Troubleshooting guide

Why Is an Air Conditioner Not Cooling Enough?

Airflow, refrigerant charge, dirty coils, controls and electrical faults: how to separate the common causes of poor cooling, with the temperature-split check.

Mechanical, HVAC & Fluids · Last reviewed October 15, 2026

Cooling depends on three things working together: enough airflow over the evaporator coil, the right refrigerant charge, and clean coils that can move heat. A fault in any of them reduces cooling.

Refrigerant work is regulated and requires certification in many places, and the equipment has high voltage and rotating fans. Handling refrigerant and electrical diagnosis belong with qualified technicians following local rules.

Symptom	Likely cause	What to check
Weak airflow, ice on the indoor coil	Dirty filter, blocked return, slow blower, or dirty evaporator coil	Filter, return grille, blower speed setting and coil condition. Let the ice melt before testing
Normal airflow, warm air, ice on the suction line	Low refrigerant charge (a leak)	A technician checks the pressures and looks for the leak. Adding refrigerant without finding the leak is not a repair
Outdoor unit not running, indoor fan runs	Tripped breaker, failed contactor or capacitor, or a safety switch open	Breaker, disconnect, contactor and capacitor by a qualified person. Condensate overflow switch
Outdoor unit runs, but air is only slightly cool	Dirty condenser coil, failed condenser fan, or recirculating hot air	Condenser coil and fan. Clear plants and debris from around the unit
Cools at first, then warms	Frozen coil, or the compressor overheating on a thermal limit	Airflow and coil condition. Compressor current and discharge temperature
Room does not reach setpoint on hot days	Undersized system, high internal or solar load, leaky ducts, or open windows	Load calculation, duct leakage and insulation, thermostat location

The temperature-split check

With the system running for 10 to 15 minutes, measure the air temperature entering the evaporator (return) and leaving it (supply). The difference is the temperature split. A common healthy range is about 15 to 22 °F (8 to 12 °C) in normal humidity, depending on the equipment and the conditions.

Split well below the normal range with normal airflow: possible low charge, a compressor problem, or very high load.
Split above the normal range: airflow is too low (dirty filter, blocked coil, slow blower), which also risks coil freezing.
Do not read the split in the first minutes after start-up, or when the coil is iced.

Check against the sensible heat formula: at 1,200 CFM and a 20 °F split, sensible cooling is 1.08 × 1,200 × 20 = 25,920 Btu/h, about 2.2 tons of sensible capacity.

Write a useful problem statement

Equipment type, size and age. Thermostat setting and the actual room temperature.
Return and supply temperatures, and outdoor temperature.
Whether both the indoor and outdoor units run, and any ice or water.
Last filter change and maintenance date.

A second example: duct leakage

A system is supposed to deliver 1,200 CFM at a 20 °F split, which is 1.08 × 1,200 × 20 = 25,920 Btu/h of sensible cooling.
If 20% of the air leaks out of the ducts into an attic, only 960 CFM reaches the rooms.
Delivered cooling = 1.08 × 960 × 20 = 20,736 Btu/h, which is 20% less.

The equipment can be working correctly and the house can still be too warm, because the air never reaches the rooms. Duct leakage and uninsulated ducts in hot spaces are common causes.

Preventive checks

Change or clean the filter on the interval the manufacturer gives.
Keep the outdoor coil clear of leaves, grass and debris, and clean it when it is dirty.
Have a technician check refrigerant charge, electrical connections and the condensate drain once a year.
Keep supply and return grilles unblocked by furniture and curtains.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

How often should the filter be changed?

Follow the equipment instructions. Common intervals run from one to three months, and sooner with pets, dust or heavy use.

Does low refrigerant mean there is a leak?

In a sealed system, yes. Refrigerant is not consumed, so a low charge means it escaped somewhere.

Why does the indoor coil freeze?

Low airflow (dirty filter, blocked return, slow blower) or low refrigerant charge lets the coil temperature fall below freezing, so moisture on it turns to ice. Turn the system off and let it melt before looking for the cause.

Does a bigger air conditioner fix poor cooling?

Not by itself. An oversized unit can short-cycle and remove less moisture. Check the load, the ducts and the airflow first.

How often should a system be serviced?

Many manufacturers recommend a yearly check by a qualified technician, in addition to regular filter changes by the owner.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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