Your AC is on. The thermostat shows cooling mode. Two hours later, the house is still 80 degrees and the system is running in a loop. This problem is different from an AC that’s completely dead. The system is doing something. The challenge is figuring out whether it’s doing enough, and what’s getting in the way.
Start with the things that take two minutes and cost nothing. Then work toward the more complex causes. This guide follows that order and covers the technical details most homeowners never get from a service tech, because knowing what’s actually happening inside your system is the fastest way to figure out what you’re dealing with.
Check the Thermostat Fan Setting First: On vs Auto
Before anything else, check the fan setting on your thermostat. Most have two options: Auto and On. When the fan is set to On, it runs continuously regardless of whether the compressor is running. This pushes unconditioned, room-temperature air through the vents constantly.
When the fan is set to Auto, air only moves when the compressor is actively cooling it. If the fan is set to On and the compressor cycles off, you’ll feel air coming from the vents that’s barely cool or completely neutral. It feels like the AC is running but not doing anything, because in the fan-only phase it isn’t. Switch it to Auto and run for 30 minutes before drawing any conclusions.
This setting catches a surprising number of service calls. It costs nothing to check and takes 10 seconds.
Replace the Air Filter and Do It Right Now
A partially clogged filter doesn’t just reduce airflow. According to the US Department of Energy, a dirty air filter can reduce AC efficiency by 5 to 15 percent. In severe cases, a fully blocked filter starves the evaporator coil of warm return air, drops the coil temperature below 32 degrees Fahrenheit, and freezes it solid. The system keeps running. Air still moves. But it’s barely cool because the coil is encased in ice.
Use a MERV 8 to MERV 11 filter. Anything rated MERV 13 or higher restricts airflow too much in most residential systems and contributes to the freezing problem you’re trying to solve. On a system running 12 to 16 hours a day in summer, a standard 1-inch filter can load up in three to four weeks. Check it monthly during peak season.
One thing no one tells you: running a system with no filter at all is not the solution when you run out of filters. Without a filter, dust and debris coat the evaporator coil directly. That buildup is far harder and more expensive to clean than swapping a filter.
Do the Temperature Split Test Before Calling Anyone
This is the single most useful diagnostic step a homeowner can do, and almost no one does it because it’s never explained. It takes a $10 thermometer from a hardware store.
Hold the thermometer in the return air grille (where air is pulled into the system) and note the temperature. Then hold it directly at a supply vent (where cold air comes out) and note that temperature. The difference is called the Delta T, or temperature split.
A healthy, properly functioning central air system should produce a Delta T of 14 to 22 degrees Fahrenheit. If the air coming out of your vents is 75 degrees and the air going in is 80 degrees, your Delta T is only 5 degrees, which means the system is delivering almost no cooling. If the Delta T is above 22 degrees, the system may have severely restricted airflow, often from a clogged filter or a partially frozen coil.
This single test tells you more than most homeowners learn from an entire service call. A technician who checks the Delta T first is doing their job correctly.
How to Diagnose a Frozen Evaporator Coil
If the system has been running for hours without cooling, go to your indoor air handler and look for frost or ice on the refrigerant lines or the unit itself. Frosted lines are the tell. If you see ice, do this specific sequence:
Turn the system fully off at the thermostat, not just to Fan Only. Running the fan across a heavily frozen coil before the ice is fully off adds strain and can cause water overflow as it melts too fast. Full off. Leave it for two to four hours. Put towels under the air handler if there’s visible ice buildup.
After thawing, replace the filter, then turn the system back on. If it refreezes within a day or two, the cause isn’t just airflow restriction. Low refrigerant or a failing blower motor are next.
Three things cause a freeze-up: restricted airflow from a dirty filter or closed vents, low refrigerant charge, or a blower motor that isn’t moving enough air volume even when running. Sometimes two stack on top of each other and the freeze happens faster than either cause alone would produce.
The Outdoor Unit: Fan, Compressor, Contactor, and Capacitor
Go outside while the system is calling for cooling. Two components should be running: the fan at the top of the condenser unit and the compressor. You’ll hear the compressor as a low hum or vibration from the bottom of the unit.
If the fan is spinning but the compressor is silent, or neither is running while the indoor blower is on, you have an electrical failure on the condenser side. The two most common causes are the capacitor and the contactor relay.
The capacitor stores charge and delivers the voltage boost needed to start and run both the compressor and the condenser fan. Most residential systems use a dual-run capacitor with three terminals: C (common), HERM (compressor), and FAN. When it fails, the fan may spin slowly or the compressor may hum without starting.
A licensed tech tests this with a multimeter set to capacitance mode. The reading should be within 6 percent of the rated microfarad (MFD) value printed on the capacitor. A reading more than 6 percent below rated MFD means the cap is failing under load even if it looks marginal on the bench. The capacitor costs $10 to $50 in parts. Installed, the total runs $150 to $350. For a full breakdown of what different HVAC repairs cost before you agree to anything, read our HVAC repair cost guide.
The contactor relay is the electrical switch that connects grid power to the compressor and condenser fan when the thermostat calls for cooling. You’ll find it inside the electrical panel on the outdoor unit. When it fails, neither the fan nor the compressor will run even though the indoor air handler is operating normally. A contactor typically costs $10 to $30 in parts, with an installed cost of $100 to $250. Contactors also accumulate pitting and carbon buildup on the contact surfaces over time, which causes intermittent failure, meaning the system works sometimes and not others. If your AC seems to randomly stop cooling and then work fine again, the contactor is a strong suspect.
Do not open the electrical panel on the outdoor unit yourself unless you know how to discharge a capacitor. Even with power disconnected at the breaker, a capacitor holds a dangerous charge. A technician discharges it through a resistor before touching any terminals.
How to Identify Your Refrigerant Type from the Unit Nameplate
This matters because the refrigerant type determines repair cost and, if the system is old enough, whether repair even makes financial sense.
Find the metal nameplate sticker on your outdoor condenser unit. It lists the model number, serial number, voltage, and refrigerant type. Common refrigerants in residential systems:
R-22 (Freon): Systems manufactured before 2010. R-22 is being phased out under EPA regulations. Supply is constrained and the cost per pound has increased significantly. If your R-22 system has a refrigerant leak, the cost of the refrigerant charge alone can run $400 to $1,000 or more depending on how much is needed. For most R-22 systems over 12 years old with a significant leak, replacement is the financially smarter decision.
R-410A (Puron): Systems manufactured roughly 2010 through 2024. Far more common and less expensive to recharge. A refrigerant recharge on an R-410A system with a diagnosed and repaired leak typically runs $250 to $500.
R-454B or R-32: Systems manufactured from 2025 onward under the AIM Act phase-down. Newer refrigerants with lower global warming potential. If you’re shopping for a new system now, this is what you’ll be buying.
Knowing which refrigerant your system uses before you call a technician means you can research cost expectations and ask better questions.
What a Hard Start Kit Is and When Technicians Recommend It
A hard start kit is an electrical add-on that gives the compressor an extra surge of starting current on top of what the capacitor provides. It’s typically a package of a start capacitor and a potential relay.
Compressors are hardest on their electrical components during startup. On aging systems, a compressor that struggles to start smoothly puts strain on the capacitor every time it cycles, shortening the capacitor’s life and sometimes preventing the compressor from starting at all on hot days when grid voltage is slightly lower.
A hard start kit runs $50 to $100 in parts and $150 to $300 installed. Technicians recommend it when: the compressor is struggling to start consistently, the capacitor has failed more than once in a few years, or the system is 8 years or older and the technician wants to extend compressor life.
It’s not a replacement for a failed capacitor, but it’s a legitimate, cost-effective way to reduce compressor wear on a system that’s otherwise working well and doesn’t yet warrant full replacement.
Low Refrigerant: Leak Points, UV Dye, and Why the Charge Alone Fixes Nothing
An AC running on low refrigerant runs long cycles without reaching setpoint. It may cool your home somewhat at night and fail to keep up on hot afternoons. The air at the vents feels cool but not cold. If you have a digital thermometer and do the Delta T test described above, you’ll see a split below 14 degrees.
Refrigerant doesn’t get used up. If yours is low, it’s leaking. The most common leak points are:
Schrader valve cores on the service ports (the same type of valve stem as a car tire). These corrode and lose sealing ability over time.
Flare connections where the copper refrigerant lines connect to the condenser and the air handler. A flare that wasn’t seated perfectly during installation will seep slowly for years.
Evaporator coil return bends, especially on aluminum evaporators. Aluminum coils are increasingly common in newer systems because they’re cheaper to manufacture, but they’re more susceptible to formicary corrosion caused by formic acid in household air (from cleaning products, wood, and normal organic matter). The corrosion pits through the aluminum over time.
For slow leaks, a technician uses UV dye, which is injected into the refrigerant circuit and fluoresces under a UV light. For faster leaks, an electronic refrigerant detector pinpoints the location. A recharge without finding and fixing the leak source is money wasted. The system will be low again in weeks or months, and continuing to run with low refrigerant accelerates compressor wear significantly.
If you end up needing a contractor for any of this, our guide on how to compare contractor bids the right way will help you evaluate every quote you receive.
Ductwork: The Silent Cause Most Homeowners Never Check
The US Department of Energy estimates that typical homes lose 20 to 30 percent of conditioned air through duct leaks and disconnections. If a flex duct has separated from a metal collar in your attic, you’re air conditioning that attic space while the living area stays warm.
You don’t need special equipment to do a basic duct check. Go into your attic or crawl space while the system is running. Feel along duct runs for air escaping at connections. Look for flex duct that’s sagged, kinked, or pulled away from a boot or collar. A disconnected duct connection is sometimes visible from the attic hatch without even going in.
Also check the supply registers themselves. A supply register that’s partially blocked behind furniture, covered with a rug, or in a room where the door is kept closed all day reduces effective airflow to the rest of the house.
Signs of duct problems without going into the attic: rooms at the far end of the duct run that are always warmer than rooms near the air handler, noticeably higher energy bills without a change in usage, or dusty registers that suggest air is being pulled from unconditioned space.
Why Your AC Might Work at Night but Struggle in the Afternoon
If the system cools fine overnight but can’t keep up on hot afternoons, the issue is almost always one of three things: the system is low on refrigerant, the outdoor unit is struggling to reject heat effectively, or the home’s thermal envelope is allowing solar heat gain faster than the equipment can remove it.
Afternoon solar load through west-facing windows, a dark roof absorbing heat through a poorly insulated attic, and air infiltration through gaps around windows and doors all add heat load that wasn’t factored in when the system was originally sized. These aren’t equipment failures, they’re building performance issues, but they show up as HVAC problems.
If your system is properly charged and the outdoor unit is clean and unobstructed, an energy audit that checks attic insulation levels, window performance, and air sealing is worth doing before deciding the HVAC equipment needs to be replaced.
An Oversized System Short-Cycles and Never Finishes the Job
A system that’s too large for the space cools the air near the thermostat sensor quickly but doesn’t run long enough to dehumidify the air or equalize temperature throughout the house. It shuts off early, cycles back on soon after, and the house never really stabilizes.
You can observe this: watch how long the compressor runs per cycle. A properly sized system on a typical summer day should run cycles of at least 15 to 20 minutes. Cycles consistently shorter than 10 minutes, especially on moderate-temperature days, suggest the system may be oversized.
High indoor humidity even when the temperature is technically at setpoint is a strong indicator of short-cycling. Dehumidification happens during the cooling cycle. When the cycle is too short, moisture isn’t removed from the air and the house feels clammy at 74 degrees.
Proper sizing requires a Manual J load calculation. Any contractor recommending a system replacement without running a load calculation is guessing.
Why Choose Chilll Before You Schedule a Service Call
Chilll is a home repair diagnosis platform built for homeowners who want an independent expert on their side before any contractor shows up at the door. When your AC is running but not cooling, you shouldn’t have to pay a $75 to $150 service call fee just to find out what’s wrong, especially when the technician diagnosing the problem also benefits from recommending the most expensive fix.
What Chilll does: You connect with a licensed HVAC technician over a quick video call, show them the problem in real time, and get a clear diagnosis with a realistic cost range. No guesswork. No information asymmetry. You walk into every contractor conversation knowing what’s actually wrong and what a fair price looks like.
How It Works
- Start a free video session at Chilll.com. No app, no account, no credit card
- Show the problem to a licensed HVAC expert over live video
- Get a real-time diagnosis and fair cost range for the fix
- If you need a local pro, Chilll finds vetted ones who review your diagnosis before they arrive
Why Chilll is different from every other home repair platform
Most platforms that connect homeowners with contractors earn a percentage of the job. A $4,000 repair pays them 10 times more than a $400 repair. That creates a financial incentive to push toward the more expensive option, even when a simpler fix would do.
Chilll charges local pros a flat fee based on repair type, not job cost. A $400 capacitor replacement and a $4,000 compressor replacement pay us the exact same rate. Our only incentive is giving you an accurate diagnosis.
The pros in the Chilll network pass an eight-level vetting process. Only 1 in 5 applicants qualify. When a vetted Chilll pro shows up, they’ve already reviewed your diagnosis video. They know the problem before they walk in. No discovery, no padding, no surprises.
The video diagnosis is completely free. No account required. No app to download. No credit card. Start at Chilll.com.
What Makes Chilll Different From Other Platforms
Most home repair platforms earn a percentage of the job value. Bigger repair, bigger revenue for them. That creates a conflict of interest that shapes every recommendation they make, even when the technicians themselves are honest.
Chilll charges local pros a flat rate by repair type, not job size. A $400 capacitor replacement and a $4,000 compressor replacement pay us the same flat rate. Our only financial incentive is giving you an accurate diagnosis.
The remote experts who conduct video diagnoses are credentialed professionals in their trade. Licensed HVAC technicians with real field experience. They do not earn more when the repair costs more. Their only job is to tell you what’s wrong and what it should cost.
The pros in our local network pass an eight-level vetting process before they can take jobs through Chilll. Only 1 in 5 applicants qualify. When one of them shows up, they’ve already reviewed your diagnosis. They come with the right parts, a clear scope, and a price they’ve committed to before ringing your doorbell.
Visit Chilll.com to start a free diagnosis.
FAQ
Why is my AC running but blowing warm air?
The most common causes are a frozen evaporator coil, a non-functioning outdoor unit, or low refrigerant. Do the Delta T test first: hold a thermometer at a supply vent. If the reading is barely below room temperature, the system isn’t cooling properly at the coil level.
What does it mean if my AC fan is spinning but there’s no compressor noise?
The compressor isn’t starting. Most likely cause: a failed run capacitor on the HERM terminal of the dual-run cap. A contactor relay failure produces the same symptom. A technician confirms with a multimeter and visual inspection of the contactor surface.
Can a clogged air filter really freeze my evaporator coil?
Yes. Restricted airflow drops the coil surface below 32 degrees Fahrenheit, causing ice to form. The ice then blocks airflow entirely. Replace the filter and let the coil thaw completely (2 to 4 hours, system fully off) before restarting.
How do I know if my refrigerant is R-22 or R-410A?
Check the metal nameplate on the outdoor condenser unit. The refrigerant type is listed there. You can also call the manufacturer with your model number and they’ll confirm it.
What is a reasonable Delta T (temperature split) for a properly working AC?
Between 14 and 22 degrees Fahrenheit difference between the return air temperature and the supply air temperature. Below 14 suggests low refrigerant or airflow restriction. Above 22 suggests severely restricted airflow.
My AC is more than 15 years old and not cooling. Should I repair or replace?
If the compressor has failed, the system uses R-22, or the repair cost exceeds 50 percent of replacement cost, replacement is usually the better financial decision. A licensed technician can run the numbers on your specific system.