Why is my AC running but not cooling the house?

The most common causes, in order of likelihood, are a dirty air filter or other airflow restriction, a dirty condenser or evaporator coil, low refrigerant charge from a leak, or a failing compressor. Because these problems can produce nearly identical symptoms at the thermostat, always check filter and airflow first before assuming the system needs refrigerant.

Does a frozen evaporator coil mean I need more refrigerant?

Not necessarily. A frozen coil is a symptom, not a root cause — it can be triggered by low airflow (dirty filter, closed vents, weak blower) just as easily as by low refrigerant charge. The coil must be fully thawed before any accurate diagnosis can be made, since ice makes pressure and temperature readings unreliable.

How can a technician tell if a system is overcharged or undercharged?

By measuring suction and liquid line pressures and temperatures once the system is running under stable conditions, then calculating superheat and subcooling. Undercharge typically shows high superheat and low subcooling; overcharge typically shows low superheat and high subcooling. These values are compared against the manufacturer's target values for the specific equipment and conditions.

Can a dirty condenser coil really stop a system from cooling?

Yes. A coated or blocked condenser coil prevents the system from rejecting heat efficiently, which raises head pressure, reduces capacity, and can shorten compressor life over time. It is one of the most common and most overlooked causes of poor cooling because it requires a simple visual inspection of the outdoor unit that is often skipped.

Should I add refrigerant myself if my AC isn't cooling?

No. Refrigerant systems are sealed by design — a system that is low on charge has a leak somewhere, and simply adding refrigerant without finding and repairing that leak means the charge will drop again. Charging also requires calculating superheat/subcooling against manufacturer targets and proper recovery equipment, and is regulated (EPA Section 608 in the U.S.) — it should be performed by a licensed technician.

Engineering·9 min read·July 18, 2026

🔍 HVAC Not Cooling: Complete Troubleshooting Guide

A systematic, symptom-to-fix troubleshooting guide for an HVAC system that isn't cooling adequately — covering airflow restrictions, refrigerant charge problems, frozen coils, and compressor failure, in the correct diagnostic order.

📖Single-Family Residence HVAC Design Drawing Set (Full Access)

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Why "Not Cooling" Is Rarely One Simple Problem

"The AC is running but the house isn't cooling" is one of the most common service calls in HVAC, and also one of the most misdiagnosed. The symptom is identical whether the root cause is a $15 filter, a $200 capacitor, or a $1,500 compressor — which is exactly why guessing at the cause (and jumping straight to "it must need refrigerant") wastes time and money. A proper troubleshooting sequence works from the cheapest, fastest checks toward the more invasive and expensive ones, and uses actual measurements — not assumptions — to isolate the real cause.

Topics covered

HVAC not coolingAC not cooling troubleshootingfrozen evaporator coillow refrigerant chargesuperheat subcooling diagnosisHVAC compressor failure