Complete Guide to Air Conditioning Preventitve Maintenance

We have all received those advertisements in the mail for air conditioning check-ups. Preventive maintenance is a good thing. It can increase the efficiency and life of your HVAC system. What these fliers do not tell you is that these companies are just looking for a way to get through your door.

Air Conditioning

The tech will be there looking for a way to run up the bill. Basic maintenance of your air conditioning system can be done by the homeowner. This guide will show you how.


The filter is the first thing that should be looked at. Dirty filters create an air flow restriction. This makes the fan work harder and in extreme cases can cause the evaporator coil to freeze up. Filters that are missing, not the proper size, damaged or of poor quality allow dust to accumulate on the coil.

This will create the same situation as a dirty filter.

Disposable filters will usually be found either at the air handler or in a return grill. Measure the filter area to verify that the proper one has been installed. Write this measurement on the front of the air handler for future reference. When it comes to air conditioning filters you truly get what you pay for.

The cheep filters that you can see through allow massive amounts of dust to pass through. These need to be replaced often. The white filters with W shaped material are best and last longer.

Electronic air conditioning filters require the cartridges to be removed. Follow the manufacturers instructions.

A basic overview is as follows:

  1. Disconnect the power supply
  2. Remove the cover. Take note of the top and bottom for reinstallation.
  3. Remove the pre-filter and the cleaning cartridges. Take note of the position of everything that you remove. For the filter to work the pins will need to line up and air flow will need to be correct.
  4. Use a hose to wash the filters and cartridges. The water should flow from the unit side back to the dirty side. Do not push the dirt into the filter.
  5. Allow everything to dry.
  6. Reinstall in the opposite order as removal.

Air Handler

This unit is sometimes called the furnace. The air handler will be found in the garage, in a closet, in the basement or in the attic. It is the unit that all of the duct goes to. Inside the air handler you will find a fan, a heating source and some electrical parts.

Most models also have the evaporator coil in the same compartment, the exception would be gas heating systems which use a plenum on top of the furnace.

Clean the drain line first. A helper will make this easier. Find the plastic drain line that leaves the coil area. Access this line from either a T fitting or from inside the coil area. Have your helper stand outside where the drain line exits the house. His job is to stand there and watch the water.

As you pour water into the drain sludge will come out of the drain outside. Continue to flush the drain until the water exiting the drain runs clear. A common place for a back-up is the elbow outside where the drain line ends. Often all that is needed to fix a clogged drain is removing this fitting.


For stubborn blockages hook a wet/dry vac to the drain line. After the line has been sucked clean pour water into the drain to check for sludge. Some drains are equipped with an emergency float switch. This is designed to turn off the unit if the drain is clogged. If after working on a drain the air conditioner will not turn on check this.

It will be near the unit and have wires coming out of the top. The cap will pull straight out.

Inspect the evaporator coil. To do this you will need to remove the access panels. Set the screws aside in a safe place so they will not be lost. Look at the side of the coil that the air comes across first. This is where the dust will gather. Extremely dirty coils will need a professional AC tech to “pull and clean” them.

Homeowners can perform maintenance cleaning. This can be done with an aerosol coil cleaner. Simply spray the cleaner on and then rinse it off. The more advanced “in place coil clean” is done with liquid coil cleaner and either a coil rake or a stiff bristle brush.

I recommend the brush as the rakes come in different sizes and if you use the wrong one the coil will be damaged.

To clean the coils:

  1. Mix the liquid coil cleaner in a spray bottle with water.
  2. Spray the coil. The solution will start to foam.
  3. Carefully use the brush to clean the fins. To do this start at the top and slowly slide the brush with the fins down to the bottom.
  4. Rinse with clean water. When doing this keep plenty of rags handy to clean up any mess immediately.

Visibly inspect the wires and electrical parts for damage or signs of overheating. Burnt or brittle wires and components need to be replaced and the cause will need to be solved. Depending on your ability and the availability of the parts an air conditioning tech might need to be called.

Often the cause can be found by using an amp probe as described below.

Check the fan for oil ports. These have little rubber stoppers on the fan casing. When servicing pull the plug and use a light oil. A little will go a long way.

On the front panel of the furnace will be sticker that gives the electrical data. It will show voltage and amp draws of the fan (FLA) and heating elements. When checking the system all voltage and amp draws should be less than the stated maximum. If a component is above the stated maximum then a service tech should be called for replacement.

Check the voltage at the terminal block. Turn the fan on at the thermostat. Check the amp draw at the fan. Check the amp draw at the terminal block. It should be the same as at the fan. Turn the AC on at the thermostat. The amp draw should remain the same as when the thermostat is set to the fan only.

If there is a large increase usually the heat relay is stuck on. Turn the electric heat on. The amp draw at the terminal block should equal the fan plus the heat strip rating.

Check the deference in the temperature across the coil to determine if the air conditioner is working properly. Place a thermometer at the return. Record the temperature. Check the temperature as close to the coil as possible. The difference should be about 20*F.

Frozen coils are a common problem in older models. This is usually caused by a leak. Other causes could be dirty coils and filters, fan not working properly or an air restriction. If the filter and coil are clean then the homeowner should check the duct work.

If the duct is not crushed and everything is clean then a service tech will need to be called for a leak check. The tech will hook gauges up outside to check the freon level. If it is low then they will use a hand held devise to check for freon leaks. Most often the leak will be found at the evaporator coil.

Older units might not have replacement coils and a new air handler will need to be bought. Heat pumps should have both inside and outside units replaced at the same time if the SEER rating does not match.

Servicing a gas furnace should be left to qualified technicians. The regulator and igniter’s are easy to miss diagnose and potentially dangerous.

Condensing Units

The condensing unit will be found outside. This is the heart of an air conditioning system. It pushes liquid freon to the inside unit and sucks vapor freon back to the compressor. From the compressor the liquid freon is cooled in the condenser coil before starting over.

Freon can not be purchased by a homeowner so if a system needs a boost a tech will need to be called. Checking freon levels require a set of gauges and the correct pressures will depend on the ambient air.

Homeowner maintenance is limited on the outside unit. Remove any grass or debris that is around the coil. Clean the condensing coil with an aerosol coil cleaner. Checking fan and compressor amp draws can be done following the same directions as the air handler. The fan should have an oil port near the top.

Access to the motor depends on the brand. Check the low voltage wires for cracked or missing insulation. A cheap fix here would be wrapping them with electrical tape.

Capacitors, contactors and fans are the most common replacement items. Often the capacitor will swell on top or leak oil. Many times I have been called to a house for a second opinion only to find a capacitor that has swollen. Dishonest ( or incompetent ) service techs try to sell units when all that is needed is a capacitor.

Package Units

These are the systems where the air handler and condenser are both in one unit. These are common on Mobile homes and commercial buildings. Another type would be window size air conditioners. These are easy to work on because everything is in one spot. It is not cost effective to service the window size units.

The only thing that should be done with these is a simple coil clean. The larger units are maintained the same as split systems except everything is in one spot.

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