AC Condenser Unit: Types, Maintenance, Common Problems, Cost for New/Replacement Condensers

This guide is about the AC condenser types – options, how they work and more.

Understanding the Modern Air Conditioning Process

Gone are the days when people placed a fan behind a large block of ice so it would blow cooler air toward the occupants of a room. That method was used in homes and in the first commercial “air conditioning” units developed by Willis Carrier and others. There’s loads of historical detail in our AC Then and Now Guide.

Today, the technology of “conditioning” air has advanced much further with much greater benefits to us and people around the world when the temperature gets uncomfortably warm.

Today’s AC systems used can be divided into two major methods: Refrigeration and Evaporation.

The Refrigerant Method of Air Conditioning

This method is the most commonly used and most efficient type of air conditioning.

It uses a refrigerant, which is a special chemical that circulates inside a closed loop. During this process, the refrigerant is changed back and forth from liquid to vapor, and from very cold to very hot, depending on what part of the loop it is in.

For a basic explanation of this process, if we begin at the evaporator, we find that the refrigerant is a very cold liquid. As indoor (room) air passes over the surface of the evaporator coils, the cold refrigerant inside them absorbs some of the heat from the air. This leaves the air cooler, or “conditioned”, and it is then blown into the living space.

When the refrigerant arrives at the compressor, it has now been changed to a cool, low pressure vapor. It is then compressed to a high pressure and high temperature vapor and goes on to the condenser. Passing through the condenser coils, the refrigerant (now a very hot vapor) transfers much of its stored heat to the air outside of the building and at the same time cools enough so that it changes back to a liquid, though still quite warm. This then travels through a special device which quickly lowers its temperature and pressure for the cycle to repeat.

NOTE: It should be mentioned at this point that in recent years, the compressor component of most AC units on the market has been greatly improved by using an inverter/compressor instead of the standard compressor. This has been called “a quantum leap” in AC technology. 

The main difference is that the standard compressor would start whenever the indoor thermostat called for cooler air and run at full speed. This continued until the thermostat was satisfied, then the compressor would  shut off. When cool air was again needed, the unit started again, running at full speed.

In contrast, the newer inverter/compressor will run much of the time the system is operational, but its speed varies depending on the amount of cool air needed. This is the function of the inverter, which carefully regulates the speed of the compressor to just what is needed from moment to moment. 

Because the compressor is not starting and stopping every time the thermostat signals a small rise in the indoor air, the energy consumption can be as much as 30% less than the standard compressor. 

Just as your vehicle gets better fuel mileage when running at a steady speed on the highway than when in stop and go city traffic, the AC unit with an inverter-controlled compressor gets better electricity “mileage” than the one that starts and stops all the time. An added benefit of this newer technology is that the temperature of the conditioned air inside the building fluctuates less than it did with the older technology.

Many homes with existing AC units will use the older technology, while most new homes and AC units on the market use the newer technology.

The Evaporation Method of Air Conditioning

This older method uses water that moistens pads made of various materials so that air blowing across the pads evaporates some of the water, resulting in a drop in the air temperature. This cooler (and moisturized) air is then blown into the room or rooms either directly or through air ducts in the building. Additionally, the moist air evaporating from skin carries heat with it, further cooling off those in the range of the evaporative cooler. If you’re interested, we’ve compiled a current list of the best evaporative coolers available. 

These two methods of air conditioning are employed in a variety of types of AC units, which will be discussed below.

Types of AC Condenser Units – Refrigeration Method

Here is a description of each type of condenser with pros and cons. Cost information is included.

1. Standard Air Conditioner with Split Furnace as the Air Handler

How Does AC Work

This type of system is probably the most commonly employed kind of AC in many new homes, especially where a furnace is considered a necessary appliance. It is called a “whole-house” system because it works in conjunction with the house heating system. It normally has 3 components, 2 outside the building and 1 inside.

  • Outdoors, a metal cabinet contains the compressor and condenser, plus a fan to circulate air across the condenser coil.
  • Indoors, the evaporator is usually installed as part of the building’s furnace air handler (blower), that sends cool air through the duct system when called for, or warm air from the furnace during cold weather.

With this type of system, the only connection between the building and the outside cabinet is a pair of copper tubes through which the refrigerant flows, known as the line set, and an electric wire to control the compressor.

Standard AC split furnace units are usually found in sizes ranging from 18,000 to 60,000 BTUs which are capable of cooling homes as small as 600 square feet to as large as 3,000 square feet depending on the climate. There is a sizing guide and lots more information in our Central Air Conditioner Guide.

Many factors need to be considered when choosing the proper air conditioner for your home: geographical location, size and floor plan, exposure to direct sunlight, wind, and others, all discussed in our Central AC Guide linked above. Local professionals are the best source of information when planning to install Standard AC in your home, and our guide will help you discuss your AC requirements with the pros. 

We recommend getting estimates from at least three. Get your questions answered, compare systems, and choose an installer with a proven track record (see online reviews) of quality installation and customer service.


  • Because the mechanical components are all outside the building, it is quieter than those types which have the condenser in the home.
  • Various sizes are available capable of providing comfortably cool air for even fairly large homes.


  • This is one of the more expensive AC systems to purchase initially, but the plus side of this is that it is one of the most efficient AC systems.

Cost: $1,500 to $3,500 for the outdoor Condenser component only, depending on size and efficiency rating. This must be matched to the indoor evaporation coils and an air handler or furnace, if heat is also required. The average cost of installing a complete Standard AC with Split Furnace is $7,000 to $10,000.

Maintenance Recommendations:

  • Filter. Dirty or clogged filters can restrict air flow, decrease efficiency and even cause damage to the evaporator coils. The filter or filters are usually found inside the return air duct. Change at least every 4 months.
  • Evaporation coils may become dirty from accumulated dust in the duct system. Check and clean at least once a year. In fact, as we’ve discussed in many posts like our HVAC Warranty Guide, most warranties require the homeowner to provide annual maintenance – and keep proof of it – of the warranty might be voided.
  • Condenser coils (outside unit) may especially collect dirt and reduce your units’ efficiency because of exposure to the outdoor elements. Dirt, dust, leaf litter and other debris should be cleaned from these coils at least annually. Keep bushes and other plants trimmed back at least 2 feet to facilitate air flow.
  • Condensate Drain Line. This drains water from the indoor evaporation component  through a line outside your home. Running a stiff wire gently into this line a few inches annually will help keep it running free.

What Can Go Wrong?

  • Dirty Condenser Coils 
  • Clogged Drains
  • Frozen Evaporator Coils
  • Low Refrigerant
  • Fan Problems 
  •  Leaking Ducts 
  • Thermostat Problems  

With most of these problems, you may not realize that they exist, except that your AC is not cooling the way you are accustomed to. The first three on this list should be prevented if you have followed the Maintenance Recommendations. For the other problems, a trained technician will most like have to diagnose and fix the issue.

2. Heat Pump & Air Handler

The heat pump central AC works  the same way the standard AC does in the summer. But during the winter, the system basically reverses the process. The outside condenser component gets cold and the inside evaporator component gets warm. Rather than cooling your home, the heat pump warms it. Learn lots more in our Heat Pump Buying Guide.

Like the standard AC system, these units are capable of cooling even large homes.


  • Heat pumps are very efficient cooling systems, with some of the higher efficiency ratings of all AC types.
  • They heat more efficiently than gas furnaces – producing lower energy costs in most areas.


  • Most are only efficient at heating a home when the outside temperature is around 25 F or above. If you live in a climate where the temperatures drop below this point, an additional source of heat will be required. A few cold climate split system heat pumps are available, but at premium cost.
  • Sometimes these units make strange noises. This is usually in the winter when certain valves reverse the refrigerant flow to defrost the system.

Cost: $1,800 to over $4,000. This is for the outdoor Condenser component only, depending on size and efficiency rating. This must be matched to the indoor evaporator coils and a furnace or air handler, which will increase the total cost significantly. Average cost of installing a complete Heat Pump Central Air system is $10,000 or higher.

Maintenance Recommendations:

  • Air filters. Check monthly. Clean or replace if signs of wear are observed.
  • Outdoor unit. Keep it clear of snow and ice in winter; leaves, litter and other debris Spring through Fall. Maintain a 2 foot clear space around the unit to permit free air flow.
  • Outdoor coils must be kept clean. If dirty, clean according to the owners’ manual.
  • Keep shrubs pruned back at least 18 inches from all sides of the heat pump to allow for proper airflow and maintenance.

What Can Go Wrong?

  • Not cooling
  • Not heating
  • Not running
  • Running all the time

 If you have followed the Maintenance Recommendations, you have already done all that you can probably do. You might check them again, but otherwise, a trained technician should be consulted.

3. Geothermal Air Conditioner

A geothermal HVAC system is most often a combination heating and cooling, whole-house system. It works on the same refrigeration principle as those above, except that the refrigerant – or in some cases, water – is pumped through a large loop of underground pipes – a “ground loop” – which are buried from 6 to 8 feet or more deep. At this depth, the temperature of the Earth remains constant around the calendar – about 55 degrees, regardless of where you are located.

When cooling, warm air from the house is transferred to the fluid in the interior coils, which leaves cooler air behind which is then sent through the air ducts. The warmed fluid is pumped out through the underground loop where it transfers its stored heat into the ground, then comes back into the house to repeat the process.

When heating, the process is somewhat reversed, as the fluid coming from the ground has absorbed the heat from the Earth and delivers it to the indoor unit. This equipment extracts the heat from the liquid, then sends it through the air ducts as warmed air. The fluid then returns to the outside pipes to absorb more heat from the ground.


  • Because of its efficiency, this type of AC (combined with winter heating) can save anywhere from 30-70% on utility bills when replacing a standard split system / AC and furnace combination.


  • Usually the most expensive type of unit to install, primarily because of the labor and materials involved in constructing the ground loop.
  • Repairs to the loop can be difficult and costly.

Cost: The average cost of installing a Geothermal AC system is $12,000 to $30,000, with most homeowners paying about $20,000. This is for a system designed for a 2,000 square foot home. Because of the specialized labor involved, few people will install their own geothermal system.

Maintenance Recommendations:

  • Change the air filter once a month.
  • Check condensation pans for puddled water. Make sure the drain hole is completely clear of any dirt or debris.

What Can Go Wrong?

  • Wet spots on the ground over the ground loop.
  • Unit not cooling (or heating) as it normally does.

The first problem may be a leak in the ground loop and requires immediate attention to prevent or decrease pollution from refrigerant into the soil. Any other problem should be referred to a trained technician for diagnosis and repair.

4. Ductless Mini-Split Air Conditioner

Mini Split Air Conditioners

The ductless mini-split AC has an outdoor compressor and condenser in a cabinet, but as the name implies, there is no inside ductwork involved. Instead, the indoor component with the evaporator and fan (sometimes called the “head”) is usually attached directly to the outside wall of a room. The head can also be placed on an interior room wall with some additional labor involved.

A major advantage of this type of unit is that AC can be readily added to an existing building with minimal construction cost. There can also be several indoor components connected to a single outdoor component, each one providing cooling for separate areas or rooms, each with its own temperature control located on its own remote control. These indoor units can be obtained in several sizes to cool rooms from 450 to 1,200 square feet of space. In terms of BTUs, they start as small as 5,000 and can be 48,000 BTUs or larger.

The ductless mini-split AC unit can also be obtained with an integral heat pump, if heat is also needed.


  • This is a good way to add heating and air conditioning to an existing home, since no ductwork is required.
  • Many times, these units can be installed in just a few hours with just a small hole in the outside wall for the refrigerant and electric lines.
  • DIY ductless systems are available that do not require a refrigerant license. They come pre-charged.


  • The total cost of providing cooling for an entire home with these units will most often be significantly higher than a standard air conditioning system – perhaps as much as 30% higher. This is especially true when the system is zoned – multiple indoor units per each outdoor unit.

Cost: $1,650 to $9,500 is the typical range, depending on size, efficiency rating, configuration of home, number of zones, etc. The average cost of installing a mini split AC system with one or two indoor units is $5470.

Maintenance Recommendations:

  • Filters. Check and clean every 4-6 weeks.
  • Condenser coils (outside unit) must be clean and free of any dirt or debris. Make sure there is adequate space around this cabinet to allow free air flow.
  • Evaporator coils and blower wheel (inside unit) must be checked for mildew and/or mold at least quarterly. Clean if necessary.

What Can Go Wrong?

  • Refrigerant Leak.
  • Compressor/Condenser Failure
  • Clogged Condensate Line(s)

If you have followed the Maintenance Recommendations, a trained technician should be consulted to diagnose and fix these problems.

5. Window Air Conditioner

Window Air Conditioner Works

These units are one of the most popular types of air conditioners purchased and installed by homeowners and renters. They are a single unit with all three major components inside one case. This is installed by placing it on an open window, with the majority being outside the building supported by a shelf or bracket. The unit is simply plugged into an electric outlet inside the room.

A few manufacturers are also making units that either hang over the sill or are shaped like a “U”, so that the window can be closed except for a few inches, rather than the usual 12-18 inches.

Some of these compact units come with a dehumidifying function, remote controls and Wi-Fi capability.

Window air conditioners are manufactured that will produce between 5,000 and 15,000 BTU’s, or enough to cool from a little over 100 to approximately 600 square feet of space if the air can freely circulate throughout that area. They are normally considered a one-room AC unit.


  •  These are the least expensive AC units which use the refrigeration method of cooling. Can easily be installed in a bedroom or other room needing cooling.


  • Most can only be installed in a window that opens from the bottom up. In homes with windows that slide sideways, they usually will not fit the window opening, nor can a good weather seal be obtained. However, we’ve compiled the Best Vertical Window ACs for Sliding/Casement Windows.
  •  Support outside the window must be adequate to hold the unit in place. Care must also be taken to have it tilted slightly outward so that water condensed on the condenser coil can drain away.
  • Because the unit is situated in an open window, the open space on the sides of the unit must be filled in.

Cost: $140 to $700+ depending on size and efficiency rating. It is assumed that the purchaser would install the window AC. Installation by a professional would add to this cost.

Maintenance Recommendations:

  • Window Seal. The seal on each side of the unit should be such that it does not allow insects or an appreciable amount of air to enter the room from outdoors. These may need to be replaced each year as the elements will take a toll on them.
  • Filter. If this is dirty, it can cause several of the problems listed below. Check the filter once a month when the unit is in use. Clean it with a soft brush. Replace if it shows signs of wear.
  • As winter approaches, the window air conditioner should at least be covered to protect it from harsh weather, blowing leaves, etc. In very cold climates, it may be best to remove the window unit to both protect it from freezing and provide a better seal with the window completely closed.

What Can Go Wrong?

  • Water dripping from the front – inside the room.
  • Ice formation inside the unit – on the coils,
  • The unit will not start (and, yes – it is plugged in!).
  • The unit shuts off before it’s cool in the room.
  • The unit isn’t putting out cold air.

The most likely cause of the first issue is that the unit is tilted inward and the water condensed from the cooling process is running in rather than out. Reinstall the window unit to have a slight outward tilt.

If you have followed the Maintenance Recommendations, the remaining problems will need to be addressed by a trained technician.

6. Through the Wall Air Conditioner

Through The Wall Air Conditioner

As the name implies, these units are installed in the outer wall of a house or business. Very similar to the Window AC, they require a special “sleeve” which must be installed through the wall. This sleeve holds the unit in place and also has ventilation louvers on the outside to allow the warm air to be expelled out of the building. They are usually found in sizes ranging from 8,000 to 15,000 BTUs and are considered adequate for one room or possibly a small apartment.


  • Because the unit rests in a metal sleeve, no additional outside support is needed as with the window AC unit. The seal around the unit will also usually be fairly airtight.
  • Because they are mounted in the wall, there is no loss of the use of a window as in the case of a window AC.
  • They require no special wiring and can be plugged into a standard wall outlet.


  • A permanent hole must be cut in the outside wall of the building which will be larger than the space between framing studs. This means that consideration must be given to ensure that proper support of the wall is maintained.
  • When installing, the unit must tip outward slightly to drain away water accumulated in the cooling process.

Cost: $450 to $800 depending on size and efficiency rating. If installed by a professional, figure on adding approximately 30%.

Maintenance Recommendations:

  • Air Filter. Check monthly and clean or replace if needed.
  • Condenser coils. Carefully clean if needed. Also clean any dust, dirt, dead insects, etc. inside the unit.
  • Clean the water pan and make sure the drain hole and line are clear.

What Can Go Wrong?

  • Water dripping from the front – inside the room.
  • Ice formation inside the unit – on the coils,
  • The unit will not start.
  • The unit shuts off before it’s cool in the room.
  • The unit isn’t putting out cold air.

If water is draining into the room, the unit does not have enough tilt toward the outside wall of the home. The installation must be revised so that water drains out rather than in. If you have performed the Maintenance Recommendations, the rest of the problems will need to be addressed by a trained technician.

7. Packaged Terminal Air Conditioner (PTAC)

packaged terminal ac

This is the type of AC unit (often also equipped with heating capability) that is commonly used in motels, nursing homes, and other large buildings with many rooms that require individual temperature control. Normally installed beneath a window, all components are “packaged” into one unit that is inside the room. There is a large grille on the outside where the warm air is expelled. Many PTACs can be switched between cooling and heating.

In your home or office building, a PTAC might be a viable option for a room that gets significantly warmer or cooler than the rest of the building. They normally produce between 8,000 to 15,000 BTUs for cooling from 400 to 700 square feet.


  • This type of unit can be the answer if you need a ductless, all-in-one cooling and/or heating system for an add-on room, sunroom or similar situation.


  • A large opening must be cut into the outside wall which requires consideration of supporting framing.
  • PTACs also require a 230 Volt electric circuit, and cannot be simply plugged into a wall outlet.
  • The efficiency level is not as high as many other types of AC units.

Cost: $900 to $1,700 including required accessories. The additional cost of having a PTAC installed will vary greatly, depending on size of unit, wall construction, obstacles inside the wall, etc.

Maintenance Recommendations:

  • Air Filters. There are usually two. Clean and inspect monthly.
  • Clear space in front. Do not allow furniture to impede air flow.
  • Any other maintenance on these units requires the removal of the unit from the wall.

What Can Go Wrong?

  • Water leaking inside the room.
  • Not cooling properly.

If water is leaking into the room, the unit must be reinstalled so that it tilts slightly outward, allowing condensed water to drain outside the building.

If you have followed the Maintenance Recommendations, a trained technician should be consulted for any other problems.

8. Portable Room Air Conditioner

The portable room air conditioner is a unit that sits on the floor of a room and has all of its components contained inside a free-standing case. They are relatively small, and often have wheels to facilitate moving from one room to another. 

Room air is drawn into the unit and heat is extracted across the condenser coil as in the normal refrigeration method. The resulting hot air is blown out of the room through a large hose (similar to that of a clothes dryer) which goes through an outside window with a special window kit that usually comes with the unit.


  • Small and relatively light, they can be moved around as desired.
  • They use a standard 115V electric circuit – a wall outlet.


  • Often less efficient than other types of AC for reasons explained in our Portable AC Buying Guide.
  • Requires a nearby window for the removal of hot air.
  • Requires regular removal of water condensed from the air.
  • Can be somewhat noisy, especially when the fan is on high.

Cost: $275 to $650 depending on size and efficiency rating.

Maintenance Recommendations:

  • Air Filter. Check monthly. Clean and replace as needed.
  • Drain pan or bucket. Empty as needed.

What Can Go Wrong?

If you have followed the Maintenance Recommendations, check owner’s manual or contact the manufacturer for instructions concerning malfunctions.

Types of AC Condenser Units – Evaporation Method

Technically, these units do not have condenser units, since they don’t use refrigerant. 

1. Portable Evaporative Air Conditioner – also called a Swamp Cooler

The name of this type of AC is derived from the fact that it uses water for cooling rather than a refrigerant in a sealed system. The water saturates a special pad or pads over which air drawn from the room is moved by a fan. In doing this, some of the water evaporates, lowering the temperature of the air which then goes back into the room.

Portable Evaporative AC units can be found in very small “desktop” sizes, with a cooling capacity as little as 45 square feet, to very large sizes to cool as much as 3,000 square feet. Smaller units will have a water reservoir which is refilled manually, while larger models may be connected to a supply hose and sink drain.


  • These are usually the least expensive of all types of air conditioners to buy.
  •  They operate on much less energy than refrigeration-type AC units.


  • They are only efficient in low humidity areas. Dry air is required to make the evaporative process work well.
  • Even in a low humidity environment, a portable evaporative AC unit will raise the humidity inside a room excessively unless there is outside air coming in. This can cause health problems and functional problems with electronic devices nearby.

Cost: From $50 for a “desktop” unit to $350 for a one-room “tower” unit. The largest portable evaporative AC units cost upwards of $2,000. If you’re interested, we’ve compiled a current list of the best evaporative coolers available.

Maintenance Recommendations:

  • Cooling pads. Replace at least every 6 months – more often if your water is high in mineral content.
  • Water tank/reservoir. Clean and ensure that the drain line is open regularly.
  •  Keep water at a proper level.

What Can Go Wrong?

  • Water is leaking on floor. Check drain plug in tank or drain line for leaks.
  • Unit not cooling properly. Check pads for scale buildup.
  • Water level too high or low. Check and adjust float as needed.

2. Whole-House Evaporative Air Conditioner

In the arid western part of the U.S. (especially the Southwest), large, “whole house” evaporative coolers are found on the rooftops of many homes. These, of course, have a water line connected to them to give a continual supply of water. They are connected to ductwork inside the house to distribute the cooled air throughout.

A major consideration when thinking of purchasing an evaporative cooling system is that they are only effective when used where the relative air humidity is less than 50%.


  • When used in low humidity areas, evaporative systems are very energy efficient and quieter than a refrigerant type system. This will result in a lower electric cost than a unit using the refrigeration method.


  • They cannot be used in high humidity areas and require a constant supply of water.
  • They require maintenance at least twice a year, and must be shut down and drained during cold months.
  •  The evaporation method cannot lower the room temperature as much as a standard AC system can.

Cost: $350 to $2,000 depending on size. Additional materials such as ducting, vents and labor to install will increase this price by $400 to $900.

Maintenance Recommendations:

  • Cooling pads. Change at the beginning and in the middle of the cooling season.
  • Use water treatment tablets if your water is high in mineral levels.

What Can Go Wrong?

  • Not cooling properly.
  • “Musty” odor coming through the air.
  • Unit is corroded.

Almost all problems with swamp coolers are related to either water levels, water quality or the effect that water has on the pads and associated parts of the unit. Following the Maintenance Recommendations faithfully will eliminate most problems.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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