AC Running but Not Cooling Below 78°F: What's Going On?

Is it normal for an AC to stall at 78°F in Laredo heat?

Sometimes. Residential systems are sized for a "design" outdoor temperature, and Laredo afternoons can go beyond it. As a rough rule of thumb, many systems are designed to hold the indoor temperature about 15–20°F below the outdoor temperature during peak heat. On a 105°F afternoon, holding 78°F may be all a properly working system can do. Setting the thermostat to 70°F won't make it cool faster; it just keeps it running.

It's not normal if the house stays at 78°F+ in the morning or evening when it's cooler outside, if it used to keep up and now doesn't, or if the air from the vents isn't much colder than the room.

Check the temperature split (a 2-minute test)

With the AC running for at least 15 minutes, measure the air temperature at a return grille (where air goes in) and at a nearby supply vent (where cold air comes out). A kitchen or infrared thermometer works.

• About 15–20°F colder at the supply is typical for a healthy system.
• Less than about 14°F often points to low refrigerant, a dirty coil, or an airflow problem.
• More than about 22°F can mean airflow is too low (dirty filter, closed vents, blower issue).

Write the numbers down and share them with the technician. It speeds up diagnosis.

The most common causes

1. Dirty air filter. It restricts airflow and can freeze the indoor coil. Check it monthly in summer.
2. Dirty outdoor coil. Dust and debris keep the condenser from shedding heat. Gently rinse it with a garden hose (power off first), never a pressure washer.
3. Low refrigerant (a leak). Refrigerant doesn't get used up. If it's low, there's a leak that a licensed tech should find and repair.
4. Leaky or uninsulated attic ducts. Laredo attics get extremely hot, and leaks there pull hot attic air into the system. See air duct installation.
5. Undersized system or added heat load, such as an addition, new windows, or a system replaced "same size" without a load calculation.
6. Failing parts, such as a weak capacitor or a blower running at the wrong speed.
7. Sun and heat gain: west-facing windows, open blinds, and attic insulation that's thin or settled.

What you can do right now

• Replace the air filter.
• Make sure all supply vents are open and return grilles aren't blocked by furniture.
• Clear plants and debris from around the outdoor unit (about 2 feet of space).
• Close blinds on sunny windows in the afternoon.
• Pre-cool: set the thermostat lower in the morning and let it coast in the afternoon instead of fighting the peak.
• Avoid the oven and dryer during the hottest hours.

When to call a technician

Call if the temperature split is low, the system can't keep up in the morning or evening, you see ice on the lines or coil, or the outdoor fan isn't running. A licensed technician can check the refrigerant charge, coil condition, airflow and electrical parts. If the system is old or uses R-22, ask them to price a replacement alongside the repair. Call or text (956) 307-9262 and we'll connect you with a licensed Laredo contractor, or book AC repair.

This guide is general information, not a diagnosis. For safety, leave electrical and refrigerant work to a licensed technician. Laredo AC Pros is an HVAC referral service, not an HVAC contractor. We connect customers with independent air conditioning contractors licensed by the Texas Department of Licensing and Regulation (TDLR), including our current partner Twins Air Conditioning (TDLR #TACLA015293E). All HVAC work is performed, priced and warrantied by the contractor. Laredo AC Pros may receive a referral fee from contractors.