This post overlaps with a fuller one: Why Is My AC Blowing Hot Air? Check These Three First.
When an air conditioner in a Pasadena, CA home starts blowing hot air, work through the causes in this order. The first three take minutes and fix a surprising share of calls. The rest are a diagnostic visit.
Check in this order
- Thermostat. Cool mode, fan on Auto, setpoint a few degrees below the room. Replace the batteries if the screen is blank.
- Filter. Hold it to a light. No light through it, replace it. A starved coil cools poorly and can freeze solid, which turns weak air into warm air.
- Power to the outdoor unit. Breaker in the panel and the disconnect by the unit. Reset a tripped breaker once. A second trip means a part is pulling too much current, and it is time to stop.
- The outdoor unit itself. Fan spinning, compressor humming, fins not caked in dust. A unit that hums without spinning usually has a failed capacitor. Do not open its electrical cover.
- Refrigerant. Ice on the copper line, hissing, cooling that has faded over weeks. A leak has to be found and repaired by a certified technician before the charge is restored.
Two local causes
Older side-yard condensers sit against fences and under shrubs and their coils mat up with dust; a dirty coil cannot dump heat, so the air inside stays warm. And in the 1950s to 1980s ranch homes across the Valley, attic ducts run through 130°F air; a torn joint makes one or two rooms warm while the rest of the house is fine.
When to stop
Turn the system off and call if the outdoor unit is silent, a breaker trips twice, you see ice, or you hear hissing. Running a compressor on low charge or a dead capacitor is how a repair becomes a replacement.
The full guide: Why Is My AC Blowing Hot Air? Check These Three First.
Still warm after the checks? Book a $69 diagnostic, credited toward the work. Reach Boost Services at 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.