This post overlaps with a fuller one: Why Is My AC Blowing Hot Air? Check These Three First.
An AC that runs but blows hot air almost always comes down to one of five things. Three you can check yourself in a few minutes. Two need a licensed tech. Here they are in the order we check them on a Pasadena, CA service call.
The three you can check
Thermostat settings. Mode on Cool, fan on Auto, setpoint below room temperature. A fan left on On runs between cooling cycles and pushes room-temperature air, which feels warm. Dead batteries and a blank display count too.
A clogged filter. A filter you cannot see light through starves the indoor coil of air. Cooling fades, and eventually the coil ices over and the vents go warm. Replace it. In a Pasadena summer with dust and smoke days, monthly is realistic.
The outdoor unit has no power. Go outside. If the fan is not spinning and the compressor is silent while the indoor blower runs, check the breaker and the disconnect next to the unit. Reset a tripped breaker once. If it trips again, stop.
The two that need a tech
A failed capacitor, contactor, or fan motor outside. The unit hums but does not spin, or the fan runs and the compressor never starts. Heat waves kill capacitors. Do not open the electrical compartment; the capacitor holds a charge with the power off.
A refrigerant leak. Cooling that faded over weeks, ice on the copper line, a hiss. The leak has to be found and fixed before the system is recharged, and only a certified technician can handle the refrigerant.
Two more worth knowing: a coil packed with dust outside cannot shed heat, and a torn duct in a 130°F attic pulls hot air into one or two rooms while the rest of the house is fine.
The full guide: Why Is My AC Blowing Hot Air? Check These Three First.
If the quick checks did not fix it, book a $69 diagnostic, credited toward the work. Call Boost Services, 818-277-5056 or send us a message.
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.