Warm or room-temperature air from the vents means the indoor blower is doing its job but heat is not being pulled out of the air. The most useful clue is timing: did it start all at once, does the air start cold and then turn warm, or has it been getting a little worse every week? Each pattern points to a different part of the system, and two of them are worth a quick look before you pick up the phone.
If the air at the registers is cool but the house never reaches the setting, that is a different problem with different causes. See AC not cooling.
First, rule out the settings
- Mode. The thermostat should read Cool. Heat, Fan, or an Auto mode with a high heating setpoint all produce warm or room-temperature air.
- Fan switch. On Auto the blower stops when cooling stops. On On it keeps pushing air between cycles, and that air has not been cooled.
- Setpoint. Set it at least three degrees below the room temperature, or the system has no reason to cool.
- A heat pump with a new thermostat. If warm air began the day a thermostat was replaced, the reversing valve setting may be wrong, so the heat pump heats when you ask it to cool. That is a setup fix; see thermostat installation.
- After a power cut. Many outdoor units hold the compressor off for a few minutes after power returns. Give it five minutes before deciding something is broken.
What the timing tells you
| Pattern | What it usually means | What it usually takes |
|---|---|---|
| Warm from the first minute, outdoor unit silent | No power to the outdoor unit: breaker, disconnect, fuse, contactor, or a drain safety switch that opened | Reset a tripped breaker once; anything else needs testing |
| Warm from the first minute, outdoor unit hums but its fan is still | Failed run capacitor, seized fan motor or a compressor that cannot start | Turn it off now; capacitor or motor replacement after testing |
| Cold at first, warm after several minutes | Compressor or fan shutting off on heat: dirty outdoor coil, weak capacitor, tired fan motor or low charge | Coil cleaning, electrical parts or a leak search |
| Fine yesterday, warm today | Something stopped suddenly, almost always electrical | Electrical diagnosis, often fixed on the first visit |
| A little warmer every week | Refrigerant slowly escaping, or coils getting dirtier | Leak search and repair, coil cleaning |
| Weak, barely cool air and frost on the copper line | Frozen indoor coil | Thaw first, then find the airflow or charge fault |
Safe checks before you call
- Fix the settings above and give the system ten minutes.
- Pull the filter. A grey, matted filter can freeze the indoor coil; replace it.
- Check the breakers for both the indoor unit and the outdoor unit. Reset a tripped one once.
- Stand by the outdoor unit while the thermostat calls for cooling. The compressor should hum steadily, the fan should turn, and the air blowing off the top should feel warm. Warm air leaving the outdoor unit is a good sign: heat is being carried out of the house.
- Cut back plants and rake leaves away from the sides of the outdoor unit. Don’t open panels or reach inside.
Turn it off and call when
- The outdoor unit hums or buzzes but its fan does not turn.
- The breaker trips a second time. A breaker that keeps tripping is protecting a shorted motor, compressor or wire, and resetting it again can add to the damage.
- There is ice on the copper lines at the outdoor unit or frost on the indoor coil.
- Something smells burnt at the outdoor unit or the electrical panel.
- Water is spilling from the indoor unit’s drain pan.
What a technician does
Sudden warm-air failures usually live on the electrical side, so a proper visit starts there: voltage at the outdoor unit, the contactor, the capacitor measured against its rating, and the amp draw of the fan and compressor. Then the refrigerant side, where pressures and line temperatures show whether the charge is low, the compressor is pumping or something in the circuit is restricted. If the charge is low, the right fix is to find the leak, not to keep adding refrigerant; our page on refrigerant leaks explains how that is done. Our technicians are EPA-certified to handle refrigerant.
A failed compressor is the expensive outcome, and it is the one to confirm carefully before anyone recommends replacement. On an older system that is where the repair-or-replace conversation starts; our article on when to replace an HVAC system covers what usually decides it. In most homes, though, warm air turns out to be a capacitor, a contactor, a dirty coil or a leak, all handled on an AC repair visit.
Heat pumps and added cooling
Many older homes in San Rafael and San Anselmo had no central air at all, and cooling was added later with a heat pump or a ductless mini-split. On a heat pump, warm air in cooling mode adds two suspects to the list above: the reversing valve itself and the control that switches it. Both can be tested without opening the refrigerant circuit, so a wrong guess does not have to cost a recharge.