How to Repair Central Air Conditioners

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Central air conditioners have two separate components: the condenser and the evaporator. The condenser unit is usually located outside the house on a concrete slab. The evaporator coil is mounted in the plenum or main duct junction above the furnace.

Most central air conditioners are connected to a home's forced-air distribution system. Thus, the same motor, blower, and ductwork used for heating are used to distribute cool air from the air conditioning system. When a central air conditioner is operating, hot air inside the house flows to the furnace through the return-air duct. The hot air is moved by the blower across the cooled evaporator coil in the plenum and is then delivered through ducts to cool the house. When the air conditioner works but the house doesn't cool, the problem is probably in the distribution system.

Central air conditioners are made up of  the condenser unit, on a concrete slab, and the evaporator coil.

Central air conditioners are made up of two separate components: the condenser unit,
located outside the house on a concrete slab, and the evaporator coil above the furnace.

Both the evaporator and the condenser are sealed. Therefore, a professional service person should be called for almost any maintenance other than routine cleaning. Central air conditioners should be professionally inspected and adjusted before the beginning of every cooling season. However, don't let your maintenance end with this annual checkup. While there aren't many repairs you can make yourself, there are specific maintenance procedures you can follow to keep your system operating at peak efficiency.

Caution:
Before doing any work on an air conditioning system, make sure the power to the system, both to the condenser and to the evaporator assembly, is turned off.

Before you start working, let's try to narrow the scope of the job. Look for the problem you're having, and its solution, on the chart on the next page.

For more articles on home repair, check out the following links.

  • How To Repair Room Air Conditioners: Cooling units that you mount in your window have the same job as central air conditioners, but the repair principles are different. Follow these instructions to get your unit running smoothly.
  • Major Appliance Repair: If the a/c isn't the only thing in your house on the fritz, you can learn how to fix other machines in this article.
  • Small Appliance Repair: Once you've tackled the a/c, a toaster or blender seems like child's play. Find out how to fix them here.
  • Thermostat Maintenance: To make sure there's actually a problem with your a/c, you may want to check the thermostat, too. Learn how to calibrate a thermostat.

Troubleshooting Central Air Conditioners

Central air conditions will need some professional maintenance, but there are many minor problems that you can easily fix yourself. If your central a/c unit isn't working properly, look for the problem you're experiencing on this chart and see if it's a do-it-yourself job.

CENTRAL AIR CONDITIONER TROUBLESHOOTING CHART
Problem  Possible cause  Solution 
Condenser
doesn't run

 
1. No power.


 
1. Check for blown fuses or tripped
    circuit breakers at main entrance
    panel or at separate entrance
    panel; restore circuit. 
  2. Thermostat set
    too high.
2. Lower thermostat setting 5°.

  3. Motor faulty.  3. Call a professional. 
  4. Compressor
    faulty. 
4. Call a professional.
 
Uneven cooling

 
1. Distribution
    system out of
    balance. 
1. Balance system.

 
Inadequate
cooling
1. Thermostat set
    too high. 
1. Lower thermostat setting 5°.

  2. Evaporator dirty.  2. Clean evaporator 
  3. Unit too small.
 
3. Replace with larger unit;
    call a professional. 
Unit doesn't
1. Thermostat set
1. Lower thermostat setting 5°.
cool
    too high.  
  2. Condenser dirty.  2. Clean condenser coil and fins; 
        if necessary, straighten fins.
  3. Condenser unit  3. Remove debris blocking condenser;
      blocked.
    cut down weeds, grass, and vines.
  4. Evaporator dirty.  4. Clean evaporator.
  5. Compressor faulty.  5. Call a professional. 
  6. Not enough
    refrigerant in system.
6. Call a professional.
 
Condenser unit
turns on and
off repeatedly 
1. Condenser dirty.
2. Condenser unit
    blocked. 
1. Clean condenser coil and fins.
2. Remove debris blocking condenser;
    cut down weeds, grass, and vines. 
  3. Evaporator dirty. 3. Clean evaporator.



A dirty evaporator is a common problem. On the next page you'll learn how to access the evaporator and clean it properly.

Cleaning the Evaporator

The evaporator for the central air system is located directly above the furnace in the plenum. The evaporator may not be accessible, but if it is, you should clean it once a year. If the plenum has foil-wrapped insulation at its front, you can clean the evaporator; if the plenum is a sealed sheet metal box, do not attempt to open it. Here's how to clean an accessible evaporator:

Step 1: Remove foil-wrapped insulation at front of plenum; it's probably taped in place. Remove tape carefully, because you'll have to replace it later. Behind insulation is access plate, which is held in place by several screws. Remove screws and lift off plate.

What You'll Need
You'll want to have these tools on hand to clean the evaporator:
  • Screwdriver
  • Stiff brush
  • Large hand mirror
  • Household bleach
  • Wire
Step 2:
Clean entire underside of evaporator unit with stiff brush. A large hand mirror can help you see what you're doing. If you can't reach all the way back to clean entire area, slide evaporator out a little. Evaporator can be slid out even if it has rigid pipes connected to it, but be careful not to bend pipes.

Step 3: Clean tray below evaporator unit. This tray carries condensation away from evaporator. Pour 1 tablespoon of household bleach into weep hole in tray to prevent fungus growth. In extremely humid weather, check condensate drain and pan every other day. If there's much moisture in pan, weep hole from pan to drain line may be clogged. Open weep hole with piece of wire.

Step 4: Put unit back into place, reinstall plate, and tape insulation back over it.

Step 5: Turn back on air conditioner, and check for air leaks. Seal any leaks with duct tape.

You also may need to clean the condenser to get your air conditioning functioning properly. Find out how on the next page.

Cleaning the Condenser

In most air-conditioning systems, the condenser unit is located outside the house and is prone to accumulate dirt and debris from trees, lawn mowing, and airborne dust. The condenser has a fan that moves air across the condenser coil. You must clean the coil on the intake side, so, before you turn off the power to the air conditioner, check to see which direction the air moves across the coils. Here's how to clean the condenser:

What You'll Need
You'll want to have these tools on hand to clean the condenser:
  • Grass shears or pruners
  • Spray bottle of coil cleaner
  • Soft brush
  • Fin comb
  • Carpenter's level
  • Pry bar or piece of 2-by-4
  • Gravel or rocks
Step 1: Cut down any grass, weeds, or vines that have grown around condenser unit; they could be obstructing airflow.

Step 2: Clean condenser with commercial coil cleaner, available at refrigerator supply stores. Instructions for use are included. Flush coil clean (do not use hose); let dry.

Step 3: Clean fins with soft brush to remove accumulated dirt. You may have to remove protective grille to reach them. Do not clean fins with garden hose, as water could turn dirt into mud and compact it between fins. Clean fins very carefully: They're made of light-gauge aluminum and are easily damaged. If fins are bent, straighten them with fin comb, sold at most appliance parts stores. A fin comb is designed to slide into spaces between fins. Use it carefully to avoid damaging fins.

Step 4: Check concrete pad on which condenser rests to make sure it's level. Set carpenters' level front to back and side to side on top of unit. If pad has settled, lift pad with pry bar or piece of 2 x 4, then force gravel or rocks under concrete to level it.

During the fall and winter, outside condenser units should be protected from the elements to prevent leaf blockage and ice damage. Cover the condenser unit with a commercial condenser cover made to fit the shape of the unit or use heavy plastic sheeting secured with sturdy cord.

If you've cleaned everything you can and you're still not getting cool air, the problem could be the refrigerant. Learn what to do in that case on the next page.

Handling the Refrigerant

The coolant used in most air conditioning systems is a refrigerant called Freon. If the system does not contain the proper amount of Freon, little or no cooling will take place. If you suspect a Freon problem, call a professional service person to recharge the system. Caution: Do not try to charge your system's refrigerant lines.

Here's how you can repair the system's coolant lines. Examine the lines running from the condenser outside the evaporator inside the house. If the insulation is damaged or worn, it will cut down on the cooling efficiency of the unit and, therefore, should be replaced.

Replace damaged or worn coolant line insulation with new insulation of the same type as soon as possible. Follow manufacturer's instructions for installation.

Now that you know when you do and don't need to call for professional help, you'll be able to make some repairs on your own. Using the tips in this article, you'll be able to get save a few bucks and get the cool air pumping sooner.

For more articles on home repair, check out the following links.

  • How To Repair Room Air Conditioners: Cooling units that you mount in your window have the same job as central air conditioners, but the repair principles are different. Follow these instructions to get your unit running smoothly.
  • Major Appliance Repair: If the a/c isn't the only thing in your house on the fritz, you can learn how to fix other machines in this article.
  • Small Appliance Repair
  • Thermostat Maintenance: To make sure there's actually a problem with your a/c, you may want to check the thermostat, too. Learn how to calibrate a thermostat.