What it is

A rooftop-unit PM is where you earn the maintenance agreement. Done right, it catches the cheap failures before they become emergency no-cool calls in July, and it protects the capacity and efficiency the customer is paying to run. Done as a lazy filter-swap, it misses the belt about to snap, the economizer stuck open, and the loose lug heating up toward a burnt leg. This is what to actually check on a light-commercial RTU so the PM means something.

How it works

RTUs live on a roof getting baked by sun, blasted by wind, and packed with debris and birds. They have moving parts a residential split doesn't — belt-drive blowers, economizer dampers and actuators — and they run hard, often year-round. Preventive maintenance is about three goals: keep airflow up (filters, belts, coils), keep the machinery healthy (electrical connections, motors, refrigerant side), and keep the controls honest (economizer, staging, safeties). Hit all three and the unit stays reliable; skip one and that's where the summer breakdown comes from.

In the field

A real RTU PM, grouped by goal:

Airflow:

  1. Filters — inspect and replace. Commercial filters load up fast; a packed filter starves the coil, kills capacity, and can freeze the evaporator. Note the size and quantity for the customer.
  2. Belt and sheaves (belt-drive blowers) — check belt tension and condition (cracks, glazing, fraying), check sheave alignment, and confirm the blower is moving design CFM. A slipping or worn belt quietly robs capacity; a belt about to break is a guaranteed callback.
  3. Blower bearings and wheel — listen for bearing noise, check for a dirty blower wheel (cuts airflow), and verify the motor isn't running hot.
  4. Coils — inspect and clean the evaporator and condenser coils. A dirty condenser drives head pressure up and capacity down; a dirty evaporator chokes airflow. Check the condensate pan and drain for sludge and proper flow.

Machinery / electrical:

  1. Tighten and inspect electrical connections — loose lugs and contactors are a top cause of voltage imbalance, burnt legs, and single-phasing on three-phase units. Look for discoloration/heat damage.
  2. Contactors — check contacts for pitting and chatter; a worn contactor is a cheap part that prevents an expensive failure.
  3. Capacitors — test run/start caps; weak caps stress motors and cause hot-weather no-starts.
  4. Three-phase check — read all three legs line-to-line for balance, and clamp current on each leg under load against nameplate.
  5. Motors — condenser fan and blower motor amps vs. nameplate, bearing condition, mounting.

Controls / refrigerant / safety:

  1. Economizer — drive the actuator full stroke, confirm dampers move and linkage is intact, verify sensors and changeover, and check minimum-position and barometric relief. This is the most-neglected RTU component — check it every time.
  2. Staging — force each cooling/heat stage and confirm it energizes and carries load; verify anti-short-cycle timers.
  3. Refrigerant performance — take a real set of readings: suction/head, superheat, subcool, and the air temperature split. Don't just "top it off" — verify the charge is right and look for leak signs (oil).
  4. Heat section (gas/electric) — inspect burners, igniter, flame sense (microamps), inducer, and the heat exchanger; run a combustion/CO check before the heating season.
  5. Safeties — confirm high/low-pressure switches, limits, float/overflow, and any phase monitor are present and functional, not bypassed.

Normal values & targets

  • Belt deflection: roughly ½–¾" under thumb pressure on the span — adjust to manufacturer spec; replace if cracked/glazed.
  • Air temperature split: ~16–22°F across the evaporator in cooling at typical humidity; a low split flags airflow or capacity loss.
  • Three-phase imbalance: keep voltage imbalance small (rule of thumb under ~2%); investigate uneven legs.
  • Flame-sense current: ~2–6 µA on a healthy rod; clean and recheck if low.
  • Filter and coil: clean enough that static pressure is within the unit's rated ESP — measure static if airflow seems off.
  • Currents: all motor/compressor legs at or below nameplate RLA/FLA under load.

Verify everything against the unit's data plate; these are typical light-commercial targets.

Common faults caught at PM

  • Belt about to break / slipping: capacity loss now, no-cool call later. Catch it on the belt inspection.
  • Economizer stuck (open or closed): energy waste and winter freeze-ups or lost free cooling. Found only by driving it full stroke.
  • Loose/heat-damaged electrical lug: path to voltage imbalance and a burnt leg — caught by tightening and inspecting connections.
  • Dirty condenser: high head and lost capacity on the first hot day — caught by coil cleaning.
  • Plugged condensate drain: overflow, water damage, or a tripped float killing the unit — caught at the drain check.
  • Weak capacitor: hot-weather no-start — caught by testing, not waiting for failure.

Tech tips & gotchas

  • A PM is not a filter swap. The belt, economizer, electrical connections, and a real performance check are where breakdowns hide. Skipping them is how you get the July emergency you were paid to prevent.
  • Check the economizer every single visit. It's the component most likely to be silently broken and it wastes money in both failure directions.
  • Tighten three-phase connections. Heat and vibration loosen lugs over time; a loose connection becomes imbalance, then a burnt leg, then a dead compressor.
  • Take real refrigerant readings; don't "top off." Adding refrigerant without diagnosing is how a small leak becomes an annual ritual and a fried compressor. If it's low, find the leak.
  • Do the heat check before heating season, including combustion/CO — don't wait for the first cold-morning no-heat to discover the igniter or flame sensor.
  • Document everything, including filter sizes, belt sizes, readings, and anything trending toward failure. It builds trust, justifies the agreement, and lets the next tech pick up where you left off.

Safety / code notes

  • Rooftop PM is fall-hazard work — follow rooftop access and fall-protection requirements; watch edges and skylights.
  • Lock out/tag out and verify zero energy before working on three-phase line voltage; confirm the disconnect is present and within sight per NEC §440.
  • On gas/electric units, verify venting and combustion-air provisions per the applicable mechanical/fuel-gas code and run a combustion/CO check — never shortcut combustion safety.
  • Keep all safeties (pressure switches, limits, float, phase monitor) functional; a PM is the time to confirm they work, never to bypass one.