When an AC runs but won’t cool, the cause is usually one of a few simple things: a clogged air filter, a tripped condensate safety switch, a dirty or blocked outdoor unit, or a thermostat set wrong. Work through the checks below in order — most Houston homeowners find the problem before they reach the bottom of the list. If the system still blows warm after all seven, it’s likely low refrigerant or a failed component, and that’s a job for a licensed tech.
Watch how it's done
Video: Affordable Comfort. Shown for reference — not affiliated with GetHoustonLeads.
What you'll need
- A fresh air filter (correct size)
- A flashlight
- A garden hose
- A wet/dry vacuum (optional)
- A soft brush
Recommended parts & supplies
- HVAC air filter (check your size) — size is printed on your old filter, e.g. 16x25x1
- AC coil cleaner spray — for a dirty condenser or coil
- Condenser fin comb — straightens bent cooling fins
- Smart / programmable thermostat — if yours is old or unreliable
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Step by step
- 1
Confirm the thermostat is set to COOL and lower than the room
It sounds obvious, but start here. Set the mode to COOL (not just “fan” or “auto heat”) and drop the target temperature at least 3–4 degrees below the current room reading so the system is actually being told to cool. If the thermostat screen is blank, replace its batteries — a dying thermostat can leave the blower running with no cooling call.
- 2
Check and replace the air filter
A clogged filter is the single most common cause of weak or no cooling. Pull your filter out and hold it up to the light — if you can’t see through it, replace it. In dusty Houston homes with pets, filters can choke in 30–45 days. A starved system freezes up and stops cooling, so this one fix solves a surprising number of “no cool” calls.
- 3
Look for ice on the refrigerant line or indoor coil
Go to your indoor unit and the copper lines running to it. If you see ice or frost, the system is frozen — turn the thermostat to OFF and set the fan to ON to thaw it (this can take a few hours). A frozen coil almost always traces back to a dirty filter or low refrigerant. Never chip the ice off; let it melt.
- 4
Clear the outdoor condenser unit
Head outside to the big metal box. It needs to breathe. Clear away grass clippings, leaves, and anything within about two feet of it. With the power off at the disconnect, gently rinse the fins from the inside out with a garden hose (never a pressure washer) to wash off the dirt and cottonwood that Houston summers cake on. A smothered condenser can’t dump heat, so the house never cools.
- 5
Check the condensate drain and float switch
Most Houston systems have a safety “float” switch that shuts the unit down when the condensate drain clogs — which happens constantly in our humidity. If the indoor unit hums or the blower runs but the compressor won’t kick on, a full drain pan may have tripped it. Find the white PVC drain line and clear the clog with a wet/dry vacuum on the outdoor end. Emptying the pan and clearing the line often brings the system right back.
- 6
Reset the breakers and the outdoor disconnect
Find your electrical panel and look for a tripped AC breaker (it will sit between “on” and “off”). Flip it fully off, then back on. Some systems also have a pull-out disconnect box next to the outdoor unit — make sure it’s seated. If a breaker trips again immediately, stop: that points to an electrical fault, not a reset you should keep forcing.
- 7
Give it 15–20 minutes, then feel the vents
After the checks above, let the system run and hold your hand to a supply vent. Air coming out should feel a solid 15–20 degrees cooler than the air going into the return. If it’s cold at the vents but the house still won’t cool, you may have a duct or insulation issue. If the air itself isn’t cold after all of this, the likely culprit is refrigerant or the compressor — time for a pro.
When to call a pro
Call a licensed HVAC tech if, after these checks, the air at the vents still isn’t cold — that usually means low refrigerant (a leak that needs finding and sealing, not just “a top-off”), a failing compressor, or a bad capacitor. Also call right away if a breaker keeps tripping, you smell burning, or the outdoor unit is buzzing but the fan won’t spin. Refrigerant work legally requires a certified technician, and running a system low on charge can destroy the compressor — the most expensive part in the whole system.
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AC Running But Not Cooling — FAQ
Why is my AC running but blowing warm air?
How long should I wait for my AC to cool the house after a fix?
Is it safe to rinse my outdoor AC unit with a hose?
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Try the fix →Ice on Your AC? How to Safely Thaw and Fix a Frozen Unit
Seeing frost on the copper lines or ice on the indoor coil? Here’s how to thaw it safely and fix the two things that cause it.
Try the fix →AC Leaking Water Inside? Clear the Condensate Drain Yourself
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