Short answer
Warm air from the vents usually means the outdoor unit isn't running: a tripped breaker, a pulled disconnect, a failed capacitor or contactor, or a thermostat set to the wrong mode. Check the thermostat, both breakers, and the filter first. If the outdoor unit still won't start, or it hums without the fan spinning, turn the system off and call.
What's probably happening
Your system has two halves. The indoor unit (an air handler in a closet, attic, or crawlspace, or a furnace) moves the air. The outdoor unit is what actually takes the heat out. When you feel air but it's warm, the outdoor unit is usually the one that has stopped.
The outdoor unit has no power
The indoor and outdoor units are usually on separate breakers. The outdoor one can trip on its own, and the house looks normal otherwise. There is also a service disconnect in a small box on the wall near the outdoor unit that can get bumped or pulled.
A failed capacitor or contactor
These are the parts that start the outdoor motors. When a capacitor weakens, the unit may hum and not start, or the fan may sit still while the compressor tries. Capacitors take a beating on hot Upstate afternoons, and a weak one is a common summer failure.
The thermostat is in the wrong mode
A thermostat set to fan, or to heat, or a setpoint above the room temperature, will move air without cooling it. Heat pump owners sometimes find it switched to heat after a cool morning.
A frozen coil or low refrigerant
If the airflow is weak as well as warm, the indoor coil may be iced over. Low refrigerant can cause that, and low refrigerant always means a leak somewhere.
What you can check yourself in five minutes
- Look at the thermostat. It should say cool, the setpoint should be below the room temperature, and the fan should be on auto. If the screen is dim or blank, replace the batteries.
- Go outside and see whether the outdoor unit is running. Listen for the fan and the compressor.
- Check your breaker panel. Find the breakers for both the indoor unit and the outdoor unit. If one is tripped, switch it fully off, then back on, once.
- Check the disconnect box on the wall by the outdoor unit. Make sure the pull-out block or switch is fully in the on position. Don't open anything beyond that.
- Pull the air filter. If it's gray and packed, replace it.
- Look for ice on the copper lines or the outdoor unit. If you see ice, turn cooling off and run the fan only so it can thaw.
- Check the drain. Many systems have a float switch on the drain line or in the pan under the indoor unit that shuts cooling off when water backs up. A full pan is a sign of a clogged drain.
- Give it a few minutes after any power change. Many systems wait a short time before restarting the outdoor unit to protect the compressor.
If one of those gets it running and it stays running, you don't need us. Keep an eye on it for a day.
When to stop and call
- A breaker trips again after you reset it once. Don't keep resetting it. Something is drawing too much current.
- The outdoor unit hums or clicks but the fan doesn't turn. Turn the system off at the thermostat and don't touch the unit.
- You smell something burning, or see scorch marks near the outdoor unit. Turn it off at the thermostat and the breaker.
- Water is near the indoor unit's wiring or a nearby outlet. Shut the power off first.
- You've done the checks and it's still blowing warm.
Leave panels closed, and leave the capacitor alone. It can hold a charge after the power is off. If you're in Spartanburg or Greenville County and the checks didn't solve it, call Alpine Cool at (864) 702-0777 and tell me what you found.
Common questions
Why is my AC fan running but the air coming out is warm?
The indoor fan is working, but the outdoor unit is not doing its part. The most common reasons are a tripped breaker or pulled disconnect at the outdoor unit, a failed capacitor or contactor, or a thermostat set to fan only or heat.
Is it OK to keep running the AC while it blows warm air?
No. Turn the thermostat to off. If the outdoor unit is trying to start and can't, running it longer can damage the compressor. If the coil is frozen, running it makes the ice worse.
Can I add refrigerant myself?
Please don't. Refrigerant isn't used up like gas in a car. If it's low, there is a leak, and adding more without finding it just puts off the same problem. Handling refrigerant also requires certification.
The outdoor unit hums but the fan doesn't spin. What does that mean?
That often points to a failing capacitor or a stuck motor. Turn the system off at the thermostat and leave the unit alone. Capacitors hold a charge even with the power off, so this one is for a technician.