Every AC system is sized and rated to handle a certain outdoor temperature — its "design condition" — and cool the house to a comfortable indoor setpoint under those conditions. During a genuine heat wave, when it's 100, 105, or higher outside, you're often well past what the system was ever designed to fully overcome. A perfectly healthy AC in that situation often can't hit 72 degrees; it may only manage a 15–20 degree drop below the outdoor temperature, running close to nonstop the whole time. That's normal physics, not necessarily a broken system.
The catch is that a real fault — low refrigerant, a dirty coil or filter, an undersized system, or leaky ducts — makes this normal strain dramatically worse, and a heat wave is exactly when a marginal problem stops being tolerable. Here's how to tell the difference.
What to check and do while the heat wave lasts
- Change the filter if it's due. A restricted filter costs you exactly the airflow and capacity you can't spare on the hottest day of the year — check it before anything else.
- Keep the outdoor unit clear, and shaded if you can manage it. The condenser rejects heat to the outside air, so a unit sitting in direct sun works harder than one with some shade. Never block airflow to create that shade — a shade cloth or umbrella positioned a few feet away is fine, right up against the unit is not.
- Close blinds and shades on the sun-facing side of the house. Direct solar heat coming through glass adds real load to what the AC has to remove. Closing up the sunny side during peak afternoon hours makes a noticeable difference.
- Keep doors and windows closed while the AC runs. Every trip in or out lets conditioned air escape and hot air in — during a heat wave, that matters more than usual.
- Don't set the thermostat drastically below what you actually want. Setting it to 60 won't cool the house any faster than setting it to 72; the AC runs at the same capacity either way, and an unrealistic setpoint just means it runs longer chasing a number it may never reach.
- Hold off on major heat-generating appliances during peak hours if you can. Running the oven or the dryer during the hottest part of the afternoon adds to the load your AC is already struggling with.
Normal strain vs. a real problem
Every AC is rated to a design condition, and a genuine heat wave can push well past it. A healthy system asked to fight 105-degree heat may only manage a 15–20 degree drop below the outdoor temperature, running close to continuously the entire time — that alone doesn't mean anything is broken. Expect the house to run warmer than normal on the worst days, and expect the system to be loud and constant, simply because it's working at its limit.
What isn't normal, even during extreme heat, is a house that stays 8 to 10 degrees or more above the thermostat setpoint and isn't closing that gap at all as the day cools into evening — that points to a real problem stacked on top of the weather, not just the weather. So does ice anywhere on the indoor coil or the refrigerant line, a new rattle, squeal, or grinding noise, or a system that's short-cycling — turning on and off in short bursts instead of running a full cycle.
If you're seeing any of those, the extreme heat is exposing a fault that was probably already there: low refrigerant from a leak, a dirty evaporator coil restricting airflow, or a failing capacitor that can't keep the compressor running steadily under load. In our pricing, a capacitor is about $120–$350 to replace and a refrigerant leak repair with recharge runs roughly $180–$650 — both far more manageable fixed the moment you notice the pattern than after the compressor gets pushed to failure trying to compensate.
When to call it in
If your AC is running constantly but the house is only a few degrees off target and slowly catching up, that's the heat wave, not a broken system — keep it running and give it time. If the gap is large and not closing, or you see ice or hear something new, that's worth a call before the next heat wave, not during it.
Triton repairs central AC, heat pumps, and mini-splits across the inland Bay Area — Livermore, Pleasanton, Dublin, San Ramon, and Hayward — where heat waves hit hardest. Licensed techs, upfront pricing, and a diagnostic fee credited toward the repair when you approve the work.
Book online or call (888) 993-5699. We'll confirm whether it's the heat or a real fault and get it fixed either way.
Related pages
- HVAC Repair in the SF Bay Area — our full heating & cooling hub
- AC Not Cooling / Blowing Warm Air? — when it truly isn't cooling at all
- AC Short Cycling (Keeps Turning On and Off) — a real fault, not normal heat-wave strain
- AC Repair Cost Guide — typical part-by-part pricing
- HVAC & AC Repair in Pleasanton — Tri-Valley service area
Frequently asked questions
Why does my AC run constantly during a heat wave but the house still feels warm? On an extreme-heat day an AC can be working at its limit and still only manage a 15–20 degree drop below the outdoor temperature, rather than hitting the exact thermostat setpoint. Running nonstop and losing a little ground is often normal physics during a genuine heat wave, not proof something is broken.
How do I know if it's just the heat or an actual AC problem? Watch the gap between the thermostat setpoint and the actual house temperature over several hours. A few degrees off that slowly closes as it cools off outside is normal strain. A large gap — 8 to 10 degrees or more — that isn't closing, or ice on the coil, a new noise, or short cycling, points to a real fault.
Will setting my thermostat lower make the house cool faster during a heat wave? No. The AC cools at the same rate no matter what the thermostat is set to; setting it far below the temperature you actually want just means it keeps running once it gets there, or runs a very long time trying to reach a number it may not hit at all on the hottest days.
Does shading the outdoor AC unit actually help? It can, since the outdoor unit is rejecting heat into the surrounding air and works a bit easier out of direct sun. The key is doing it without restricting airflow — a shade cloth or umbrella set a few feet away is fine, but nothing should touch or crowd the unit itself.
Should I keep running my AC nonstop during a heat wave, or will that damage it? A healthy system is built to run for long stretches on the hottest days and running it isn't what damages it. What's worth stopping for is ice on the unit, a new noise, or short cycling — those are signs of an actual fault, not signs that the AC is simply working hard.
What's the most common real cause of an AC losing ground during extreme heat? Low refrigerant from a leak and a dirty or restricted evaporator coil are the two most common, both of which cut the system's cooling capacity right when it has the least margin to spare. A weak capacitor that can't keep the compressor running steadily under load is also common.