AC running but not cooling: what to check before you call anyone
The compressor is humming, air is coming out of the vents, and the house keeps getting warmer. This is the most common summer call an HVAC company gets, and a meaningful share of those visits end with a technician replacing a filter or hosing off a coil. Work through the list below first. It takes about twenty minutes and costs nothing.
The twenty-minute checklist
Pull the air filter and look through it
Hold it up to a light. If you cannot see light through it, the system is being starved of the airflow it needs to move heat, and it will underperform no matter how healthy everything else is. A badly clogged filter can also drive the evaporator coil below freezing. This is the single most common cause of a no-cooling complaint — see how often filters actually need changing.
Confirm the thermostat is actually calling for cooling
Set it to cool, drop the setpoint several degrees below room temperature, and make sure the fan is set to auto rather than on. A fan stuck on “on” blows room- temperature air between cooling cycles, which feels exactly like a system that has stopped working. Replace the batteries while you are there — a dying thermostat battery produces genuinely strange behavior.
Check both breakers
Most systems have two: one for the indoor air handler or furnace, one for the outdoor condenser. If the outdoor breaker has tripped, the indoor fan still runs and still blows air, which is why the system seems alive. Reset it once. If it trips again, stop — a repeatedly tripping breaker is a fault, not an inconvenience.
Look for ice
Inspect the copper refrigerant lines where they enter the indoor unit, and the indoor coil if it is visible. Frost or ice means shut the system off at the thermostat, set the fan to on, and give it several hours to thaw completely. Running a frozen system risks the compressor. Ice is a symptom of restricted airflow or low refrigerant, so it will come back until the underlying cause is fixed.
Clear and rinse the outdoor unit
The condenser rejects your house’s heat to the outside air, and it cannot do that buried in cottonwood fluff, grass clippings, or shrubbery. With the power off at the disconnect, clear a couple of feet around all sides, remove debris from the top, and rinse the fins gently from the inside out with a garden hose. Never a pressure washer — the fins bend, and bent fins choke airflow permanently.
Find the condensate drain
Air conditioning pulls a surprising amount of water out of the air. If the drain line clogs, many systems have a float switch that shuts cooling down deliberately to prevent an overflow — so a clogged drain looks like a dead system. A wet-vac on the drain outlet often clears it.
Walk the vents
Closed or blocked supply registers, a rug over a return, furniture pushed against a grille. Closing vents in unused rooms does not save money in most residential systems; it raises static pressure and makes everything work harder.
Symptoms that mean stop and call
Some things are not diagnostic puzzles. Call a licensed technician if you have a breaker that trips repeatedly, a burning or acrid electrical smell, a loud grinding or shrieking from either unit, an outdoor fan that will not spin while the compressor hums, or ice that returns within a day of thawing. Any of those can turn a repair into a compressor replacement if the system keeps running.
The same is true of the refrigerant conversation. A sealed system does not consume refrigerant. If a technician says you are low, the correct next question is where the leak is and what it costs to repair it — not simply what a recharge costs. Recharging a leaking system is buying the same refrigerant twice.
- Replacing the filter and clearing the vents
- Rinsing the outdoor coil with a garden hose, power off
- Clearing a clogged condensate line
- Resetting a breaker once, and thermostat batteries
- Anything involving refrigerant, by law in most places
- Repeated breaker trips or burning smells
- Capacitor, contactor, or compressor diagnosis
- Ice that returns after a full thaw
Why this keeps happening in July
Air conditioners rarely fail without warning; they fail without anyone noticing the warning. The capacitor that dies on the first ninety-degree day was usually marginal in May. The coil that cannot reject heat was dirty in April. A spring service visit before the first heat wave — coil cleaning, capacitor check, drain clear, refrigerant charge verified — is genuinely cheaper than an emergency call in the middle of a heat advisory, when every technician in your area is booked four days out.
It also helps to know what you own. Tonnage, model, filter size, install date, and whether the compressor is still inside its manufacturer warranty all change how you should respond to a quote. A five-year-old system with a ten-year parts warranty is a different decision than a fifteen-year-old one.
What homeowners argue about online
The argument that will not die on Reddit is whether closing vents in unused rooms saves money. It feels obviously true — heat fewer rooms, spend less — and it is one of the more reliably counterproductive things you can do to a residential system.
The reason is that your blower moves the same air regardless. Close registers and that air has nowhere to go, so static pressure in the ducts climbs sharply. What follows is more leakage at every duct seam, a blower working against resistance it was never sized for, and in cooling season reduced airflow across the coil, which is one of the ways coils freeze in the first place. The system runs longer to reach setpoint, which is the opposite of the goal. Zoning a house properly is a real thing, but it is done with equipment and dampers designed for it, not by shutting registers.
The other one worth settling: refrigerant is not a consumable. Threads full of people getting an annual “top-up” are describing a leak nobody has repaired. A sealed system that is low is a system with a hole in it, and paying to refill it each summer is paying for the same refrigerant on a subscription.
Write it down while it’s fresh
The filter size is the small one that matters most. It is printed on the edge of the filter you just pulled out, and it is the detail people stand in a hardware store aisle trying to remember. Capture it once.
- Outdoor unit make, model, and serial from the data plate
- Indoor air handler or furnace model, and the exact filter size
- Install date and installer, plus the parts and labor warranty terms
- Date of every filter change and every service visit
- What the technician found and what they replaced
- Where both breakers and the outdoor disconnect are
This is the part almost everyone skips, and it’s the part that pays. The next time this happens — or when a warranty claim, an insurance adjuster, or a buyer’s inspector asks — the answer is already written down. That is what homwarden is built to hold: maintenance, appliances, warranties, receipts, and records, filed with the equipment they belong to.
Common questions
Why is my AC running but not cooling the house?
The most common causes are a clogged air filter choking airflow, a dirty or blocked outdoor condenser coil, a tripped breaker on the outdoor unit, a frozen evaporator coil, or low refrigerant from a leak. The first three are homeowner-checkable. The last two generally require a technician.
Should I turn off my AC if it is not cooling?
Yes, if you see ice on the refrigerant lines or the indoor coil. Running a system with a frozen coil can damage the compressor, which is the single most expensive component. Switch the thermostat to off, set the fan to on to help the ice melt, and let it thaw fully before restarting.
How often does an air conditioner need refrigerant?
A sealed system should never need refrigerant added on a schedule. Refrigerant is not consumed. If a system is low, there is a leak, and topping it up without finding and repairing that leak means paying for the same refrigerant again next season.
What temperature difference should I feel at the vents?
Technicians commonly look for a meaningful drop between the air entering the return and the air leaving a supply vent, often cited in the range of roughly 15 to 20 degrees Fahrenheit. A much smaller difference points to an airflow or refrigerant problem worth diagnosing.
General maintenance guidance only. It is not a substitute for an inspection, a licensed contractor, an engineer, your insurer, the manufacturer’s instructions, or local code. When the stakes are gas, fire, structure, or standing water near electricity, stop and call a professional.