Autumn Economizer Tune-Up: Catch the Lies Before Heat Season
- Alex Khachaturian
- Sep 1
- 6 min read
September is when the economizer starts lying to you.
On paper, it’s your best friend: “free cooling,” lower kWh, happy utility bills. In real life? A stuck damper, a drifting sensor, a lazy high-limit, and suddenly you’re reheating outside air in October while Finance asks why the meter spiked. I’ve walked into too many buildings where the BAS graphic swears everything is perfect, OA 55°F, RA 72°F, MA 60°F, but the space is muggy, the boiler is waking up, and the AHU sounds like it’s training for a marathon.
This is the shoulder-season trap. And the way out is simple: an economizer tune-up before heating season.
Why an economizer tune-up now (not after the first frost)
The weather is in the “lies zone.” Cool mornings + warm afternoons + stubborn humidity = the perfect cocktail for false confidence and drift.
Your sensors took a summer vacation. UV, heat, vibration, and time all nudge OA/RA/MA sensors off truth.
Linkages loosen when no one’s looking. A tiny slip at the damper turns into a 20% outdoor air leak all winter.
Bad high-limit logic wastes real money. If your dry-bulb/enthalpy cutouts are off, you’ll either miss free cooling or over-ventilate and reheat.
The Truth–Lie Map: where economizers fool you
Sensors lie (OA/RA/MA temperature and humidity, CO₂, mixed-air stratification).
Mechanics lie (slipping linkages, bent blades, wrong actuator rotation, failed end-stops).
Logic lies (reversed high-limit, no low-temp lockouts, minimum OA hard-coded wrong, no anti-short-cycle).
Data lies (trending on 5-minute intervals masks short-cycling; no alarm deadbands = noise).
Your tune-up exposes the lies and resets truth before winter bills arrive.
Prep before you drive (15 minutes that saves an hour onsite)
Pull last 14 days of OA/RA/MA temps, humidity if available, damper command vs damper feedback, supply air temp, and reheat valve command.
Check current high-limit strategy (dry-bulb setpoint? enthalpy? which sensor is bound?).
Note the minimum OA target by design or code (what’s required vs what’s implemented).
Grab the AHU submittal or sequence if you have it.
Create a one-page checklist (copy the sequence below).
Field tune-up sequence (the fast, no-excuses version)
Goal: leave with verified sensors, proven mechanics, sane logic, and alarms/trends that keep it that way.
Safety & setup
Lock out anything you’re opening.
Pop panels, find the OA damper, RA damper, linkage, actuator, end-stops, and any blade seals.
Verify where OA/RA/MA sensors physically sit. If you can’t see an MA probe, you’re already suspicious.
Sensor sanity checks
Bring a reliable handheld temp/RH reader. Stand in OA, RA, and MA locations. Compare your handheld to BAS values.
Acceptable drift: within ~1–2°F and ~3% RH for comfort work. If more, calibrate in software and note a replacement.
If the MA sensor is near a coil face or stratified area, move your handheld across the section, if it swings >5°F, call out stratification and plan for a multi-point sensor or averaging probe.
Damper & actuator truth test
Command 0% OA. Confirm OA damper blades fully closed, RA open. Feel for leakage; flashlight helps. Note % leakage if you can estimate.
Command 100% OA. Confirm blades fully open, RA closed. Watch linkage, any lag, slip, or chatter? Tighten and re-pin as needed.
Slowly sweep from 0% → 100% in 10% steps. Verify feedback tracks command. Any dead zone (e.g., command moves but blades don’t) means linkage or actuator sizing is wrong.
Confirm actuator rotation direction and end-stop settings match reality.
High-limit & lockout verification
Identify current high-limit type: dry-bulb or enthalpy.
Dry-bulb cutout commonly lives in the mid-60s°F to ~70°F range, confirm you’re not trying to “free cool” with 75°F OA.
For enthalpy, confirm sensor whereabouts and that you’re using the OA value (not RA by mistake). Check the curve/threshold used.
Verify low-temp lockout to protect coils (e.g., do not allow 100% OA below your site’s threshold).
Confirm compressor/economizer changeover logic: economizer first, compressors assist when OA can’t carry the load.
Minimum OA set & prove
Identify design ventilation rate. Convert to damper position only if you have a calibration chart; otherwise, measure airflow or verify with CO₂ levels post-occupancy.
Set Min OA in the BAS. Command AHU to occupied. Measure MA temp vs RA/OA, sanity-check that your “10%” isn’t secretly 30%.
Label the damper position as “calibrated” with date and your initials in the BAS notes.
Control sequence exercise
With OA below high-limit, command a call for cooling. Confirm economizer opens before mechanical DX.
Introduce a simulated high OA (raise the OA value or change setpoint) and confirm economizer closes and compressors take over.
Kill the call for cooling, ensure economizer returns to Min OA and doesn’t hover half-open.
Trend and alarm hardening
Set 30-second sampling for damper cmd vs fbk and MA temp for at least 24 hours (you can relax later).
Add alarms: (a) cmd/fbk mismatch > 15% for 5 minutes, (b) MA out of plausible range (outside RA↔OA bounds), (c) economizer active above high-limit, (d) Min OA below setpoint for >10 minutes.
Document & handoff
Snap photos of damper position at 0/50/100%, actuator label, and sensor locations.
Drop a three-bullet owner summary: what you found, what you fixed, and the money-leak you avoided.
Common failure patterns (and the quick fix)
“OA 55°F all day.” Classic failed or defaulted OA sensor. Replace or rebind to the real point; set an alarm on flatliners.
“Economizer stuck at 30%.” Linkage slip or end-stop mis-set. Tighten hardware, recalibrate feedback, re-prove 0/100.
“Always calling for reheat in the morning.” Economizer leaking OA during warm-up. Enforce Min OA = 0% during warm-up; lock out OA under low outdoor temps.
“Short cycling compressors when it’s 68°F outside.” No true first-stage economizer or bad changeover, fix sequence order.
“CO₂ low but energy high.” Over-ventilation. Re-confirm Min OA; coordinate DCV setpoints and sensor placement.
Sensor calibration: do it once, then make it stick
Use your handheld as the local truth; adjust BAS offsets to match.
If an offset exceeds ~3–4°F, stop “fixing in software”, replace the sensor.
If MA readings swing wildly with damper position, install an averaging MA probe or relocate away from stratification and coil shadow.
High-limit strategy, plain English
Dry-bulb is simple and robust: free cool when OA is cool enough (e.g., ≤68°F, confirm your site standard).
Enthalpy is smarter in humid climates but only if the RH sensor is accurate and maintained.
Whichever you use, your economizer must drop out above the limit, and alarms should prove it.
Minimum OA that’s actually minimum
Don’t hard-code a random 10%. If the design calls for a flow rate, translate it to damper position via measurement at least once. If you can’t measure flow, validate through steady-state CO₂ during occupancy (well-placed sensor, properly averaged). Tag the verified damper position in the BAS and on a label at the panel.
DCV, economizers, and reheat: stop the tug-of-war
If DCV opens OA based on CO₂ while the economizer is also modulating for cooling, you can end up over-ventilating and reheating. The fix:
Prioritize economizer when OA is below the high-limit.
Let DCV raise Min OA only when economizer is not active or when CO₂ exceeds your threshold persistently (e.g., >900 ppm for 10 minutes).
Add a hard cap on combined OA position to protect against runaway.
Easy math to prove the dollars
Rough cut: If your economizer leaked an extra 1,500 cfm of OA for 12 hours/day at a 20°F winter delta-T, the boiler is reheating 360,000 Btu/hr (1,500 cfm × 1.08 × 20°F) ≈ 4.3 therms/day. That’s $4–$6/day per unit (local rates vary), multiplied by weeks of heating season and fleets of AHUs. You’re not “tuning”, you’re printing money.
Your 30-minute autumn economizer tune-up checklist (copy/paste)
Verify OA/RA/MA sensor locations; compare with handheld; calibrate/replace.
Command 0%/100% OA; tighten linkage; confirm actuator rotation and end-stops.
Prove cmd vs fbk in 10% steps; eliminate dead zones.
Confirm high-limit type and threshold; prove drop-out above limit.
Set/verify Min OA per design; label in BAS; take photos.
Exercise economizer-first sequence; ensure compressors assist, not lead.
Add/confirm trends & alarms (cmd/fbk mismatch, MA plausibility, high-limit violation, Min OA low).
Document three bullets: find/fix/save.
Book Recommendation
Control Systems for Heating, Ventilating, and Air Conditioning
Written by: Roger W. Haines & Douglas C. l-Iittle
Best for: Translating HVAC control theory into sequences that actually work in real buildings, great for commissioning and troubleshooting.
What you’ll get: Clear explanations of control strategies, sensor behavior, loop tuning, economizer logic, and reset schedules
How to use it: This week (45–60 min): Read the chapters on sensors, loops, and sequences; audit one AHU against its written sequence and note mismatches.
Field drill:
Draw a simple state diagram for supply fan/heat/cool; verify each sensor placement and unit/scale; check reset logic vs trend data.
Pro tip:
Convert your most-used sequences into a one-page checklist and tape it inside the panel door.
Gear Recommendation
Fluke 971 Temperature Humidity Meter
Buy it: Fluke 971 Meter
Best for: Fast room spot-checks to verify what your BAS says, perfect for comfort complaints and sensor validation.
What it is: A handheld temperature/RH meter with quick response and backlit display; ideal for comparing live room conditions against BAS points.
How to use it: Let it equilibrate 30–60 seconds; hold at chest height away from your body; sample center of room and near the return; avoid exhaled breath; log readings.
Field drill: Do a 5-room audit, record BAS vs meter. Flag >±1.5°F or >±5% RH and build a correction/replace list.
Pro tip: Keep fresh batteries and store in a case; annual check with a 75% salt solution keeps RH accuracy honest.
Aftercare: keep it honest through December
Weekly glance: MA temp trend banded between OA and RA, damper cmd ~0% nights/Min OA occupied, feedback tracks.
Monthly nudge: Re-verify Min OA position; season creep can push things off target.
First freeze alert: Confirm economizer lockouts and warm-up Min OA = 0% until supply is happy.
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