What it is

A packaged rooftop unit (RTU) puts the whole HVAC system — compressor, condenser, evaporator, blower, and usually the heat source — into one cabinet that sits on the roof. Residential splits separate the indoor and outdoor halves; an RTU is "packaged" because it's all in one box, with ducts dropping straight through the roof curb into the building. They're the workhorse of light commercial: strip malls, restaurants, small offices, retail. If you're moving from residential into commercial, the RTU is the first thing to get comfortable with.

The refrigeration cycle inside is the same one you already know. What's different is the packaging, the heavier blower, the economizer, the controls, and frequently three-phase power and multiple stages.

How it works

Everything lives in one cabinet divided into sections. Return air comes up from the building, passes through filters, gets conditioned (cooled by the evaporator or heated by the heat section), and the blower pushes it back down into the supply ductwork. The condenser section rejects heat to the outdoor air through its own coil and fan(s), exactly like the outdoor half of a split — it's just sitting a few feet away in the same box.

Most light-commercial RTUs are "gas/electric" (gas heat, electric cooling) or "electric/electric" (electric-resistance or heat-pump heat, electric cooling). A big differentiator is the economizer: a damper section that can bring in outdoor air for "free cooling" when conditions allow.

In the field

Walk the cabinet, then walk the sequence.

The major sections you'll find:

  1. Return/filter section — return air enters, passes the filter rack. Commercial filters and racks are bigger; check them every PM.
  2. Blower section — often a belt-drive blower with a motor, belt, and adjustable sheave (newer units may be direct-drive ECM). This moves serious CFM against real duct static.
  3. Evaporator (indoor) coil — cools and dehumidifies the supply air; condensate drains to a pan and out through the curb.
  4. Compressor(s) — one or more, often scroll. Multiple compressors or a multi-stage compressor give staged cooling.
  5. Condenser section — outdoor coil and condenser fan(s) rejecting heat.
  6. Heat section — a gas heat exchanger with burners/inducer/igniter (gas/electric) or electric heat strips, or heat-pump operation.
  7. Economizer — outdoor-air and return-air dampers with an actuator and controls, plus a barometric relief or power exhaust to dump building air.
  8. Controls — a control board or unit controller, contactors, transformer, safeties, and the low-voltage interface to the thermostat or building controls.

Sequence on a call for cooling (typical single/two-stage):

  1. Thermostat/controller calls for cooling (Y1) and fan (G).
  2. Blower energizes (indoor fan).
  3. The economizer logic checks first: if outdoor air is suitable for free cooling, the dampers modulate open and mechanical cooling may stay off or run to supplement.
  4. If mechanical cooling is needed, stage 1 compressor and condenser fan energize. On rising demand (Y2), stage 2 brings on the second compressor/stage.
  5. Safeties (high/low pressure, etc.) supervise throughout and will park the stage if tripped.

Sequence on a call for heating (gas/electric):

  1. Thermostat/controller calls for heat (W1).
  2. The gas heat section runs its own startup — inducer proves draft, pressure switch closes, igniter heats, gas valve opens, flame proves on the sensor.
  3. After a delay, the blower energizes to move the heated air.
  4. Second-stage heat (W2) brings on additional heat capacity if equipped.

Normal values & targets

  • Belt-drive blower: belt should deflect roughly ½–¾" under thumb pressure on the span; too loose slips, too tight wears bearings. Set fan speed via the sheave to hit design CFM.
  • Airflow: commercial cooling is often set around ~350–450 CFM per ton depending on the application; verify against the unit and the design.
  • External static pressure: RTUs are rated against a design ESP; high static (dirty filters/coils, closed dampers, bad ducts) drops airflow and capacity. Measure it.
  • Refrigerant side: same R-410A targets as any system — superheat and subcool per the application, suction/head appropriate to conditions.
  • Three-phase: check that all three legs are present and reasonably balanced (large voltage imbalance between legs cooks motors).

Verify everything against the unit's data plate and the building's design; these are typical light-commercial ranges.

Common faults & what they mean

  • Low airflow / weak cooling: dirty filters, plugged coils, slipping or broken belt, closed economizer damper, or high duct static. Measure static and airflow before blaming the charge.
  • One stage works, the other doesn't: stage-2 compressor, its contactor, or the controller's staging output.
  • Economizer stuck open in cold weather: brings in too much cold outdoor air, freezes coils or overworks heat — a very common RTU complaint (covered in depth in the economizer article).
  • Heat won't run: same furnace sequence stalls as residential — pressure switch, igniter, flame sense — plus three-phase/control issues unique to the unit.
  • Tripping on high head: dirty condenser, failed condenser fan, or recirculating hot air on the roof.

Tech tips & gotchas

  • It's the same refrigeration cycle. Don't be intimidated by the cabinet — superheat, subcool, and pressures mean exactly what they always do. The new stuff is the blower, economizer, staging, and three-phase.
  • Rooftop conditions are brutal. Sun, wind, debris, and birds. Check for nests, sun-baked capacitors, and condensers recirculating their own hot discharge against a parapet.
  • Belt-drive blowers need PM the residential world doesn't: belts, sheave alignment, bearings, and CFM setting. A slipping belt quietly kills capacity.
  • Know the controls before you start pulling wires. Some RTUs have simple 24V boards; others have unit controllers or talk to a building automation system. Get the sequence/wiring for the specific unit.
  • Static pressure is your friend on commercial. It catches airflow problems that masquerade as refrigerant problems.

Safety / code notes

  • Roof work is fall-hazard work — follow rooftop access and fall-protection requirements; mind skylights and edges.
  • Gas/electric units are combustion appliances: verify venting and combustion-air provisions per the applicable mechanical/fuel-gas code and test for safe combustion (CO) — never reproduce or shortcut venting.
  • Three-phase line voltage is lethal — lock out/tag out, verify dead, and confirm the disconnect is present and within sight per NEC §440.
  • Confirm the unit is properly supported and curbed; condensate must drain per code, not onto the roof membrane.