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Common problems
AC runs all the time
The AC runs for hours without reaching the setting, or barely shuts off even in moderate heat. Long runs aren’t always a problem, but long runs while the house stays warm usually are.
What it usually means
- A correctly sized system is chosen for a typical hot day in your area (the design day), not the hottest day on record. In a heat wave it may run nonstop and let the house drift a few degrees above the setting. That alone doesn’t mean it’s broken.
- Variable-speed systems are designed to run for long stretches at low speed, which dries the air better and uses less energy. Check whether yours is one before worrying about run time.
- Long runs on ordinary days, warm air at the vents or a rising bill point to lost capacity: a dirty filter or coil, low refrigerant from a leak, leaky ducts, or more heat getting into the house than the system was sized for.
Where it’s most common
Our climate research lists this as a common problem in these metro areas. Each guide covers what homes there need, local audit programs and rebates.
- Hot and humid Dallas–Fort Worth · Greater Houston · Miami · Tampa · Orlando · San Antonio · Austin
- Hot and dry Phoenix · Riverside
- Mixed New York City · Atlanta · Washington, DC · Philadelphia · Charlotte · Baltimore · St. Louis
- Cold Denver
Somewhere else? Find your metro guide.
Likely causes, by climate
The same symptom has different causes in Miami, Phoenix and Minneapolis. Find your climate below; the tag marks the climates where this problem is most common.
Hot and humid Florida, Texas and the Gulf Coast Common here
The AC handles heat and moisture, and runs most of the year. Leaky attic ducts, a dirty outdoor coil, low refrigerant and attic heat are the usual causes, and an aging system loses capacity, too.
Hot and dry the desert Southwest and inland Southern California Common here
With so many afternoons above 100°F, a healthy system can run nonstop at the peak of the day. Watch the air at the vents and the bill instead: attic ducts, a dirty outdoor coil and low refrigerant are common, and an evaporative cooler loses cooling power on humid monsoon days.
Mixed the Mid-Atlantic, St. Louis and the Southeast Piedmont Common here
Long runs in a heat wave are normal. On ordinary summer days, look at ducts in the attic or crawlspace, a dirty coil and refrigerant leaks. Heat waves are when systems most often fail, so have it serviced before summer.
Cold the Northeast, Great Lakes, Upper Midwest and Colorado Front Range Common here
Central AC here is sized for shorter, milder summers. In a heat wave, a healthy system may run all afternoon, and window units in upstairs rooms often can’t keep up. If it runs constantly on mild days, look for low refrigerant, a dirty coil or attic ducts.
Marine and mild coastal the Pacific Northwest and coastal California
Many homes have no AC or added it recently. Portable and window units often can’t keep up in a heat wave, and a ductless heat pump sized for the rooms you use is the usual upgrade. Ducted AC added to an older home may have undersized or leaky ducts.
Not sure which climate you’re in? Your metro guide says, and the climate zone entry explains the map builders use.
How an audit confirms it
Guessing is how people end up with new equipment that doesn’t fix the problem. These tests measure the cause. Ask for them by name when you book; each guide explains what the results mean and what to do next.
- Manual J load calculation A Manual J load calculation shows how big the system should be for your house and climate, so you can tell undersized from underperforming.
- Duct leakage test Leaky ducts in a hot attic can waste a big share of the system’s output before it reaches a room.
- Attic inspection Thin attic insulation and attic heat raise the load the AC has to meet.
- Energy bill analysis Compares your summer use with past years and the weather, which shows whether something changed.
- Blower door test Measures how much hot outdoor air leaks in.
Try first (cheap or free)
- Replace the filter. A clogged filter is the most common, and cheapest, cause of weak cooling.
- With the power off at the outdoor disconnect, gently rinse dirt and debris off the outdoor coil with a garden hose (no pressure washer), and keep plants back from the unit.
- Hold your hand to a supply vent: if the air isn’t noticeably cool, or the house stays far above the setting for hours, call a technician.
- Close blinds on sunny windows, and save oven cooking and the dryer for cooler hours.
- Look up your model number: if it’s a variable-speed system, long, quiet runs are how it’s meant to work.
The fix ladder
From the cheapest step to the biggest. Work up only as far as you need to, and test again after.
- New filter, a clean outdoor coil and clear space around the unit
- A professional tune-up: refrigerant charge, airflow, indoor coil and drain
- Find and repair a refrigerant leak; topping it up without a repair doesn’t last
- Seal and insulate ducts; improve attic insulation and window shading
- Replace an aging or undersized system with one sized by Manual J; variable-speed for long, efficient runs
When to call a pro
- There’s ice on the refrigerant lines or indoor coil. Switch to fan only so it thaws, and call a technician: running it frozen can damage the compressor.
- The air at the vents is warm, or the outdoor unit hums, clicks or won’t start.
- The AC fails in a heat wave. Don’t wait it out in a hot house: heat illness is the main risk when cooling fails, so go somewhere cool, such as a friend’s home or a cooling center, until it’s fixed.
Common mistakes
- Setting the thermostat very low to cool the house faster: it doesn’t, it just runs longer.
- Buying a bigger unit without a load calculation: oversized systems short-cycle and leave the air humid.
Paying for the fixes
Utility programs often pay toward audits, duct sealing, insulation and heat pumps, and some require an audit first. Your metro guide lists the programs near you, and your state’s page covers state and federal rebates.
Words worth knowing
- Load calculation
- An engineering calculation of how much heating and cooling a house needs, room by room, used to size equipment correctly.
- Manual J (ACCA load calculation)
- The US industry standard for calculating how much heating and cooling a house needs, room by room, so equipment can be sized correctly.
- Design temperature
- The cold (or hot) outdoor temperature a heating or cooling system is sized for; colder than most winter days, but not the record low.
- Oversizing
- Installing heating or cooling equipment with much more capacity than the house needs, usually from rules of thumb, not a load calculation.
- Short cycling
- When heating or cooling equipment turns on and off frequently in short bursts instead of running in steady, longer cycles.
- Variable-speed (inverter) compressor
- A compressor that ramps its speed up and down to match the heating or cooling needed, instead of switching fully on and off.
- Single-stage, two-stage and variable-speed
- How many output levels heating or cooling equipment has: single-stage runs fully on or off, two-stage adds a low setting, and variable-speed adjusts smoothly.
- Refrigerant (R-454B, R-32, R-410A)
- The fluid a heat pump or air conditioner pumps around to move heat; newer systems use R-454B or R-32 instead of the older R-410A.
- Ton (of cooling)
- A unit of heating and cooling capacity: one ton equals 12,000 BTU per hour (3.5 kW), based on the cooling from melting a ton of ice in a day.
- EER / EER2
- Cooling efficiency measured at one hot test condition rather than averaged over a season; useful for judging performance on the hottest days.
The glossary explains every term on the site.
Common questions
Is it normal for my AC to run all day?
On the hottest days, often yes: systems are sized for a typical hot day in your area, not the extreme, so they may run nonstop in a heat wave. Variable-speed systems also run for long periods by design. Constant running on ordinary days, warm air at the vents or a jump in your bill are signs something needs attention.
Does my AC need more refrigerant?
An AC doesn’t use up refrigerant. If it’s low, it’s leaking, and the leak should be found and repaired. Adding refrigerant without fixing the leak only lasts until it leaks out again.
Do I need a bigger AC?
Not necessarily. Many systems that can’t keep up are losing capacity to leaky ducts, attic heat or a dirty coil. A Manual J load calculation tells you the size the house actually needs; a bigger unit than that short-cycles and leaves the air humid.
Related problems
Sources
Where each problem is common comes from our regional climate research (NOAA climate normals, EPA, PNNL, FEMA and others), cited on each metro guide. The guidance on this page follows these sources, checked October 26, 2026:
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