The AC Tune-Up Checklist

Why a Tune-Up Is More Than a Rinse

A real AC tune-up is a performance audit, not a car wash. Anyone can spray a condenser and call it a day. The value you bring is measurement: you leave with numbers that tell the homeowner whether the system is delivering its rated capacity today and whether a capacitor, contactor, or slow leak is going to strand them in the middle of a July heat wave. Nearly every no-cool call traces back to something a thorough maintenance visit would have caught — a clogging condensate drain, a weakening run capacitor, a dirty coil driving head pressure up.

The checklist below is ordered deliberately. You correct airflow before you ever judge the charge, because low airflow skews superheat and subcooling and will trick you into adding or recovering refrigerant you never needed to touch. Work the list in sequence and your diagnostics stay honest.

Before You Touch a Panel

Start with the customer and a walk-around. Two minutes of questions saves you from missing the real complaint — “it cools fine but the upstairs never gets there” is a duct or airflow story, not a charge story.

• Ask about comfort complaints, noises, short cycling, and prior repairs.
• Confirm the thermostat calls for cooling and is not sited over a heat source or in direct sun.
• Check the filter first — a loaded filter is the single most common root cause of service calls and it changes everything you measure downstream.
• Eyeball accessible ductwork for disconnected boots, crushed flex, and obvious leakage at plenums.
• Pull the outdoor disconnect before removing panels, and verify it is dead with your meter — never trust the breaker alone.

Air Side: Filter, Coils & Drain

The air side is where most preventable failures live. Handle it methodically.

1. Filter: Replace or clean it. Match the MERV rating to what the equipment can tolerate — an oversized MERV jump chokes airflow and drives static pressure through the roof.
2. Condenser coil: With the unit de-energized, clear leaves and grass, then rinse from the inside out so debris exits the way it came in. Use an approved non-acidic coil cleaner on caked fins. A dirty condenser is a classic high-head-pressure, poor-cooling complaint, most common in spring.
3. Evaporator coil: Inspect for dirt and biofilm. A fouled evaporator restricts airflow and can ice up. Note it now even if a full cleaning is a separate ticket.
4. Condensate drain: Clear the primary drain line and trap — algae and sludge here trigger float-switch shutoffs and water damage. Flush it, confirm it flows, and test that the float safety switch actually opens the low-voltage circuit.
5. Blower: Check the blower wheel for dirt loading; a caked wheel silently loses a third of its airflow.

Static Pressure & Airflow

Total external static pressure (TESP) is the blood-pressure reading of the duct system, and it takes 90 seconds with a manometer. Drill test ports upstream and downstream of the air handler — after the filter and coil — and add the absolute values of the two readings.

Airflow itself should land near 400 CFM per ton for standard residential cooling — a 3-ton system wants roughly 1,200 CFM. If you measure CFM from a known heat load, use the sensible-heat relationship:

Refrigerant Charge Verification

Only now — after airflow is confirmed good and the coils are clean — do you judge the charge. Connect your gauges, let the system run and stabilize for 10–15 minutes, and read pressures against a PT chart for the specific refrigerant on the nameplate.

Reading the pattern: low subcooling with high superheat signals an undercharge or restriction — find the leak before you add refrigerant, because topping off a leaking system is both illegal under EPA rules above threshold leak rates and a guaranteed repeat visit. High subcooling points to an overcharge or a condenser airflow problem.

Electrical & Component Testing

Electrical failures are the parts that leave people without cooling overnight, and most of them announce themselves on a meter weeks in advance.

• Run/start capacitor: Discharge it, then measure microfarads. Replace it if the reading is more than 6% off the rated value stamped on the can. A weak capacitor is one of the most common AC failures and cheap insurance to swap.
• Contactor: Inspect the contacts for pitting and welding. Pitted or arcing contacts cause hard starts and burned wiring — a leading cause of no-cool calls.
• Amp draw: Record compressor and condenser fan amps under load and compare to the RLA/FLA on the nameplate. A compressor pulling near locked-rotor amps is failing.
• Connections: Torque the lugs and check low-voltage terminals. Loose connections generate heat and nuisance faults.
• Motors: Confirm the condenser fan and blower spin freely and quietly; listen for bearing noise.

Target Ranges Cheat Sheet

Tape this to the inside of your truck. These are the numbers a healthy residential cooling system should return during a summer tune-up. Manufacturer data always wins where it differs.

Documentation & Handoff

The visit is not finished until the numbers are written down. A tune-up that logs superheat, subcooling, delta-T, static, capacitor MFD, and amp draws creates a baseline — next season you will spot a slow-leaking system or a fading capacitor by trend, not guesswork. It also protects you: documented readings are your record that the system left in spec.

• Record every measured value, not just pass/fail.
• Flag developing failures — a capacitor at 5% off, a lightly pitted contactor — so the customer can plan a repair before the breakdown.
• Give the homeowner a written summary and a filter-change reminder cadence.
• Note refrigerant type and any leak observations for EPA recordkeeping compliance.