On a 110-degree Tucson afternoon, an AC that runs nonstop and can’t pull the house below 80 may be working at full capacity rather than failing. Temperatures that high push most systems past the conditions they were designed for. Warm air from the vents, ice on the lines or a sudden drop in cooling point to a real problem.
Signature Heating, Cooling & Plumbing is a veteran-owned, family-owned HVAC company in Tucson, and “my AC can’t keep up” is one of the most common calls we get every summer. Sometimes the system is doing all it can against extreme heat. Other times there’s a repairable problem hiding behind the weather. This guide will help you tell the difference, and if something seems wrong, our AC repair team can find the cause.
Why does an AC struggle when it’s 110 degrees outside?
An air conditioner works by moving heat from inside your home to the outdoor air. The hotter it is outside, the harder that job gets. At the same time, your home is absorbing heat faster through the roof, walls, windows and attic ductwork.
Residential systems in Tucson are sized using a summer design temperature, which is the outdoor temperature a load calculation plans around. That design temperature is well above 100°F here, but it’s typically below the 110-plus-degree peaks we see during the hottest stretches of summer. When the outdoor temperature climbs past the design point, a properly working, correctly sized AC may run continuously and still lose a few degrees through the late afternoon. That’s not a failure. It’s a system running at its limit.
Is the 20-degree AC rule true?
Not the way most people hear it. The common belief is that an air conditioner can only cool a home 20 degrees below the outdoor temperature, so 110 outside means 90 inside. That isn’t how it works.
The 20-degree number actually comes from temperature split, sometimes called delta T. That’s the difference between the air going into your system at the return vent and the cooled air coming out of a supply vent. It measures what’s happening across the indoor coil, not how much cooler your house can be than the outdoors. How cool your home gets depends on the system’s capacity, how much heat the house absorbs, and the condition of the equipment and ducts.
How do you check your AC’s temperature split?
A temperature split check won’t diagnose your system, but it can give you a useful clue before you call.
- Let the AC run for at least 15 minutes so the readings settle.
- Hold a thermometer at a return vent and note the temperature.
- Hold it in a supply vent close to the indoor unit and note that temperature.
- Subtract the supply reading from the return reading.
For example, 78°F at the return and 59°F at the supply gives you a 19-degree split. Many systems land somewhere around 15 to 20 degrees under normal conditions, but the right target depends on indoor humidity, airflow and the equipment. A split well below that range can point to low cooling capacity, while an unusually high split can point to restricted airflow. A normal split doesn’t rule out duct leaks or other problems, so treat it as a screening check.
What makes a Tucson AC fall behind in extreme heat?
If your AC is running but the house won’t cool down, one or more of these is often part of the picture.
Dirty outdoor coil
The outdoor condenser coil releases heat from your home into the air. Tucson’s dust, pollen and debris build up on the coil and make it harder for the system to shed heat, which matters most on the hottest days.
Hot attic ductwork
In many Tucson homes, ducts run through an attic that can get far hotter than the outdoor air. Leaky or poorly insulated ducts lose cooling before the air reaches your rooms, and rooms farthest from the air handler usually feel it first.
Afternoon sun on west-facing windows
Large windows that take direct afternoon sun add a lot of heat. If certain rooms get noticeably warmer after 2 p.m., solar heat gain may be a bigger factor than the AC itself.
Monsoon humidity
During monsoon season, your AC spends part of its capacity pulling moisture out of the air. That can lower the temperature split and make the house feel stickier at the same thermostat setting.
A large thermostat setback
Letting the house warm to 88 while you’re at work and then asking the AC to bring it down to 75 at 4 p.m. sets it up to fall behind. During extreme heat, a smaller setback or none at all usually keeps the house more comfortable.
What signs mean your AC actually has a problem?
Extreme heat alone doesn’t prove anything is broken. These symptoms do deserve a call to a technician:
- A sudden, noticeable drop in cooling
- Warm air coming from the vents while the system is running
- Ice on the refrigerant lines or indoor coil
- Weak airflow from several vents
- New grinding, buzzing or rattling noises
- A breaker that keeps tripping
- Poor cooling on days when it’s only in the 90s
If you see ice, switch the thermostat from cooling to off and let the system thaw before running it again, since running a frozen system can damage it. If a breaker trips more than once, leave it off and call for service rather than resetting it again.
What can you do to help your AC on a 110-degree day?
A few simple steps can help your system hold its ground through the hottest part of the day.
Check the air filter. A clogged filter restricts airflow, and Tucson dust fills filters faster than in many other places. Check it monthly during summer.
Keep the outdoor unit clear. Trim plants back and clear away leaves and debris. If the coil looks dirty, turn off power to the unit and rinse it gently with a garden hose from the inside out. Skip the pressure washer, which can bend the fins.
Close blinds on sunny windows. Shading west- and south-facing windows before the afternoon sun hits them keeps a lot of heat out.
Use ceiling fans. Fans don’t lower the temperature, but the moving air makes a room feel cooler, so you may be comfortable with the thermostat a degree or two higher.
Keep supply vents open. Closing vents in unused rooms doesn’t push more cold air elsewhere. It raises pressure in the ductwork and can reduce airflow through the whole system.
Save heat-producing chores for evening. Running the oven, dryer and dishwasher after sunset keeps extra heat out of the house during peak hours.
When should you schedule AC maintenance in Tucson?
Spring is the best time, before the first 100-degree week. An AC tune-up can catch weak capacitors, dirty coils and airflow problems while you still have time to fix them, instead of discovering them during a July heat wave. Maintenance won’t make an undersized system bigger, but it keeps a correctly sized system running at the capacity it was built for. Learn more about our AC maintenance service.
Could your AC be too small for your home?
It’s possible. If your system struggles every summer, even when it checks out as working properly, sizing may be the issue. The right size depends on your home’s cooling load, which accounts for insulation, windows, orientation and ductwork, not just square footage. Our guide on what size AC you need for a Tucson home explains how that’s calculated. A bigger system isn’t automatically better, since an oversized AC brings its own comfort problems.
Should you repair or replace an AC that can’t keep up?
Find the cause first. A dirty coil, a failing capacitor, duct leaks or a refrigerant problem can all make a system fall behind, and all of them can be repaired. If your system is older and has had repeated or major failures, it may be time to compare repair costs against a new system. Our guide on whether to repair or replace your AC walks through that decision.
Frequently Asked Questions
How cool should my house be when it’s 110 outside in Tucson?
There’s no single number every home should reach. A properly working AC may run continuously and hold the house somewhere in the upper 70s, depending on the home’s insulation, windows and equipment. A home that keeps getting warmer through the afternoon, or won’t cool at all, should be checked.
Why does my AC run constantly in the Tucson summer?
On extreme-heat days, continuous operation is often normal because the system is working at full capacity. If it’s also blowing warm air, icing up or making unusual noises, call for service.
What is a good temperature split for an air conditioner?
Many systems fall around 15 to 20 degrees under normal conditions, but the correct target depends on indoor humidity, airflow and the equipment. A technician can compare your reading to the right target for your system.
Is my AC broken if it can’t reach the thermostat setting?
Not necessarily. On the hottest days, even a healthy system may not reach an aggressive setting like 72. A sudden change in performance, warm air or ice are signs something else is going on.
AC Struggling to Keep Up?
We’ll tell you honestly whether your system is fighting the heat or fighting a problem. Signature Heating, Cooling & Plumbing provides AC repair, tune-ups and replacement for homes and businesses across Tucson, Oro Valley, Marana, Vail and nearby communities. Call (520) 201-8043 or request a free estimate. If your AC stops cooling altogether on a 110-degree day, we offer 24/7 emergency service.
