AC Running All Day in August: Normal, or a Problem?

Every August we get a version of the same call. The AC has been running since lunchtime, it has not shut off once, and the homeowner wants to know whether that is going to break something. The honest answer is that it depends on what the thermostat says.

Air conditioners are sized for a design day, not the worst day

A correctly sized system is not built to cool the house effortlessly on the hottest afternoon of the year. It is sized to hold the house at temperature on a typical hot day with some margin. On a run of triple-digit afternoons, with the sun on the west wall and the attic well over 130 degrees, a properly sized system may need to run continuously to hold the set point. That is by design. A system large enough to cycle comfortably on the worst day would short-cycle and leave the house clammy the other 350 days.

Around Rockwall and Rowlett there is a second factor. Air off Lake Ray Hubbard carries more moisture than air on the other side of Dallas, and removing that moisture is a large part of the work the system does. Long run times in humid weather are the system pulling water out of the air, which is exactly what you want it to do.

The test: is it holding, or losing?

Look at the thermostat in the late afternoon. If it reads the set point or within a degree or two of it, the system is working as hard as it was designed to and it is keeping up. Continuous running in that situation is normal and does not damage the equipment. Compressors are built to run for hours.

If the reading is climbing through the afternoon, three, four, five degrees above the set point and still rising at 7pm, the system is losing. That is when something is wrong, and it is worth a call.

What makes a system lose

• A dirty filter or a matted evaporator coil. Airflow drops, the coil gets too cold, and in the worst case it ices over and the system stops cooling altogether.
• A dirty outdoor coil. Cottonwood fluff, grass clippings and dust on the condenser fins stop the unit rejecting heat. Gently hosing the coil from the inside out often makes a visible difference the same afternoon.
• Low refrigerant. A system that is a little low will run and run and never quite get there. It does not use up refrigerant; if it is low, there is a leak.
• Leaky or uninsulated attic ducts. Cold air escaping into a 140-degree attic before it reaches the rooms is one of the most common reasons a system cannot keep up in our area.
• A weak capacitor or a compressor that is not starting properly, so the outdoor fan runs but the unit is not actually compressing.
• A thermostat that has been set several degrees lower than usual in the hope of catching up. This does not help and it hides the real picture.

Things that help without touching the equipment

Close blinds on the west side after noon. Run the dryer and the oven in the evening. Make sure supply registers are open and not blocked by furniture. Change the filter if it has been more than a month in heavy use. None of these are repairs, but on a marginal day they are the difference between holding and losing.

When to call

Call if the house is losing ground, if you see ice on the copper line at the outdoor unit or on the indoor coil, if the outdoor fan runs but the air at the registers is not cold, or if the system has started tripping its breaker. Those are faults, not weather. Our techs check refrigerant charge, static pressure, capacitor values and coil condition on the same visit, so you get an answer rather than a top-up.

Family-owned in Rockwall, licensed and insured, and we answer 24/7 through the hottest weeks of the year. TX HVAC License TACLB2360E. (469) 338-5533.