AC Running But Not Cooling Your Las Vegas Home? Here's What's Usually Wrong
An AC that runs continuously but can't bring the indoor temperature down to the thermostat setting is one of the more frustrating situations in a Las Vegas summer. The system sounds like it's working — the fan blows, the outdoor unit runs — but the house stays at 82°F when the thermostat says 74°F. This is a distinct problem from an AC that won't turn on, and the causes are different.
First: is it a heat load problem or an equipment problem?
Before assuming something is wrong with the AC, it's worth considering whether the system is simply being asked to do more than it's designed to do. On a 115°F afternoon when the attic is 150°F, a system that's sized for typical conditions may not keep up during the peak 2–5 PM hours. If the system eventually catches up in the evening as temperatures moderate, that may indicate a capacity edge case rather than a malfunction.
However: if the system can't reach set temperature even on a moderate 90°F day, that's a clear indicator of an equipment or system issue, not just peak-hour demand.
Most likely causes when the AC runs but won't cool
Low refrigerant charge
The most common cause of a running-but-not-cooling system. A system with low refrigerant can circulate air, run the fan, and sound completely normal — but with insufficient refrigerant, the heat transfer that makes cooling possible is impaired. The system runs longer cycles trying to compensate. Eventually it may also cause the evaporator coil to ice over, which then reduces airflow and makes things worse. Low refrigerant always means a leak somewhere in the system.
Dirty condenser coils
The outdoor condenser unit rejects the heat removed from your home by passing air across refrigerant coils. When those coils are heavily fouled with desert dust, the heat rejection efficiency drops. The system can still run but operates at higher-than-designed pressures, which reduces cooling output and increases compressor stress. This is extremely common in Las Vegas and often goes unnoticed between annual tune-ups.
Restricted airflow indoors
A heavily clogged air filter or a blocked return-air grille reduces the air volume moving through the indoor coil. Less airflow means less heat transferred from the home's air to the refrigerant — the system runs but removes less heat per hour than it should. Check the filter first, every time. In Las Vegas summer, a filter that looked fine three weeks ago may now be significantly loaded.
Failing compressor
A compressor that's operating but not at full capacity — weak valve plates, worn internal components — may run and sound normal while producing reduced compression. This results in a system that operates, circulates refrigerant, but doesn't move enough of it to meet the cooling load. Diagnosing a weak compressor requires measuring system pressures; it's not detectable by sound alone in early stages.
Undersized system for the actual load
If the system has always struggled during hot weather rather than deteriorating from a previously better performance, undersizing is worth considering. Equipment that was correctly sized 15 years ago may now be undersized if the home has added square footage, changed window configurations, or lost attic insulation over time.
What a technician checks during this kind of diagnosis
A proper diagnosis of a running-but-not-cooling system involves: measuring supply and return air temperatures (to assess actual heat removal), checking system pressures (to assess refrigerant charge and compressor function), inspecting the filter and indoor coil, and checking the outdoor condenser coil visually and measuring outdoor temperatures. This isn't a guessing exercise — each measurement tells part of the story, and skipping steps leads to misdiagnosis.
What doesn't help
Turning the thermostat lower doesn't make the system cool faster — it just makes it run longer. Closing vents in unused rooms doesn't increase cooling in occupied rooms; it often creates pressure imbalances that reduce overall performance. The system needs to be fixed, not worked around.