My AC runs all day in July and still can’t keep up. What’s actually causing it?

What does it mean when the AC can't keep up on the hottest afternoons?

Residential cooling equipment is sized to a design condition, not to the worst day imaginable. The design temperature for a region is a statistically hot afternoon that the area exceeds only a small fraction of hours each summer. On a day well above that condition, a correctly sized, correctly working system is expected to run continuously and hold a few degrees above its setpoint. That is not failure. That is the design working as intended at the edge of its envelope.

So the first question is not whether it runs constantly. It is whether the gap is small and only on extreme days, or large and on ordinary ones.

• Probably normal: the system runs nearly nonstop on a triple-digit afternoon and holds within two or three degrees of setpoint, then recovers after sunset.
• Probably a fault: it cannot hold setpoint on an ordinary afternoon near the normal August high, or the gap widens through the season, or it used to hold and no longer does.
• Definitely worth a call: the air from the registers is not cold, the outdoor unit is iced, or the indoor coil is frozen.

Take two measurements before you call anyone. Put a thermometer in the return air grille and another in a supply register nearby, and note the difference. Then write down what the thermostat reads versus what a separate thermometer in the same room reads. Those two numbers turn a vague complaint into a diagnosis a technician can start from.

Which mechanical faults should a technician rule out first?

In rough order of how often they turn out to be the answer:

• Air filter. A loaded filter starves the blower. On systems with tight filter racks and high-MERV media, this happens faster than owners expect.
• Outdoor condenser coil. Cottonwood, grass clippings and dust mat the fins. The unit cannot reject heat it cannot move air across. This is the single most common cause of a system that worked in May and struggles in July.
• Indoor evaporator coil. Dirt on the coil face insulates it and reduces both cooling and dehumidification. It usually requires a technician to see and to clean.
• Refrigerant charge. A sealed system does not consume refrigerant. If it is low, it is leaking, and a technician who tops it off without finding the leak has sold you the same repair again next summer.
• Blower speed and capacitor health. A weak run capacitor or a blower set to the wrong tap quietly costs capacity.
• Thermostat location. A thermostat on a wall with sun exposure, above a lamp, or near a supply register reads a temperature the rest of the house does not have.

Ask for the numbers, not the verdict. A competent technician will tell you the suction and liquid pressures, the superheat or subcooling, the temperature split across the coil and the total external static pressure. If the answer to what is wrong is a shrug and a quote for a new system, get another opinion before you spend five figures.

How much of the problem is hiding in the ductwork?

A great deal of it, in most houses. Duct systems in North Texas typically run through the attic, which is the worst possible environment for them, and they were often installed with more attention to speed than to airflow.

The things that go wrong:

• Leakage. Supply leaks dump cooled air into the attic. Return leaks pull attic air into the system, which means the equipment is partly cooling the attic and delivering the result to your living room.
• Undersized returns. Air cannot be supplied faster than it can be returned. Many homes have a single central return grille sized for a smaller system than the one now installed.
• Excess static pressure. Long flex runs, sharp bends, undersized trunks and restrictive filters all raise resistance. Blowers respond by moving less air, which shows up as poor cooling that no amount of refrigerant will fix.
• Crushed or disconnected runs. A boot that fell off in the attic can take a room out of the system entirely.

A technician who measures total external static pressure at the air handler will find these problems. One who does not will keep looking at the refrigerant circuit, because that is where the gauges are. Duct repair is unglamorous, and in a house that never cooled well from day one, it is frequently the entire answer.

Was the system ever sized correctly for this house?

The industry standard for sizing residential equipment is a Manual J load calculation, published by the Air Conditioning Contractors of America. It accounts for the actual house: square footage, orientation, glass area and type, insulation levels, infiltration, duct location and internal gains. It takes real effort and it produces a real number.

What gets used instead, frequently, is a rule of thumb based on square feet per ton. Building science sources have criticized this for decades, because it ignores everything about the house that matters and it errs toward oversizing.

Oversizing is not the safe mistake people assume. An oversized system satisfies the thermostat quickly and shuts off, which is called short cycling. The consequences are consistent: poor dehumidification because the coil never runs long enough to wring water out of the air, uneven temperatures because rooms far from the air handler never get their share before the cycle ends, more wear from frequent starts, and a house that feels clammy at its setpoint.

Undersizing produces the opposite symptom, the one that brought you to this page. If your system is genuinely too small for the load, it will run continuously and lose ground on ordinary hot days, not just extreme ones.

Before anyone replaces equipment, insist on a Manual J for the house as it exists today. If you have added insulation, changed windows, finished an attic space or built an addition, the load is not what it was. A replacement sized off the old nameplate simply repeats whatever decision was made years ago.

Why does everything feel worse at exactly the same hour across the metroplex?

Because it is. Cooling demand across North Texas peaks in the late afternoon, when solar gain on every building has been accumulating all day and outdoor temperatures are at their highest. ERCOT, the grid operator for most of Texas, set its all-time summer peak demand record of 85,508 MW on August 10, 2023. On September 6, 2023, ERCOT issued a Level 2 Energy Emergency Alert and asked Texans to voluntarily conserve.

That coincidence matters for your house in two practical ways. First, your equipment is being asked for its maximum output at the moment its own operating conditions are worst, because a condenser rejects heat less efficiently into hot outdoor air. Second, electricity you use at that hour is the most expensive electricity on the system. The EIA's 2024 figures put the Texas average residential price at 14.94 cents per kilowatt hour.

The strategy that follows from this is pre-cooling. Rather than holding one setpoint all day, drop the thermostat a degree or two in the late morning while the equipment still has capacity in reserve, and let it drift up slightly during the worst three or four hours. The building's mass carries you part way through. It does not repair anything mechanical, and it is not free, but it converts a losing battle at 5pm into a manageable one.

Once the equipment checks out, how do you lower the load itself?

If a technician has verified charge, cleaned both coils, measured static pressure, repaired the ducts and confirmed the equipment is sized to a current load calculation, and the house still drifts on hot afternoons, then the equipment is not the problem. The load is.

Load reduction means less heat entering the building in the first place, and it comes from the roof, the attic, the walls, air leakage and the glass. Glass is where we work. Its Prestige Series is rated by 3M at 50% to 62% total solar energy rejected, with infrared rejection reaching 97% and a range of light levels to choose from. Cutting that much solar energy on the sun-exposed elevations lowers the peak load the equipment has to meet at the hour it is already struggling.

The honest limits of that: film cannot add tonnage. It cannot compensate for an undersized system, it will not offset a refrigerant leak, and on a house whose ducts leak into the attic it is treating the smaller of two problems. It reduces demand. It does not create supply.

The case where load reduction genuinely shines is the house that is one size short. A system slightly outmatched by its load can often be brought back into balance by removing solar gain, which is a far cheaper conversation than replacing equipment that otherwise works. Our sun control film options are worth pricing at that point, not before.