An AC unit’s main task is to condition air. During the cooling cycle, heat is extracted by “moving” air (the indoor fan) through the indoor cooling coils to remove heat from the air.  The heat is transferred to a gas (commonly called refrigerant or sometimes Freon) inside the coil’s tubing, which is then pumped by the compressor into a similar coil outside where air is again passed through the coil to extract the heat.  The cooled refrigerant then returns for another round.

Obviously anything that restricts air flow through either coil cuts down on efficiency.  A dirty filter is culprit number one.  (We run into this simple problem more often then you would think.)  The filter’s function is to keep the cooling coil from becoming clogged with dirt and lint.  (A clogged coil cannot pass air.)  Be sure to keep the filter clean.  The filter has the added benefit of cleaning the air we live in!  Virtually all systems combine heating and cooling in one unit.  Both heating and cooling use the same air flow system, and hence, filter maintenance is a year round duty.

Use your eyes and ears to check the return air filters.  There is at least one filter for each unit in your house. Large systems often have more than one filter. Listen for a whistling sound coming from the filter. This is often an indication of too much restriction.   Take filters out and look at them while holding them up to a light. A dirty filter won’t pass much light.

 Check your filters more during peak cooling or heating seasons.    Typically a unit runs many more hours in the heat of the summer or the dead of winter.  A unit on the second floor may not take in as much dust as a unit on the ground floor where filters might need service more often.  Houses with lots of people typically experience more dust then with fewer people.  Become familiar with your home’s “personality”.  A typical unit will need a new filter each month during extreme temperatures but might not need one for several months during moderate weather in the fall or spring.

 In a pinch, if you don’t have a new filter on hand, you can use vacuum cleaner to temporarily reuse an existing filter.  This is better than restricted air flow.  But NEVER EVER run a unit without a filter.  The cost to clean the evaporator will be more than the cost of 50 filters!  When you buy filters, get extras so you will have them on hand when needed.  (I store my spare filters against the back of the return air plenum so I will have them when I need them.)

 Likewise check outside unit’s coil.  It typically does not have a filter but it can still become plugged with dirt, leaves, or other debris killing its efficiency and causing to eat electricity.  It can even shorten the life of the expensive compressor.  When running, there should be a steady and smooth flow of hot air from the top of the unit. If dirty, the unit can be easily washed with a hose and water.  If you are handy you can take on this chore yourself or call for service. Regular cleaning is always good.  A careful examination of the coil’s fins will tell you when service is necessary.

 To clean the unit yourself, turn unit off at the thermostat. Use a soft brush to clear any trash, paper, leaves or clippings from the outdoor unit’s surfaces.  Then use a water hose with moderate pressure and a spray nozzle to clean directly through the coil’s fins washing away any accumulated dirt.  You should be able to see light through the coil at any point on its surface when you are looking straight through the side of the unit. In some cases a mild soap can help.  Use liquid dishwashing soap.  A few ounces to a gallon of water, sprayed from a squirt bottle is safe and works well, especially if you have lots of mold or sticky debris.   If you have a unit with protective or decorative metal or plastic grills covering the sides, a complete cleaning might involve removal of the covers first.