AC Not Cooling Your House? 5 Fixes Before You Call HVAC

An AC not cooling on a hot day feels like an emergency. Before you book a technician, check the simple causes first: the thermostat, the filter, ice on the coil and the outdoor unit. You can check all of it in half an hour without opening anything electrical. Only a frozen coil takes longer, because the ice has to melt.

Look at what the system is doing. Warm air from the vents while the outdoor unit stays silent points to power, the thermostat or a safety switch. Cool but weak airflow points to the filter or ice. Ice on the copper pipes or on the indoor unit means a frozen coil.

Start With These 30-Second Checks

1. 1Set the thermostat to COOL and a few degrees below the room temperature. Replace its batteries if the display is weak or blank.
2. 2Go outside: is the big fan on top of the outdoor unit spinning, and is the unit humming?
3. 3Feel the air at a vent close to the indoor unit. Weak airflow points to the filter, closed vents or ice.
4. 4Look at the copper pipes at the indoor unit for frost or ice.

1. Check the Thermostat

A wrong setting is the cheapest fix there is, and an easy one to miss.

1. 1Set the mode to COOL, not HEAT or FAN.
2. 2Set the temperature at least 3 degrees below the current room temperature.
3. 3Set the fan to AUTO. ON keeps the fan blowing between cycles, which can feel like warm air.
4. 4Replace the batteries, and on a smart thermostat check that no schedule or eco mode overrides your setting.

2. Replace the Air Filter

A clogged filter starves the system of air. The house cools slowly, the coil can freeze, and the system works much harder. ENERGY STAR recommends checking the filter every month and changing it at least every three months.

1. 1Turn the system off at the thermostat.
2. 2Find the filter: in a return grille on a wall or ceiling, or in a slot next to the indoor unit or the furnace.
3. 3Note the size printed on its edge and the arrow that shows the airflow direction, then pull it out.
4. 4Slide in a new filter with the arrow pointing toward the unit. A basic 16x25x1 filter costs around $6.
5. 5Turn the system back on and check the airflow after half an hour.

3. Thaw a Frozen Coil

Ice on the indoor coil blocks the air, so the AC runs but barely cools. Usually the cause is too little airflow, from a dirty filter or closed vents, and sometimes low refrigerant.

1. 1Switch cooling off at the thermostat and set the fan to ON, so the blower moves warm room air over the coil.
2. 2Lay towels around the base of the indoor unit for the meltwater. Do not chip at the ice: the coil's fins and tubes are thin and easy to puncture.
3. 3Wait until all the ice is gone. That can take a few hours, sometimes longer.
4. 4Replace a dirty filter, open all the vents and set the fan back to AUTO before you switch cooling back on.
5. 5If the coil freezes again, call a technician: that usually means low refrigerant or a fan problem.

4. Clean Around the Outdoor Unit

The outdoor unit gets rid of the heat from inside. Leaves, grass clippings and dirt on its fins trap that heat, and the AC cools less.

1. 1Turn the AC off at the thermostat, then pull out the disconnect's handle or switch it to OFF, and turn off the AC's breaker in the panel.
2. 2Clear leaves and debris from the top and the sides, and trim plants back so there is open space around the unit, about two feet.
3. 3Rinse the fins gently with a garden hose. Never use a pressure washer, it bends the thin fins.
4. 4With your hands clear, put the disconnect back on and switch the breaker on, wait a few minutes, then switch cooling on.

5. A Tripped Breaker or Safety Switch

If the indoor fan runs but the outdoor unit stays silent, or nothing runs at all, the power may be cut.

1. 1Set the thermostat to OFF and make sure nobody is touching the indoor or outdoor unit. Then check the panel for a tripped AC or furnace breaker and reset it once by pushing it fully off, then on.
2. 2If it trips again, leave it off and call a technician, see breaker keeps tripping for what a repeat trip means.
3. 3Look at the condensate drain from outside the indoor unit, without opening any panel. Many systems have a float switch that shuts the AC off when the drain backs up, see AC dripping water inside.
4. 4Some furnaces have a power switch that looks like a light switch near the unit. Make sure it is on, and that the disconnect next to the outdoor unit is on too. Then switch cooling back on at the thermostat.

When to Call a Technician

This Old House puts common AC repairs at about $150 to $650, a refrigerant leak repair at $200 to $1,500 or more, and a new compressor at $1,000 to $3,000 or more. None of these are owner jobs:

1. 1The outdoor unit hums but its fan does not spin: often a failed capacitor, which holds a dangerous charge.
2. 2Ice that comes back after a thaw and a new filter, a hissing sound, or oily spots on the pipes: likely a refrigerant leak. Refrigerant may only be handled by technicians with EPA certification.
3. 3A breaker that trips again after one reset.
4. 4Warm air even though the filter is new, the coil is free of ice and both units run.