual causes:

• A jammed or seized outdoor-air damper
• Broken or disconnected linkage between the actuator and the damper
• A dead actuator or a disconnected wire
• A failed or out-of-calibration outdoor-air or return-air sensor
• A failed controller, or control settings that were never right
• Equipment installed or wired incorrectly in the first place

The same material also lists a two-by-four wedged in the damper, which says something about how some comfort complaints get fixed.

Sensor placement matters too. In the Oregon study, outdoor-air sensors on the same roof sometimes disagreed by more than 25 degrees, thrown off by sun and building exhaust under the hood. An economizer that reads the outdoor air as warmer than it is changes over to the compressor early, and nothing on the unit says so.

Stuck shut or stuck open: what each one costs

Stuck shut is the quiet failure. You lose the free cooling, so the compressors run through hours when they should be resting. Depending on where the damper stuck, the space may also be getting less ventilation air than it was designed for.

Stuck open shows up in the extremes. In a West Texas summer, a wide-open outdoor damper feeds hot air straight onto the cooling coil. In winter, the unit heats outdoor air it should never have brought in. PNNL’s trend example of a damper stuck fully open carries a two-word label: heating waste. In Dallas, an open damper on the wrong day also brings in humidity the unit has to wring back out.

Abilene and Lubbock are economizer country. Dallas takes more care.

On ASHRAE’s climate zone map, Taylor and Lubbock counties are zone 3B, which is dry. Dallas County is 3A, which is humid. That difference changes how an economizer should be set up.

ASHRAE Standard 90.1-2013, which the Texas energy code closely tracks on economizer controls, allows a simple fixed dry-bulb changeover up to 75 degrees outdoors in dry zones like 3B, but only up to 65 degrees in humid zones like 3A. The Oregon researchers recommend dry-bulb sensors in dry climates, and enthalpy control, which accounts for moisture, where humidity is the concern.

The practical point: a changeover left at a default, or set without the local climate in mind, can shut the economizer off on days it should be running. In Abilene and Lubbock, that throws away free cooling. In Dallas, the opposite mistake brings in air that is cool but wet.

What the energy code expects on newer units

Texas has used the 2015 International Energy Conservation Code statewide for commercial construction since November 2016. It requires economizers on individual cooling units of 54,000 Btu/h (4.5 tons) and up, with some exceptions, in every climate zone except the hottest one, zone 1. Abilene, Lubbock, and Dallas are all in zone 3, so a rooftop unit that size permitted here since then was most likely required to have one.

The same code requires air-cooled packaged units with an economizer to carry fault detection and diagnostics. The IECC’s fault detection section spells out what the controller has to catch: a failed air temperature sensor, not economizing when it should, economizing when it should not, a damper that is not modulating, and excess outdoor air. On newer units, the fault history is the first thing to read.

How to check a commercial rooftop economizer before winter

Ask for these on your next maintenance visit, and ask to see the readings:

1. A functional test, not a look. The technician calls for cooling, overrides the changeover if needed, and watches the dampers drive fully open and back to minimum.
2. Mixed-air temperature against outdoor and return. PNNL’s rule of thumb: with the outdoor damper wide open, mixed air should read within about 4 to 5 degrees of outdoor air, and at minimum position within about 4 to 5 degrees of return air. A bigger gap points to a failed actuator, loose linkage, or damaged damper blades.
3. The changeover setting and control type, checked against the building’s actual climate zone.
4. Sensor readings checked against a trusted thermometer, with the outdoor sensor out of direct sun and exhaust air.
5. The minimum position. PNNL flags a minimum above about 20 percent as a possible source of excess outdoor air, though the right number depends on the ventilation the space needs.
6. Fault codes on units that have fault detection.

It is one line on a real commercial HVAC maintenance checklist, and it deserves readings on the report, not a checkmark. If the unit is due for replacement anyway, economizer requirements can come into scope with the new equipment; see what drives rooftop replacement cost.

When to call us

If your compressors run on cool mornings, if nobody can say when the economizers were last tested, or if fault codes have been piling up, get them checked before heating season. Our commercial HVAC crews handle rooftop repair and controls troubleshooting in Abilene, Lubbock, and Dallas, and our preventive maintenance agreements are built around your equipment, with documented readings after every visit. Send the details below, or call 888-811-3449.