Select all symptoms that apply. This narrows the likely cause before you take measurements.
Measure at a return air grille and a supply air register after the system has run at least 15 minutes.
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How to Use the AC Not Cooling Calculator
Airflow problems are the most common and easiest to check cause of poor cooling. If you see ice on the refrigerant lines or indoor coil, turn the system to fan-only or off entirely and let it thaw before further diagnosis, running a frozen system can damage the compressor. For frost-specific causes, see the evaporator freeze calculator.
The difference between return and supply air temperature is the fastest diagnostic reading with just a thermometer. A properly working system produces a 15-20°F split after 15 minutes of steady operation. Splits outside that range point toward airflow restriction or a refrigerant-side problem. See the airflow diagnosis calculator for airflow-specific diagnosis using this same measurement.
If airflow and temperature split don't explain the problem, suspect electrical (failing capacitor, contactor, compressor) or a thermostat/control fault. These require a licensed technician to resolve safely. An outdoor unit that hums but doesn't start, or a slow-spinning fan, often points to a failing run capacitor.
Why Your Air Conditioner Runs But Doesn't Cool: A Systematic Approach
An air conditioner that runs continuously but fails to cool the home is one of the most common service calls in the industry, and also one of the most frustrating for homeowners because the system appears to be "working" -- it's making noise, the outdoor unit is on, but the house stays warm. Understanding the systematic diagnostic approach technicians use can help homeowners identify simple fixes and communicate more effectively with a technician when professional service is needed.
Airflow: The Root Cause of Most Cooling Complaints
Restricted airflow across the indoor evaporator coil is the single most common root cause behind "runs but doesn't cool" complaints. When airflow is restricted -- typically from a dirty filter, but also from blocked supply or return grilles, a collapsed flexible duct, or a coil fouled with dust -- less warm indoor air passes across the coil per minute. This has two compounding effects: less total heat is removed from the home's air (reducing cooling capacity), and the coil itself runs colder than designed because it's absorbing less heat, which can eventually cause it to drop below freezing and ice over. Once ice forms, it further blocks airflow, creating a feedback loop that can escalate from "not cooling enough" to "not cooling at all" within a few hours of continued operation.
Refrigerant Charge Problems
Low refrigerant charge, almost always caused by a leak somewhere in the sealed system, reduces the mass of refrigerant circulating and therefore the system's capacity to absorb and reject heat. Refrigerant problems often present similarly to airflow problems (frosted coil, poor cooling, higher temperature split issues) which is why proper diagnosis requires distinguishing between the two using pressure and temperature measurements rather than symptoms alone. Refrigerant work is tightly regulated in Canada -- only licensed technicians with proper recovery equipment and certification may legally handle refrigerant, whether adding charge, recovering it, or repairing a leak. For pressure-based refrigerant diagnosis, see the refrigerant fault diagnosis tool.
Electrical and Component Failures
The outdoor condensing unit contains several components that commonly fail and produce a "runs but doesn't cool" symptom pattern. A failing run capacitor is extremely common -- capacitors degrade with age and heat exposure, and a weak capacitor often causes the compressor or outdoor fan motor to hum without starting, or to start slowly and struggle under load. A failing contactor can cause intermittent operation. Compressor issues (worn valves, motor winding problems) are more serious and typically require compressor replacement or full system replacement depending on age and cost-effectiveness. For a focused compressor-specific diagnostic, see the compressor diagnosis tool.
When the Problem Is Simpler Than It Looks
Before assuming a major mechanical failure, it's worth ruling out simpler explanations: a thermostat set to "Fan" instead of "Cool," a setpoint accidentally set above room temperature, dead thermostat batteries causing erratic operation, or a tripped circuit breaker or blown fuse at the outdoor disconnect. These account for a meaningful fraction of "AC not cooling" service calls and cost nothing to check before calling a technician.
Frequently Asked Questions
An AC that runs but doesn't cool usually falls into one of four categories: restricted airflow (dirty filter, blocked vents, frozen coil), low refrigerant charge from a leak, an electrical fault (failing capacitor or contactor limiting outdoor unit performance), or a thermostat/controls issue. Checking the temperature split between return and supply air is usually the fastest way to narrow down which category applies before further diagnosis or a service call.
A correctly functioning central air conditioner typically produces a 15-20°F (8-11°C) split between return air and supply air, measured after at least 15 minutes of steady operation. A split under 14°F often points to low airflow or low refrigerant charge. A split over 22°F can indicate low airflow with adequate charge, causing the coil to run colder than intended. This calculator interprets your specific split reading alongside your other symptom inputs.
Yes. A clogged filter is one of the most common causes of poor cooling. Restricted airflow across the evaporator coil reduces the heat absorbed from indoor air, which can cause the coil to drop below freezing and ice over. Once frost forms, airflow drops further in a compounding cycle that can progress from "not cooling enough" to "barely any cold air" within hours. Replacing a dirty filter is the simplest and most common fix for this type of complaint.
Any refrigerant work (suspected leaks, adding or recovering refrigerant) legally requires a licensed and certified technician in Canada, since refrigerants are federally regulated substances. Electrical diagnostics involving capacitors, contactors, or compressor windings carry shock and equipment-damage risk and are best left to a qualified technician. Simple checks -- filter replacement, clearing blocked vents, thermostat battery and setpoint verification -- are generally safe for homeowners to perform themselves before calling for service.
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