What to Check When My Central AC Isn’t Cooling

There’s nothing worse than a summer heatwave without a functioning central AC system. While many problems require the help of a professional, there are some common problems that are easy for homeowners to fix. This article will focus on helping you to diagnose problems with your home’s AC system, and walking through the steps to troubleshoot simple problems. We’ll also let you know which problems you can handle yourself, and when it makes sense to call a professional for AC repair. Ready? Let’s get started!

A Step-by-Step Guide to Diagnosing Central AC Problems

#1. IS YOUR AC UNIT RUNNING AT ALL?

Turn your AC on and set the thermostat 5-10 degrees lower than the current temperature, then wait 60 seconds. Are you getting air from your vents? Does the outside compressor and fan come on? If one of the components is not coming on, you’ll want to check to make sure that each component is getting power. Check your circuit breaker (hopefully, the air conditioning breaker components are labeled). Reset the breaker that is connected to the device not coming on. You will also want to double-check the batteries in your thermostat which are behind the faceplate for the thermostat.

#2. IS YOUR AC UNIT RUNNING, BUT NOT BLOWING AIR?

With your system running, put your hand in front of the air registers (the vents on your floors, walls or ceiling that supply conditioned air). Do you feel air being blown? If it’s completely stagnant, with no airflow at all, it’s likely you have a problem with your air handler (blower). When was the last time your filter was changed? Double-check your filter and the condition of your indoor unit.

#3. IS YOUR AC BLOWING AIR THAT ISN’T COOL?

Reduce your thermostat temperature 5-10 degrees, so that it’s much colder than the current house temperature. Is your thermostat set to cool and is below your house's set point? When you put your hand in front of the register, does the air feel cold to the touch? If the AC is blowing, but not cooling the air at all, then it is likely that you have a problem with your outdoor condenser. Double-check that the fan and compressor are running.

#4. IS YOUR AC JUST NOT COOLING YOUR HOUSE ENOUGH?

Set your thermostat for two degrees colder than the current setpoint and allow the system to run for 90-minutes. Has the temperature in the home decreased (even by one degree)? Depending on the current outdoor temperature it may take over an hour of run time to decrease the indoor temperature one degree. Be patient! If the thermostat set point does not change in 90 minutes of run time double-check both the indoor and outdoor unit operation. Be aware these systems have designed parameters — Residential air conditioning systems are designed and sized to maintain 75 degrees (indoors) on a 95-degree day. Are your expectations reasonable for what temperature you are hoping to achieve?

What to Check if Your AC Isn’t Running at All

#1. CHECK TO MAKE SURE THE AIR HANDLER AND COMPRESSOR ARE ON AND PLUGGED IN

There are two separate pieces of your AC system, the condenser, which cools the air and is usually outside, and the air handler, which moves the air around your house and is usually inside. First, make sure that any power switches on or around the units are switched on. Then check your breaker box to make sure that neither has thrown a breaker and that both circuits are receiving power.

Does the fan come on then?

If not, try replacing the batteries of the thermostat. Even if the screen and indicators are still working, there are still cases when low power draw from the batteries can cause the thermostat to function improperly.

If it still doesn't work, double-check all wiring connections on the backplate of the thermostat. This may require an advanced level of troubleshooting. Begin by removing the faceplate for the thermostat you access all wiring connections.

Did you upgrade your thermostat to a WiFi-enabled/Smart thermostat? These types of thermostats require a common wire as a source of power? Was there a “C” wire in place prior to upgrading your thermostat? If not, you will need to run an additional wire.

IT DIDN’T WORK, NOW WHAT?

If none of these things work, it’s likely that you have a faulty thermostat, motor or compressor. This means that it will need to be fixed or replaced, and you’ll need to call a professional.

What to Check When Your AC is Running, But Not Blowing Air

#1. CHECK TO MAKE SURE THE AIR HANDLER IS ON AND PLUGGED IN

Is the air handler on? If the compressor outside your home is running but the air handler isn’t, you may be cooling air that isn’t going anywhere. If you don’t hear the air handler running, double-check that it is on and receiving power. Is there a switch on or near the unit that has been accidentally turned off? Next, check your breaker box to make sure that the breaker hasn’t been thrown.

If neither of these things gets your air handler running then it’s likely to be an issue with your condensate pump switch. If you’re a DIY type of person, this is something you may be able to fix with some online research, but in many cases, this is best left to a professional.

#2. LISTEN TO YOUR AIR HANDLER

If your air handler is running but not blowing air, there are two likely scenarios; either a broken AC blower belt or an issue with the motor itself. If the air handler is humming or buzzing, but not making the mechanical sounds of a motor running, you’re most likely talking about an issue with the motor itself. This is definitely a case where you’ll need a professional.

If the motor is running, but it is not moving any air, then you’ll want to look to see if the blower belt is broken. This is something that a DIY savvy homeowner could tackle themselves, but it does require opening up the air handler and replacing parts, so most homeowners will probably look to a professional for this as well.

What to Check When the AC is Blowing Warm Air

What to Check When the AC is Cooling, But Not Enough?

#1. MAKE SURE AIR REGISTERS ARE CLEAR OF BLOCKAGE

Check to make sure that your air registers (the vents that blow the cold air into the rooms of your home) are open and free of blockage. Sometimes these get closed, or dust can build up over time and limit airflow. Remove the registers with a screwdriver and make sure that they are clear of dust and blockage.

#3. MAKE SURE YOUR AIR HANDLER IS CLEAR OF BLOCKAGE

Refrigerant problems can cause the indoor unit and evaporator coil to freeze. Believe it or not, but the coil inside your air conditioning system will become one large block of ice. The longer the air conditioning system runs under these conditions the larger the block of ice. If you are experiencing low or no airflow, check for frost formation on the copper lines leading to the indoor evaporator coil. Chances are if ice is forming on these lines the coil is one giant ice cube.

Also, check for water on the floor around the air handler. This can be a sign that the condensate drain is clogged and may need to be cleared. You may want to call a professional, but there are some DIY resources on solving this problem online.