Why is my AC blowing warm air?

An AC blows warm air most often because of a thermostat setting, a clogged filter, a frozen coil, a tripped breaker, a dirty outdoor unit, or low refrigerant from a leak. Some of those you can check yourself in a few minutes, and others need a technician. If you have worked through the simple checks and the air is still warm, Triad Heating and Air can diagnose the system through its AC repair service.

During a long, humid Tulsa summer, a warm-air problem is more than an annoyance. Here is how to sort it out in a sensible order.

What should you check first?

Start with the things that cost nothing:

• The thermostat. Make sure it is set to cool, not heat or fan only, and that the set temperature is below the room temperature. A fan set to "on" blows unconditioned air continuously, which can feel warm between cooling cycles.
• The batteries. A dead thermostat battery can cause odd behavior or a blank screen.
• The air filter. A badly clogged filter chokes airflow and can cause the coil to freeze.
• The breaker. Check the panel for a tripped breaker for the indoor unit and the outdoor unit. Reset it once. If it trips again, stop and call for help.
• The outdoor unit. Confirm it is running. If the fan is not spinning or the unit is silent, something is wrong.

Could the filter really cause warm air?

Yes. A dirty filter restricts airflow across the evaporator coil. With too little warm air passing over it, the coil gets colder than it should and can freeze. Once ice builds up, cool air cannot pass through, and the system seems to blow warm or weak air. Replacing the filter regularly, often every one to three months in cooling season, is one of the simplest ways to prevent this.

What does a frozen coil look like?

You may see ice on the copper refrigerant line outside or on the indoor coil if you can view it. You may also notice weak airflow from the vents, water dripping, or a hissing sound. If you suspect ice:

• Turn the AC off at the thermostat
• Set the fan to "on" so air moves over the coil and helps it thaw
• Leave it off until the ice is fully melted, which can take hours
• Replace the filter and check the vents are open

If the coil freezes again after thawing, the cause is not solved. Low refrigerant, a failing blower or a blocked return duct could be behind it.

How does a dirty outdoor unit affect cooling?

The outdoor condenser releases the heat pulled out of your house. When its fins are packed with dirt, cottonwood fluff, grass clippings or leaves, it cannot shed heat well, and the system struggles. Shrubs growing too close do the same. A gentle rinse with a garden hose from the inside out, with the power off, and clearing vegetation to give it room can help. Do not use a pressure washer, which bends the delicate fins.

What if the problem is low refrigerant?

Refrigerant does not get used up like fuel. If the level is low, there is a leak somewhere in the system. A system low on refrigerant cools poorly, may ice up, and can run for hours without reaching the set temperature. Adding refrigerant without finding the leak is a temporary fix at best. Ask any technician to explain where the leak is and what it would take to repair it, and to describe the options in writing, especially on an older system where replacement may be worth comparing. See the AC replacement page if the unit is aging.

Can an electrical part cause warm air?

Often. Two parts fail commonly:

• The capacitor. It helps start the compressor and fan motors. When it weakens, the fan may run while the compressor does not, so air blows but stays warm.
• The contactor. It is a switch that sends power to the outdoor unit. If it fails, the outdoor unit may not turn on.

Both are routine repairs on a healthy system, though they should be handled by a technician because capacitors can hold a dangerous charge even when the power is off.

When should you stop and call a professional?

Call for service if any of these apply:

• The breaker trips repeatedly
• You see ice that returns after thawing
• The outdoor unit hums but the fan or compressor will not start
• You hear grinding, squealing or clicking from the equipment
• You smell burning
• The air stays warm after the simple checks above

Do not keep running a system that is icing up or tripping breakers, since that can turn a small repair into a damaged compressor.

Should you worry about the thermostat wiring or fuses?

Occasionally the cause is electrical on the low-voltage side. A blown fuse on the indoor control board, a loose thermostat wire, or a failed float switch on the drain pan can all shut the cooling off while the fan keeps running. These are not something to chase with a screwdriver, since the cabinet holds live voltage. If the thermostat is set correctly, the filter is clean and the breaker is fine, a technician can test these quickly and tell you which part failed.

What can you do to prevent it?

Change filters on schedule, keep the outdoor unit clear, and have the system checked in spring before the heat arrives. Triad Heating and Air recommends AC maintenance in spring as part of a twice-yearly routine, and you can read more on the HVAC maintenance page.

If you are in Tulsa or one of the nearby towns on the service areas page and the simple checks do not fix it, call (918) 992-2105 and describe what you are seeing.