Symptom
AC blowing warm air from the vents
Air is moving, the blower clearly works, and what comes out is room temperature or warmer. The indoor side is fine. Something outside has stopped removing heat.
What warm air actually tells you
The blower is working, so indoor power and the fan motor are fine. Warm supply air means the refrigeration cycle is not removing heat, and in the large majority of cases the reason is outdoors, not indoors.
The three usual explanations are that the compressor is not running, the system has lost most of its refrigerant charge, or something has cut power to the outdoor unit while the indoor unit carries on regardless.
Rule these out in two minutes
- Thermostat is on cool, not heat, and the fan is on auto rather than on
- Go outside. Is the outdoor unit running at all?
- Check the breaker panel for a tripped double breaker serving the outdoor unit
- Check the outdoor disconnect box next to the condenser has not been pulled
- Listen for a hum from the outdoor unit with no fan movement, which points at a capacitor
A thermostat left on fan "on" blows room-temperature air between cycles and is mistaken for a fault surprisingly often.
Call (866) 427-9264Causes
Working from most to least common
The outdoor unit has no power
The indoor blower and the outdoor condenser are usually on separate circuits. When the outdoor breaker trips or the disconnect is pulled, the blower keeps circulating air with nothing cooling it. This is the single most common explanation for genuinely warm air, and the easiest to check. If the breaker trips again after being reset once, stop resetting it and call. Repeated tripping is a fault report, not a nuisance.
A failed run capacitor
The capacitor gives the compressor its starting push. When it fails, you frequently get a condenser fan that spins normally while the compressor sits there humming or silent, so from outside the unit looks like it is working. Inside, the air is warm. It is among the least expensive repairs on a system and among the most common in a climate with this much runtime. See capacitor replacement.
A burnt contactor
The contactor is the relay that switches power to the compressor and condenser fan. Its contacts pit and burn over years of switching. When they fail to close, the outdoor unit does nothing at all. When they weld closed, the outdoor unit runs continuously regardless of the thermostat, which produces its own set of odd symptoms.
Severe refrigerant loss
A system that has lost most of its charge will run without producing meaningful cooling. Partial loss produces weak cooling and often a frozen coil. Substantial loss produces warm air and, in many systems, a low pressure safety cutting the compressor out entirely. Either way the refrigerant went somewhere, and finding where is the actual repair. See refrigerant leak repair.
Compressor failure
A compressor that has failed internally may hum, draw current and go nowhere, or may be locked out by its overload protector. The outdoor fan may still run. This is the expensive answer and it is the one a proper diagnosis rules in or out with gauges rather than guesswork. See compressor replacement.
A frozen coil that has blocked completely
A coil encased in ice stops air passing through it. What reaches the vents is whatever the blower can push around the blockage, which arrives warm. If you can see ice, switch cooling off and run the fan alone to thaw it, then get the underlying cause found. Freezing is always a symptom of something else. See frozen AC coil.
Telling them apart
What to listen for outside
Standing next to the outdoor unit for thirty seconds is genuinely informative, and it is something you can report accurately over the phone.
- Nothing at all, complete silence. No power reaching the unit. Breaker, disconnect, contactor or control wiring.
- A hum with no fan movement. Power is present but something is not starting. Capacitor is the leading candidate.
- Fan spinning, no deeper compressor note underneath. Fan circuit fine, compressor not running. Capacitor, overload or compressor.
- Everything running normally, air out the top warm rather than hot. The unit is running but not moving much heat. Points at charge.
- Loud grinding or screeching. Switch it off and call. Continuing to run it makes a mechanical failure worse.
The difference between a fan-only hum and a compressor running is a low note you feel as much as hear. If you are unsure, say so on the phone rather than guessing.
Worth knowing before you call
- Whether the outdoor unit is silent, humming, or running
- Whether the outdoor fan blade is turning
- Whether any breaker has tripped, and whether it tripped again
- How long ago the air went warm
- Whether there is ice anywhere on the copper lines
24/7 emergency HVAC service. Office monday - saturday, 9:00am - 6:00pm.
Related pages
AC not cooling
Cool air that is not keeping up, which is a different problem.
AC not turning on
When nothing runs at all, indoors or out.
Capacitor replacement
The most common cause of this exact symptom.
Compressor replacement
The expensive possibility, and how it gets confirmed.
Refrigerant leak repair
When the charge has gone and needs finding rather than topping up.
Emergency AC repair
If the house is heating up fast and it cannot wait.
Questions about warm air from the vents
Is warm air a different problem from weak cooling?
The fan on my outdoor unit is spinning but the air inside is warm.
Could the thermostat be causing this?
Can I damage the system by leaving it running?
Telling a technician what the outdoor unit is doing usually narrows this to one or two possibilities before anyone arrives.
Call (866) 427-9264Warm air and a house heating up
Most causes of this are inexpensive. A few are not. A diagnosis tells you which one you have rather than which one you fear.