Rooftop economizer check: 6 tests before cooling season

In a boiler room, an economizer captures heat from flue gas before it goes up the stack. On a roof, it does the opposite: it throws the doors open to outdoor air to reject heat, and shuts the compressor off while it can. Same word, opposite job — and one more contrast worth noting. The boiler-room version announces itself the moment it stops working. The rooftop one can be dead for three years without a single alarm reaching the BAS. That is why a rooftop economizer check belongs on the calendar rather than in the reactive queue.

What a rooftop economizer actually does

A rooftop unit constantly blends two air streams: what it pulls back from the building and what it draws from outside. The economizer sets the proportion. When the control judges outdoor conditions favourable, it opens the outdoor air damper, closes the return damper by the same amount, and lets outside air do the compressor’s work. That is free cooling: the only energy spent is the supply fan.

Three parts are enough to break it — a damper blade, an actuator, two or three sensors. When one fails, the unit keeps cooling normally, just on the compressor instead of on free air. Comfort does not change. The bill does.

Before a rooftop economizer check: tools and safety

Tools required

• a calibrated duct probe thermometer, or two matched probes to read outdoor, return and mixed air at once;
• a hygrometer if the control is enthalpy-based;
• a multimeter able to read the actuator command signal;
• a data logger or access to BAS trends;
• a camera for the linkage and the configuration plate, before and after adjustment.

⚠ Safety — Lock out the unit before reaching into the damper section: the blade closes under spring return, and a fan restarting while a hand is in the mixing box gives no second chance. Rooftop work also falls under fall-protection rules.

Step 1 — Read the configuration before touching the damper

Before commanding anything, record what the unit is supposed to do: changeover type, threshold, minimum outdoor air position, documented sequence of operations. On many economizer controls the dry-bulb changeover point is selected from a handful of preset positions on the board — an installer setting, usually made once and never revisited. This is also where you sort out the two changeover logics:

Neither is superior in the abstract: enthalpy control captures more hours through humid July nights, dry-bulb fails silently less often. What matters is knowing which one is installed before you read a single measurement.

Step 2 — Inspect the hardware before trusting the electronics

Unit locked out, open the section. Look at the intake and its screen: clogged with cottonwood fluff or grease carryover from a neighbouring kitchen exhaust, it strangles outdoor air flow without any sensor noticing. Then the linkage: play at the joints, a loose set screw on the shaft, a blade that no longer seats flat on its seal. Operations guidance puts these hardware checks on a quarterly cycle and sensor calibration on an annual one — a sensible rhythm here, provided the full inspection lands before cooling season.

Step 3 — Stroke the actuator from 0 to 100%

Command the full stroke, then watch the blade, not the screen. This is the test half of all maintenance reports skip, because it requires eyes on the damper while someone else drives the command. Three things to see: the blade starts when the command starts, it actually reaches the stop at 100%, and it returns to zero without binding.

An actuator spinning free is the most common failure: the motor runs, the feedback signal looks right, and the blade does not follow because the set screw has slipped on the shaft. From the BAS, everything is fine; from the roof, the damper has not moved in two seasons.

Step 4 — Verify the sensors that make the decision

An economizer does not decide from perceived comfort — it decides from two or three numbers. Compare each sensor against your calibrated instrument, under real conditions. Two traps recur:

• The sun-baked outdoor air sensor. Mounted on a south-facing metal panel, it reads several degrees high in the afternoon — and blocks free cooling exactly when it would pay most.
• The ageing humidity sensor. On enthalpy control, humidity drift produces large changeover errors in both directions, which is why annual calibration is not optional.

Step 5 — Force changeover and follow mixed air

The test that settles it. Drive the unit into economizing mode, then read mixed air temperature. The principle is arithmetic: mixed air is the average of the two streams, weighted by each one’s share. With the damper half open on a 12 °C day and return air at 23 °C, mixed air should fall to roughly the midpoint. If it stays pinned at 23 °C, the blade has not moved, whatever the control displays — and you now have the number that Step 3 only made you suspect.

Then check the reverse: when the control reverts to mechanical cooling, the damper must return to minimum position. An economizer left open while outside is warmer than inside costs more than one stuck shut.

Step 6 — Check minimum position and full closure

Two verifications a Montreal winter makes mandatory. Minimum outdoor air position is what ventilates occupants when the economizer is idle: too low and indoor air quality suffers; too high and you heat outdoor air for nothing from November to March. Closure must be tight: a damper left open a few degrees at −20 °C throws freezing air across a coil and, at best, trips a low-limit lockout on the morning start.

This is more than good practice. The updated chapter I.1 of the Quebec Construction Code — Building Energy Efficiency — has been in force since 13 July 2024, and any building whose construction began after 12 January 2025 must meet the requirements carried over from the National Energy Code of Canada for Buildings 2020. Outdoor air control is treated there as a performance item, not an accessory.

What should you record so the test is worth something next year?

Four lines, provided they are always the same four: changeover type and threshold, actuator stroke (reached or not), measured error on each sensor, and mixed air temperature observed at a given opening alongside both inlet temperatures. They turn “economizer checked — OK”, which proves nothing, into a baseline next year’s visit can be compared against — and into supporting evidence when an optimization project has to put numbers on expected savings for a program such as Hydro-Québec’s Efficient Solutions.

Field case: Brossard, three rooftops and a damper that never moved

Service centre in Brossard, three rooftop units serving offices and a sales floor. The manager reported summer electricity bills climbing for two years with no comfort complaints and no service calls — the hardest profile to diagnose, precisely because nothing breaks.

Economizer check on all three units, in May. Unit A: compliant. Unit B: outdoor air sensor mounted on the south panel with no solar shield, reading well high in the afternoon — so the changeover threshold was never crossed between 1 p.m. and 6 p.m., the hours of peak cooling load. Unit C: actuator spinning free, set screw slipped on the shaft; the blade sat at minimum position and mixed air tracked return air at every commanded opening.

Corrections: outdoor sensor relocated into shade with a radiation shield, shaft re-indexed and tightened on unit C, thresholds revised on all three. No major parts, one morning with two technicians. June mixed-air trends showed what the previous year’s report — stamped “economizers verified” — never had: two units out of three had stopped free cooling.

Three roofs, one morning: where to start

Take the building whose summer electricity bill has drifted the most, pull the last maintenance sheet for its rooftop units and look for one thing: a numeric mixed air temperature. If it is not there, “economizer checked” covers nothing, and that is the unit next visit should start with — before the first heat wave, not during it. The rest of rooftop preventive work, filters, belts and coils, is covered in our article on the rooftop unit maintenance program.

A repaired economizer does not photograph well: it only shows up across twelve months of consumption. Closing that gap between what a roof looks like and what a bill says is exactly what Montréal Combustion’s technicians are after — a recorded number beats a ticked box, because it is the only thing a building manager can defend in front of a budget.