The vents are blowing and the outdoor unit hums, but the house sits at 84 and climbing. Between June and September that’s the call we take more than any other, and it almost never has an exotic cause. Seven failures account for most of it, and four you can rule out yourself in ten minutes.
Answer two questions before you touch anything
Is the indoor blower moving air out of the vents? Is the outdoor fan spinning?
If either answer is no, you have an electrical fault or a tripped safety switch, not a refrigerant problem. If both run and the air still comes out warm, the system has lost heat transfer, which points at airflow or charge.
That one split saves a lot of people a service call they never needed. A thermostat left in fan mode moves plenty of air, cools nothing, and fools smart people every summer.
The seven causes we find on no-cool calls
A wrong thermostat setting or a tripped breaker comes first, and it happens more than anybody admits. A loaded filter is second. Airflow starves, the indoor coil runs colder than it should, and the system ices over.
Three is that frozen coil itself, which looks like total failure and is usually a symptom of something upstream. Four is a failed run capacitor or contactor. The compressor and the outdoor fan both lean on that capacitor to start, and our heat is hard on them. We broke that down in why AC capacitors fail.
Five is a condenser coil matted with live oak litter and grass clippings, trapping the heat your system is trying to dump outside. Six is low refrigerant charge. Seven is duct leakage with undersized returns, the quiet one, where good equipment throws its capacity away into a superheated attic.
What field studies found inside real houses
The U.S. Department of Energy reviewed 44 field studies of residential HVAC systems and found 70 to 90 percent carried at least one performance-compromising fault. Count duct leakage and that climbs to 90 to 100 percent.
Karen Fenaughty and Danny Parker of the Florida Solar Energy Center put numbers to it, citing lab work by Neal and O’Neal. A 20 percent undercharge on a three-ton system cuts sensible cooling capacity by roughly 24 percent at 95 degrees outside. Their review also found field airflow averaging 317 cubic feet per minute per ton against a recommended 400, costing another 10 percent of rated capacity.
Tom Downey and John Proctor of Proctor Engineering Group measured 13,258 units and found 57 percent outside specification on refrigerant charge.
Why our run hours turn a small fault into a dead system
None of those faults kill a system in a mild climate. They cost a little efficiency and nobody notices. Here they stack up.
Our cooling season runs close to six months, with August highs near 96 and long stretches of triple digits. The Energy Information Administration puts cooling at 27 percent of home energy costs in the hot-humid South, against 12 percent nationally. A compressor starting twenty times a day for six months wears out faster than one starting five times a day for three.
CPS Energy adds its own signal: the peak capacity charge applies June through September on usage above 600 kilowatt-hours. That’s exactly when a 10 percent capacity loss stops being invisible.
A system rarely dies in July. It dies of something that started in April and nobody bothered to measure.
The ten-minute check before you pick up the phone
No tools required:
- Set the thermostat to cool and the fan to auto, drop the setpoint five degrees, and listen outside.
- Replace the thermostat batteries even if the screen still lights up.
- Pull the filter and hold it toward a light. If nothing comes through, you found your airflow. Clear the return grille too.
- Check the panel for a tripped double breaker, then the disconnect box beside the condenser.
- Walk the outdoor coil. Grass clippings and shrubs pressed against the fins trap heat.
- Feel the large copper line outside. Ice means shut down and run the fan alone for a few hours.
The same symptom looks different by neighborhood
Ask why your AC isn’t cooling and the honest answer shifts with the age of the house.
Alamo Heights and the 1920s masonry
The stock here runs to 1920s and 1930s Craftsman and Spanish revival, masonry and stucco with original framing. Duct was added decades later, undersized and jammed into chases nobody planned for. In these homes we check the return path before we ever look at the condenser. Heavy mature shade cuts the cooling load, which helps, but it also means more debris landing on an outdoor coil that nobody thinks to rinse.
Balcones Heights and the postwar small lots
Balcones Heights is mid-century housing on tight small lots, and the geometry does real damage. A condenser wedged into a narrow side yard ends up breathing its own discharge air, head pressure climbs, and the house keeps gaining while the system works harder. Give the unit the clearance its manufacturer asks for before anybody blames the compressor. Equipment here also skews old enough that part availability shapes what the AC repair looks like.
Leon Valley and the 1950s originals
Leon Valley dates to the 1950s, so most of these houses are on their second system by now. Partial replacements are the pattern: a new condenser bolted to an old indoor coil, or a coil swapped without matching the metering device. A mismatched pair cools fine at 8 a.m. and gives up by 4 p.m. Ask what was actually replaced and when, because that history explains more here than any single reading does.
Low charge always means a leak
Refrigerant isn’t fuel. It moves through a sealed loop and never gets consumed, so a system reading low is leaking somewhere. Topping it off without finding the leak buys a few weeks and leaves the compressor running underfed.
Refrigerant type is a real cost driver now. Under the EPA’s HFC phasedown, production sits at 60 percent of the historic baseline through 2028 and drops to 30 percent for 2029 through 2033, headed toward an 85 percent cut by 2036. The EPA is clear that you can keep repairing an existing R-410A system, compressor and condensing unit included. Anyone charging or recovering refrigerant has to hold EPA Section 608 certification, and venting it breaks federal law.
What a real no-cool diagnosis covers
Swapping parts off a symptom is a coin flip. These get tested first:
- Static pressure across the air handler. It shows whether ducts, filter, or coil are choking airflow. Most calls skip it.
- Superheat and subcooling. How charge gets verified against the manufacturer’s spec, not by how the copper feels.
- Capacitor and contactor under load. A capacitor can read fine at rest and collapse when the compressor calls. Pitted contacts cause the no-cool that clears before the tech arrives.
- Condensate drain and float switch. A plugged drain shuts the system off by design. Clearing it is a repair, not a mystery.
- Duct integrity and return sizing. Leaks and starved returns are why sealing often beats a new condenser on an older home.
- Outdoor coil and fan motor. A packed coil drives head pressure up and mimics low charge on a gauge set.
When it isn’t the air conditioner at all
Sometimes the equipment is fine and the house is the problem. A west wall of glass with no film will beat a healthy system on an August afternoon. So will a unit oversized at install that short cycles and never pulls humidity down. Neither shows up on a gauge set.
Better to hear that up front. We’re family-owned, licensed in Texas as a Class A air conditioning and refrigeration contractor under TACLA00102222C, EPA 608 certified, and BBB A+ rated. We’d rather fix a capacitor than sell you a system you don’t need. Every repair carries our 90-day warranty, and financing is available when replacement really is the honest answer.
Bring us the symptom and we’ll test the full cooling cycle, then explain what failed in plain English. Need an air conditioning repair company in San Antonio? Call or text 210-763-4698, Monday through Saturday, 7 a.m. to 7 p.m.