Packaged rooftops

Typical package: outdoor condenser fans and coil, compressors, indoor blower, evaporator coil, filters, heat section (gas, electric, heat pump, or dual fuel), and controls. Economizers bring outdoor air for free cooling when conditions allow.

• Curb must match openings; leaks cause rain and air bypass.
• Ductwork attached with proper flexible connectors where required.
• Condensate drained with trap where needed; winter freeze protection in cold climates.
• Electrical and gas per code; roof safety and fall protection mandatory.

Wrong damper setup either dumps unconditioned air into the space or never uses free cooling. Sensors (dry-bulb, enthalpy) must be calibrated. Many “high bills” are economizers stuck open or closed.

Change filters, clean coils, check belts/blowers, verify staged compressors, test heat, confirm safeties (high pressure, rollout, limit). Charge to OEM method (often weigh-in or subcooling on TXV packages).

An RTU is a self-contained weather machine bolted to the roof. The curb is its foundation; the economizer is its lungs; the compressors are its muscle.

On gas RTUs, verify manifold pressure, flame sense, rollout and limit circuits, and flue paths. A cooling ace who ignores heat leaves winter callbacks.

Many RTUs use multiple compressors or unloaders. One failed scroll may only show as weak cooling on hot days. Map contactors and which coil circuit each compressor feeds.

Harness points, safe paths, and two-person lifts for heavy panels are part of the job. Wind and wet membranes turn routine filter changes into incidents.

BAS start/stop, outside-air damper commands, and discharge-air resets may override local knobs. Bring a controls contact when the unit “has a mind of its own.”

Chronic overflow stains mean traps drying out, algae clogs, or pan rust holes. Fix water path before the interior ceiling becomes the leak detector.

### Field memory hooks

Write the key temperatures, flows, or mode checks for this topic on your job notes. Teach the helper beside you to repeat the layered order aloud before tools come out. Most callbacks shrink when the first visit follows a written order instead of a parts guess.

When OEM data exists — capacity tables, wiring diagrams, water-flow charts — photograph the rating plate and the page you used. Future you (or the next tech) should not have to rediscover the same facts under pressure.

Situation. Store complains of hot/cold swings. Economizer minimum position is 40% with no demand-control logic; return duct leak on the roof deck pulls attic-hot air into the unit.

What to think. Fix envelope of the unit and damper minimums; then judge refrigeration staging.

Conclusion. Air-path integrity first on RTUs.

### Teach-back for the helper

Before leaving the topic, have the helper explain in their own words: what is normal, what is measured first, and what must never be skipped for safety. If they cannot teach it back, review the main “How it works” blocks again with a sketch on paper.

Connect this lesson to the previous one and the next one as a chain. Field skill is sequence memory — not isolated trivia. Keep OEM charts and job photos with the work order so the next visit starts smarter.

### Common measurement habits

Use the same meter locations every time so readings compare day to day. Steady the system before judging pressures or temperatures — mid-shift and mid-defrost lie. Write ambient conditions on every note page.

If a reading disagrees with the control display, believe the calibrated handheld until sensors are proven. Controls cannot fix what they cannot see accurately.

### Safety and professionalism

Apply LOTO where energy can hurt you. Wear eye protection around refrigerants, chemicals, and coil fins. On roofs and at window units, respect fall hazards. Explain findings to the owner without jargon soup — clear next steps beat impressive vocabulary.

Dispose of oil, filters, and refrigerant legally. Cutting corners on recovery or tower chemicals is not “saving the customer money”; it is transferring risk.