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Test Your Central Air’s Cooling Ability

Test your Central Air's Cooling Ability

Sure, your fan motor pushes air out the vents, but it takes more than that to cool a house. Here’s how to tell if your AC stands for “actually cooling.”

Tools & Materials

Use this checklist when you go to the store and purchase your items.

Know Before You Shop for Supplies to Test Your Air Conditioner

  • Test your air-conditioning system after you’ve changed the furnace filter to rule that out as a potential problem.
  • You may not need the pry bar if your cooling vents fit loosely enough to lift free.
  • You can test the system yourself. However, when it comes to taking apart the indoor or outdoor portions of your central air conditioner, you should call a professional who has the parts, equipment, and experience to do service work.
  • To determine which duct is the return, place a tissue over the ducts in your home. A tissue placed over the air conditioner duct will blow away from the duct when the system is working. A tissue placed over the return air duct will be drawn into the cover.

Test Your Air Conditioner

Step 1

Turn on the air conditioner while the outdoor temperature is above 80 degrees. Set the thermostat well below the room temperature to keep the system running long enough for a test. Identify an air duct that’s closest to your air conditioner’s inside unit (usually attached to the furnace) and a return air duct. These may be in separate rooms. Remove the grilles from both.

Step 2

We used a refrigerator thermometer for this test, and we taped it just inside the air duct. Allow the system to run for about 10 minutes, and then read the temperature at the air conditioning duct.

Step 3

While the system is still running, move the thermometer to the return air duct and leave it there until the temperature reading remains steady. Subtract the return air duct temperature from the incoming duct air temperature.

Understanding the Results of Your Air Conditioner Test

You should see a temperature difference of 14 to 20 degrees between the air duct and the return air. If the difference is less than 14 degrees, your air conditioner may be low on refrigerant. A service technician can recharge the coolant, but you’ll want to make sure your pro first tracks down any leaks. That’ll cost more up front, but it will save you the price of additional service calls and wasted utility dollars.

A minor temperature difference may also be caused by a dusty evaporator coil. The coil is a grid within the air conditioner’s inside unit attached to the furnace, and it’s best serviced by a professional. When a clean coil works as it should, air passing over it cools before it flows to the rest of the house. On coils that haven’t been cleaned for several years, a coating of dust and dirt clings to the moist surfaces, insulating them and partially blocking air flow. There also could be a problem with the compressor or fan, but that’s best left to a professional to diagnose.

A temperature difference greater than 20 degrees doesn’t necessarily mean all that cold air is staying in your rooms. If the filter is new — you already changed it, right? — this temperature difference could be a sign of a problem with restricted airflow in the ductwork. Again, you’ll need a pro with the right diagnostic tools to track down these problems.

Change Your System's Filter Frequently

So you’re probably thinking, “Can’t I do any other service work besides changing the filter?” Don’t underestimate the importance of that. A dirty filter acts like a wall in your air duct. So write a reminder on your calendar to check the filter each month, and then change it every one to three months, depending on the brand of filter and how many shedding pets and smokers live in your house.