It’s a hot Bay Area afternoon, and you’re expecting your AC to deliver cool air, but it’s not. You’re not alone. “My AC is blowing hot air,” and “my AC is only blowing hot air from certain vents,” are some of the most common calls we get at Calvey Heating and Air, especially when temperatures climb across Mountain View, San Jose, and the rest of the South Bay. Here are seven reasons your AC is blowing hot air, and what to do about each one.
Key Takeaways
- Hot air from your AC usually comes down to one of seven fixable causes.
- Causes like a tripped breaker or wrong thermostat mode are often an easy fix.
- Others, like a refrigerant leak or frozen evaporator coil, need a licensed technician.
- Running an AC that’s blowing warm air can damage the compressor so if you’re in doubt, shut the system off and call.
7 Reasons Your AC Is Blowing Hot Air (and What to Do)
Some of the reasons your AC is blowing hot air are more common in the Bay Area. Some you can quickly handle yourself, and others need a licensed technician.
1. Thermostat Set Incorrectly or Malfunctioning
Your system should be set to cool, not fan or heat. If you have a ductless mini-split, common in Bay Area additions and remodeled rooms, also confirm the remote is set to cooling mode.
If the thermostat is set correctly and the AC is still blowing warm air:
- Check the batteries and replace them if needed.
- Remove the thermostat panel and check that it’s properly connected. Faulty wiring is the cause of many AC issues.
- Check the circuit breaker and, if needed, reset it.
2. Clogged or Dirty Air Filter
If your AC is blowing hot air,it’s often a filter problem. The standard guideline is to change your filters every one to three months, but the Bay Area Air Quality Management District regularly issues Spare the Air alerts during wildfire season, when smoke particulates can clog a filter in weeks, not months. It’s one of the most common reasons we see service calls spike in September and October for residents whose AC is blowing hot air.
Check your filter by pulling it out and holding it up to the light. Can’t see light through it? Replace it.
A dirty filter also causes:
- Decreased energy efficiency.
- Reduced indoor air quality.
- Excessive strain on the compressor.
For more on when and how to replace your filter, see our guide.
3. Outdoor Condenser Unit Not Running
Your air conditioning system has an indoor air handler and an outdoor condenser. The condenser is responsible for removing heat from the house. If it isn’t running, that’s likely why you have a central air unit blowing hot air.
Walk outside and see if the fan is spinning. If not, there may be an issue with the safety switch, motor capacitor, or circuit wiring. Also, check your circuit breaker. If the AC breaker has tripped, reset it and see if the condenser comes back on. If it trips again right away, it could be a sign of an underlying electrical issue. Read more about what to do when your outside AC unit isn’t running.
4. Low Refrigerant or a Refrigerant Leak
Low refrigerant is usually caused by a leak and it’s one of the more serious causes of a house AC blowing warm air. When levels drop, the system can’t cool effectively.
Watch for these signs:
- Higher energy bills than usual, even though the house isn’t getting any cooler.
- Pools of fluid around the outdoor unit can indicate a refrigerant leak.
- A hissing or bubbling sound near the refrigerant lines.
Refrigerant issues must be handled by a certified technician; it’s not a DIY fix. If you suspect a leak, turn the system off and call us. In our experience, low refrigerant points to a leak, and topping it up without finding the source isn’t a lasting solution.
5. Frozen Evaporator Coil
A frozen evaporator coil can block airflow, so the air coming out goes warm or stops. Bay Area homes near the coast, particularly in Marin and parts of Santa Clara, often have it worse. Higher indoor humidity accelerates freeze-over when airflow is even slightly restricted, faster than you’d see in a drier climate. Signs of a coil issue can be:
- Little to no airflow from the vents, even though the system is running.
- Warm air that gets warmer the longer the system runs.
- Frost on the refrigerant lines outside.
- Water pooling under the air handler.
If you’re seeing these signs:
- Turn the system off. Running it makes the problem worse.
- Switch the fan to ON (not AUTO) to thaw the ice over a few hours.
- Replace the filter once the ice clears.
If it refreezes after thawing, there’s an underlying issue and you should call a technician.
6. Dirty Condenser Coils
The condenser coils on your outdoor unit release heat from inside your home. They can get dirty over time, especially in areas with cottonwood trees or heavy pollen, and your AC blowing hot air is often the result.
You can use a garden hose to gently rinse the outdoor unit (from the inside out). Our membership plan includes an annual cooling visit where we clean the condenser coil as part of the full inspection.
7. Clogged Condensate Drain Line
Your AC removes humidity from the air as it cools. That moisture drains, and when the condensate line gets clogged, many systems are designed to shut the cooling function off to prevent water damage.
Look for pooling water or a full drain pan.
When to Turn the System Off and Call for AC Repair
Some problems are safe to troubleshoot yourself, but others need a technician. Call us if:
- You’ve spotted a pool of fluid near the outdoor unit.
- Resetting the circuit breaker didn’t help or it tripped again right away.
- You notice burning smells coming from the vents.
- You hear loud banging, grinding, or hissing sounds.
- The coil keeps refreezing after thawing.
- None of the troubleshooting above resolved the issue.
If you’re not sure about the cause of your AC issue, it’s safest to shut it off and call us. If replacement is on your mind, it’s also worth knowing whether you can replace your AC unit without replacing your furnace. If you run a compromised system, you might find yourself with a costly replacement instead of a simple service call.
Schedule AC Service With Calvey Heating and Air
With flat-rate pricing, 20+ years of local experience, and no-pressure diagnostics, Calvey Heating and Air is the go-to choice in Mountain View, Palo Alto, San Jose, and throughout the South Bay. If your system is beyond repair, we can walk you through your AC installation options.
When it’s time to schedule a service appointment, call Calvey and our technicians will get your home cool again.
Frequently Asked Questions
How do I know if my AC compressor is broken?
A clear sign that the compressor isn’t working is when the air conditioner is blowing hot air. Listen for banging, clanking, or grinding sounds, which point to loose internal components. Any of those warrants a call to a technician before running the system further.
Is it safe to keep running my AC if it’s blowing hot air?
No, it’s best to turn it off. Running the AC when it’s blowing warm air puts strain on the compressor, especially if the cause is low refrigerant. Shut it down, troubleshoot what you can from this list, and call a technician if the problem isn’t obvious.
Can I fix my AC blowing hot air myself?
Yes, in some cases you can do a DIY fix. Resetting a tripped breaker, replacing a clogged filter, or correcting a thermostat setting are all things most homeowners can handle without a service call. But refrigerant leaks, frozen evaporator coils that keep refreezing, and anything involving the compressor or electrical components require a licensed technician.
