Field guide
AC Running but Not Cooling: What to Check, In Order
A system that runs without cooling is rarely a mystery. It is almost always airflow, charge, or load — checked in that order.
The complaint is the same on every call: the unit is running, the thermostat is calling, air is coming out of the registers, and the house is not getting cold. Guessing at this point usually means adding refrigerant to a system that never had an airflow problem solved.
Work the causes in the order below. Each step either clears a whole category of fault or points directly at the next measurement.
Step 1 — Confirm the system is actually running as intended
- Indoor blower and outdoor condenser fan both running, compressor drawing current.
- Thermostat calling for cooling, not fan-only, and the setpoint below the space temperature.
- No tripped breaker, blown low-voltage fuse, or open safety switch that is cycling the unit off.
Step 2 — Airflow, before anything refrigerant-side
Measure total external static pressure across the air handler and compare it to the rated maximum. A restricted system delivers less air, drops suction pressure, and imitates an undercharge perfectly.
- Filter loaded, or a filter with a higher pressure drop than the system was designed for.
- Dirty evaporator coil, collapsed flex duct, closed or blocked returns.
- Blower speed tap set too low for the installed tonnage.
Step 3 — Temperature split as a quick sanity check
| Supply-to-return split | What it usually means |
|---|---|
| Well below the expected range | Low capacity: charge, compressor, or a restriction |
| Within the expected range | The equipment is producing capacity; look at load, duct losses and run time |
| Well above the expected range | Too little air moving across the coil — return to Step 2 |
The expected split depends on indoor wet-bulb and dry-bulb conditions, so read it against the manufacturer's chart rather than a single memorised number.
Step 4 — Refrigerant side, only once airflow is proven
With airflow verified, superheat and subcooling finally describe the charge instead of describing a restriction. Use superheat on a fixed-orifice system and subcooling on a TXV system, against the charging chart for the measured conditions.
Step 5 — Load and duct losses
- Supply ducts running through a hot attic with poor or damaged insulation.
- Duct leakage pulling attic air into the return.
- Equipment genuinely undersized for the load, or a load that changed after the system was installed.
Frequently asked questions
- Why is my AC running but not cooling the house?
- The most common causes, in the order a technician should check them, are restricted airflow (loaded filter, dirty evaporator coil, undersized or crushed duct), an incorrect refrigerant charge, and duct or load problems such as leaking supply ducts in a hot attic. Airflow is checked first because a restriction makes a correctly charged system look undercharged.
- Should I add refrigerant if the air is not cold?
- Not before airflow is measured. Adding refrigerant to a system whose real fault is a restriction leaves it overcharged, and the complaint returns as soon as the restriction is corrected.