Why won't my air conditioner turn on?

Replace the HVAC system thermostat

The thermostat controls the heating and cooling system in your home. The thermostat senses the temperature inside your home and cycles the system on and off to maintain the set temperature. When your HVAC system control thermostat quits working, replace the thermostat.

Clear the condensate drain

Your central air conditioner or heat pump has a pipe that drains condensate water that drips from the evaporator to the outside of your home. If the drain pipe or condensate drain system gets clogged, water overflows the drain pan under the evaporator and starts dripping below the inside air conditioner unit. The overflowing condensate can cause water damage in your home.

Many central air conditioners have a drain float switch that shuts the system down if the drain is clogged and water backs up under the evaporator. If this switch trips, the air conditioner will shut down to avoid water damage.

If water drips from below the inside air conditioner unit and/or the air conditioner shuts down because the drain float switch tripped, then it's time to unclog the drain line.

You may be able to remove the clog with a shop vac. If not, you can use a plumber's snake to clear the drain line.

Watch our video Air Conditioner Not Cooling: Float Switches and Drain Pipe Clogs to learn more about fixing and preventing condensate drain system clogs.

To keep the condensate drain system from clogging up in the future, pour a cup of liquid bleach down the drain vent tube every month. The bleach kills mold and mildew that can build up inside the drain pipe and hose. To keep from killing the grass in your yard, stick a pan under the drain hose outside to catch the bleach as it runs out. And always remember to protect your eyes and skin when handling bleach and other harsh chemicals.