An air conditioner is sized for a normal hot day, not the worst one. In Pasadena, CA and the San Fernando Valley, that design day is roughly the mid-90s. When it is 108°F outside, a correctly sized, perfectly working system may run without stopping and hold the house in the high 70s, and that is the system doing what it was built to do. The question is whether yours is healthy and simply outmatched, or whether something is wrong that is costing you the last 5 degrees.
How much cooling to expect from a working system
A central AC pulls a fixed amount of heat out of the house per hour. The house gains heat through the roof, the walls, the windows, and every gap in the ducts. On a design day, the two roughly balance and the system cycles on and off. On a heat-wave day, the house gains heat faster than the system can remove it, so it runs continuously and the indoor temperature settles wherever those two rates meet.
A useful rule of thumb from the trade: a healthy system drops the air passing over the indoor coil by roughly 15 to 20 degrees. If the air at a supply register is about that much cooler than the air at the return grille, the refrigerant side is working. What happens after that, in the attic and through the walls, is a separate question.
The causes that are actually fixable
Before you accept "it's just hot," rule these out. Most of them show up at the same time as a heat wave because that is when the margin runs out.
- Dirty filter or dirty coils. A clogged filter cuts airflow, so the coil cannot deliver its full capacity. A dirty outdoor coil cannot shed heat, and capacity drops when it is hottest outside. These are the first two things a tech checks because they are the cheapest.
- Ducts in a 130°F attic. Most 1950s to 1980s ranch homes across the SGV and SFV have their supply ducts running through the attic. On a heat-wave afternoon, the attic can sit above 130°F. Thin or crushed insulation on the ducts means the air picks up heat on the way to the rooms. Leaks are worse: a disconnected boot or a split at a joint dumps cooled air into the attic and pulls hot attic air into the return.
- Low refrigerant. A system that is a little low works fine in May and falls apart in August. Low charge means the coil runs warmer and the split at the registers drops. Refrigerant does not get used up; if it is low, there is a leak.
- Undersized or oversized equipment. An undersized system was never going to hold 72 in a heat wave. An oversized one short-cycles and never runs long enough to pull humidity out, so the house feels warmer than the thermostat says. Both are sizing problems, not repair problems.
- The house itself. Single-pane windows on the west side, a dark roof, no attic insulation, and a 1920s bungalow with no wall insulation all add load. These are outside our trade, but they are why two identical systems in two different houses perform differently.
- Thermostat placement. A thermostat in a hallway that stays cool will shut the system off while the bedrooms are still at 82. A thermostat in direct sun does the opposite.
What you can check yourself
- Read the split. Hold a kitchen thermometer in the return grille for a minute, then at the nearest supply register. A difference of roughly 15 to 20 degrees means the refrigerant side is likely fine. A difference under 10 points at airflow or charge.
- Change the filter. Then check it again in two weeks, because heat-wave dust and smoke load it faster.
- Look at the outdoor unit. Clear anything within two feet. If the coil fins look gray or matted, that is your capacity loss. You can rinse the coil gently from the outside with a hose after cutting power at the disconnect, but leave chemical cleaning to a tech.
- Walk the registers. Is one room noticeably weaker than the rest? That points at a duct problem on that run.
- Close the blinds on west-facing windows by noon. Not glamorous, but it removes real load.
- Stop chasing the setpoint. Setting the thermostat to 65 does not cool faster. The system runs at one speed. It just runs longer.
- Pre-cool. Set the thermostat a few degrees lower in the morning while the outdoor air is cooler and the system has capacity to spare, then let it drift up in the afternoon. This is what the state grid operator asks for during a Flex Alert, and it works in your favor either way.
When to call a licensed tech
Call if the split at the registers is low, if the outdoor unit is running but the fan is slow or the line set is iced, if one room is far behind the others, or if the system used to hold the house fine and now does not. A tech will measure the actual charge, check the coil temperatures, and put a manometer on the ducts to find where the air is going. Our AC repair visit covers the refrigerant and airflow side. If the ducts are the problem, and in an attic install they often are, duct repair that seals the joints and reinsulates the runs can add back more capacity than any part on the outdoor unit.
If the system is old, tests fine, and still cannot keep up, that is a sizing conversation. A load calculation on the house tells you what it actually needs, and an AC replacement sized to that number performs differently from one picked off the old unit's data plate.
Common questions
Is it bad for the AC to run all day?
No. Continuous running in a heat wave is normal and is easier on the compressor than short cycling. What hurts the system is running with a dirty coil or low charge, because the compressor works hotter than it should for hours. If it runs all day and the split at the registers is healthy, let it run.
Should I turn it off during the hottest part of the day to let it rest?
No. Once the house heats up, the system has to remove all of that heat again, and it will be doing it at the worst time of day. Keep it running and pre-cool in the morning instead.
Would a bigger system fix this?
Sometimes, but not as often as people expect. If the ducts are leaking a third of the air into the attic, a bigger unit just leaks a third of its bigger airflow into the attic. Fix the delivery first, then size the equipment. A tech who suggests a bigger unit without looking in the attic is guessing.
If your system is running nonstop and the house is not getting there, book a $69 diagnostic. We tell you the fee before booking and credit it toward the work. Boost Services Heating and Cooling answers at 818-277-5056 or send us a message and we will usually be out within a few hours.
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.