What it is

Commissioning is the deliberate process of proving a brand-new system actually works to spec before you leave. It is not "turn it on and feel the vent." You verify the electrical, set the airflow, confirm the charge by the right method, check operation across the controls, and write down your numbers. The install isn't done when refrigerant is in it — it's done when you've measured it and documented it.

How it works

A new system can be installed perfectly and still run wrong because airflow is off, the charge is a hair high, or a wire is loose. Commissioning catches those before they become a callback or a dead compressor. The sequence matters: airflow and electrical have to be right before you trust any refrigerant reading, because superheat and subcooling are only meaningful when the air side is correct. Charge a system with a dirty filter or a kinked duct and you'll "fix" a problem that's really an airflow problem.

In the field

Work it in this order — each step sets up the next:

  1. Electrical first. Confirm the breaker and disconnect are correct for the nameplate, the whip and wire gauge match, connections are tight, and the unit is grounded. Verify incoming voltage is in range before startup.
  1. Air side before refrigerant. Confirm the blower is set to the right speed tap or ECM profile for the tonnage, the filter is in and clean, registers are open, and the coil is clear. Measure total external static pressure across the air handler — high static means the duct system is fighting you and every refrigerant number will be skewed.
  1. Set and verify airflow (CFM). Target roughly 350–400 CFM per ton for cooling (lower end in humid climates for better dehumidification). Use a measured method, not a guess — static pressure plus the blower table, or a flow measurement.
  1. Start it and let it stabilize. Run the system 15–20 minutes so pressures and temperatures settle before you read anything. Snapshot readings on a system that hasn't equalized are garbage.
  1. Verify the charge by the right method. TXV system → confirm by subcooling (the TXV controls superheat for you). Fixed-orifice/piston → confirm by superheat using the indoor wet-bulb / outdoor dry-bulb relationship. Adjust to the manufacturer's target, not a generic number.
  1. Check the temperature split. Measure return-air and supply-air temperatures; a typical cooling split lands around 16–22°F depending on indoor humidity. Way outside that points back to airflow or charge.
  1. Exercise the controls. Cooling, heating/aux (or gas heat), fan-only, and on a heat pump, force a defrost or at least confirm the reversing valve swaps. Confirm the thermostat staging and any safety (high/low pressure, float switch) actually works.
  1. Condensate and drainage. Confirm the primary drain flows, the trap is correct, and the safety switch (float) kills the unit when you test it. A new install that floods a ceiling is a brutal first impression.
  1. Document everything. Write down voltages, amps, static pressure, suction/liquid pressures, superheat, subcooling, the split, and the final charge weight if you weighed it in. That record is your proof and your baseline for the first service call.

Normal values & targets

Residential cooling ballparks — always defer to the equipment's commissioning sheet:

  • Airflow: ~350–400 CFM/ton (closer to 350 for humidity control).
  • Total external static pressure: design target around 0.5" w.c.; many residential systems run higher, but much above ~0.8" w.c. signals a duct restriction.
  • Cooling temperature split (return vs supply): roughly 16–22°F, tighter as indoor humidity rises.
  • Subcooling (TXV charge target): commonly 8–12°F — but use the model's specified value.
  • Superheat (fixed-orifice charge target): set by the wet-bulb/dry-bulb chart for the day; total compressor superheat usually lands ~10–20°F when correct.
  • Line voltage / control voltage: ~240V nominal line, ~24V control, both in healthy range under load.

Common faults & what they mean

  • Low split with correct charge: too much airflow, or low load — check CFM and filter, not the charge.
  • High split, coil sweating/icing: too little airflow (dirty filter, closed registers, undersized return) — fix air before touching refrigerant.
  • Can't hit subcooling target, high superheat: undercharge or a metering-device restriction.
  • High subcooling, high head: overcharge or a condenser that can't reject heat (dirty coil, recirculation).
  • Float switch doesn't kill the unit when tested: miswired safety — fix it now, not after a flood.

Tech tips & gotchas

Don't chase refrigerant numbers on a system with bad airflow. It's the most common rookie trap: the split looks wrong, so you add or pull charge, when the real fix was a crushed flex duct or a filter the size of a sponge. Always confirm the air side first.

Let it run before you read it. A system that's been on five minutes hasn't equalized; your superheat and subcooling will drift as it settles. Give it the full warm-up.

Test the safeties for real. Trip the float switch by hand, confirm the high-pressure cutout is wired, watch the reversing valve actually move on a heat pump. "It probably works" is how comebacks happen.

Leave the documentation with the system and a copy on the ticket. When you or another tech comes back for the first service, those startup numbers are gold — you instantly know what changed.

Take a photo of the data plate and your gauge readings. It takes ten seconds and settles any "what was the charge?" question forever.

Safety / code notes

  • Confirm the breaker/fuse size and conductor sizing match the nameplate minimum circuit ampacity and maximum overcurrent protection — these are NEC-governed (NEC Article 440 covers HVAC equipment circuits).
  • Confirm the disconnect is within sight of the unit and the equipment is grounded per NEC.
  • Condensate drainage and any required safety pan/switch are addressed in the mechanical code (IMC §307 for condensate disposal; Indiana enforces the IMC).
  • For gas equipment, verify combustion air, venting, and manifold pressure per the appliance listing and the fuel-gas code before you call it commissioned.
  • Lock out before electrical work and prove the circuit dead with a meter you've verified.