This is the call we take most often in August, and it usually starts the same way. The thermostat is set to 72. The unit outside is running. It has been running since lunch. The house is sitting at 78 and will not come down.

The first thing worth saying is that this often does not mean your air conditioner is broken.

Your system was sized for a design day, and August beats it

Every residential system in North Texas is sized against a design temperature. It is the number an installer works from when deciding how much cooling a house needs, and around here it lands somewhere near 100 degrees.

That number is not a maximum. It is a planning figure. When the outdoor temperature climbs past it, a healthy system that is working exactly as designed will start losing ground. It can still remove heat, just not quite as fast as the house is taking it on. So it runs longer, and the gap between the thermostat setting and the actual room temperature widens through the afternoon.

A system that holds 75 on a 98 degree day and drifts to 78 on a 106 degree day is not failing. It is doing what it was built to do, at the edge of what it was built for.

That is also why the advice to drop the thermostat to 65 backfires. The system cannot reach 65, so it never cycles off, and you get a longer runtime and a higher bill for the same indoor temperature.

The one check you can do yourself

Before you call anybody, there is a single measurement that separates "hot day" from "something is actually wrong," and you can do it with any kitchen thermometer.

Measure the air going into the return vent, the big intake grille. Then measure the air coming out of a supply vent near the unit. Subtract.

On a properly working system that difference should be roughly 16 to 22 degrees.

So if the air going in is 78, the air coming out should be somewhere around 56 to 62.

Here is what the answer tells you:

A split in that range. Your system is cooling correctly. What you have is a hot day and a system at the edge of its capacity. Close blinds on the west side, keep the thermostat steady rather than chasing it down, and it will catch up in the evening.

A split well under 16 degrees. The system is not removing as much heat as it should. The usual causes are low refrigerant, which means a leak somewhere, or an outdoor coil packed with cottonwood and grass clippings. Both are fixable and neither one improves on its own.

A split well over 22 degrees. That sounds good and is not. It usually means air is not moving through the system fast enough. A clogged filter is the common cause. Restricted airflow makes the coil colder than it should be, and a coil that gets cold enough will freeze.

Turn it off if you see ice

If there is ice on the copper line running into the house, or frost on the indoor coil, turn the system off at the thermostat and leave the fan running.

Running an iced up system is how a repair becomes a replacement. The ice blocks airflow, the compressor keeps pulling liquid refrigerant it was never meant to handle, and compressors are the most expensive part of the system. Give it a few hours to thaw with the fan on, change the filter, and see whether it recovers.

If it ices up again, stop and call. That one is not a maintenance item.

What we look at when we come out

When somebody calls us with this, we are checking a short list before anything else. Refrigerant charge, because low charge is the most common real cause. The outdoor coil, because a dirty one cannot reject heat no matter how well the rest of the system works. Filter and airflow. Duct condition, because leaky ducts running through a 140 degree attic lose cooling before it ever reaches a room. And the age and sizing of the equipment, particularly on houses where somebody added square footage and never resized the system.

Most of those are repairs, not replacements. We would rather fix your system than sell you a new one, and in August most of these turn out to be a charge problem, a dirty coil, or a filter nobody has changed since spring.

When it is genuinely time to replace

There are cases where replacement is the honest answer, and we will tell you so. A compressor failure on a system past fifteen years. A refrigerant leak in the evaporator coil where the repair approaches the cost of new equipment. Or a system that was undersized for the house from the day it went in, which is common in homes that got a room addition or an enclosed patio without anyone recalculating the load.

Those are real. They are also less common than the sales pitch would suggest.

If your house will not cool and you have run the check above, call us at 972-242-1000. We answer around the clock, and our crews are out Monday through Friday and Saturday by appointment.