Air Conditioning

Credit: This resource is a revised version of the fact sheet UF/IFAS – Energy Efficient Homes: Air Conditioning (EDIS-FCS3262)[1], by Wendell A. Porter, Craig Miller, and Hyun-Jeong Lee[2].

Quick Facts

  • Heating, ventilation, and air conditioning can account for more than 40% of your utility bill.
  • For every degree setting below 78°F, you can spend up to 8% more in cooling costs.
  • The average cost of electricity in Florida is 12.09 cents/kWh (EIA, 2015).
  • Upgrading from a 10-SEER to a 14-SEER system can reduce your air conditioning costs by over 25%.

Terms to Help Get You Started

  • Air handler: The indoor unit that moves the air through the heating/cooling system
  • BTU & kWh: British Thermal Unit & kilowatt-hours
  • Cooling load & Load calculation: Measurements that calculate what size system is appropriate for a particular structure given its square footage, ductwork analysis, insulation, windows, etc.
  • Condenser: The outdoor unit that keeps the refrigerant cool
  • Heat Pump: An air conditioner equipped with a valve that lets it “switch” between “cooling mode” and “heating mode”
  • HVAC: Heating, Ventilation, and Air Conditioning equipment
  • SEER: A measure of efficiency for air conditioning units; the higher the SEER number, the more energy efficient the unit is in cooling the air
  • SHR: A measure of efficiency for the ability of air conditioning systems to remove moisture or humidity; the higher the SHR number, the less capable the system is in removing humidity
  • Supply & Return: Supply registers and ducts bring conditioned air in; return registers and ducts draw air out to be reconditioned
  • Ton: A measurement of size used to determine cooling capacity
  • Ventilation: The natural or mechanical process of supplying conditioned or unconditioned air to, or removing such air from, any space

Why should I be concerned about the efficiency of my air conditioning system?

The largest consumer of energy in the typical Florida home is the heating, ventilation and air conditioning (HVAC) system, which can account for more than 40% of home energy use and, therefore, for more than 40% of your utility bill. Energy use by your HVAC system is affected by many factors such as insulation levels, system efficiency, shading on the home, quality and sealing of the windows and doors, design and integrity of the duct system, and, of course, how the system is used.

Because of the heat and humidity, most Florida residents today rely on air conditioning systems to maintain not only reasonable comfort levels, but lower humidity levels as well. The size, efficiency, and placement of an air conditioning system, therefore, are all important. Equally critical is the air conditioning contractor selected, particularly because the operating efficiency of a system relies on proper installation to achieve its performance rating. A skilled air conditioning contractor’s keys to obtaining the designed efficiency of a system in the field include: proper sizing of the system for the specific cooling load of the home; selection and proper installation of thermostats or controls; proper installation and commissioning of the system; and, if required, a duct system designed to deliver the correct amount of conditioned air to each space within the building; and sealing and insulating all ductwork.

Isn’t bigger better?

Sizing the air conditioner close to the actual load (cooling requirement) provides better humidity control and typically results in energy savings when compared to an oversized air conditioner with the same performance characteristics. Better humidity control results from a higher percentage of run time during which the coil is operating at its coldest temperature allowing more condensate to form and flow out of the system, while energy savings result from less start and stop operation and higher comfort levels due to better humidity control. Reducing the number of cycles may also result in longer equipment life. However, close-sizing also comes with requirements. Since there should not be large excess capacity on the hottest days, the system must be properly maintained—most importantly, keep it clean. Also, energy efficient features of the house that reduce air conditioning load such as proper sealing of ductwork, windows, insulation and sealing against infiltration must actually be done and done correctly.

To assure proper sizing, the load must be calculated—not just estimated based on square feet. Load calculations are based on the exact area, orientation, insulation levels, type of construction for each component of the building envelope, as well as the heat given off by the lights, people, and equipment inside the building, etc. The standard for residential air conditioning sizing is ACCA Manual J or a method based upon Manual J. (ACCA stands for Air Conditioning Contractors of America).

Make sure your contractor conducts comprehensive load calculations before accepting a bid. Central air conditioner and heat pump capacity is generally referred to in terms of tons. A ton of air conditioning is equal to 12,000 British Thermal Units (BTUs) per hour.

What exactly is SEER?

The cooling efficiency of a heat pump or an air conditioning system is rated by the Seasonal Energy Efficiency Ratio (SEER). The SEER is defined as a ratio of the average amount of cooling per unit of electricity used. The amended Federal Standard on residential air conditioners specifies that central air conditioners and central air conditioning heat pumps manufactured after January 1, 2015, will have not less than 14 SEER for split system heat pumps. Efficiencies of some systems can be as high as SEER 22 or more. It is important to understand that even though the SEER may be high, if the system is inefficient (for example, leaky ductwork) or oversized, you will not receive the full value of the efficiency as the air distribution system is not used to determine the SEER rating.

So a high SEER system will also take care of the humidity in my house, right?

Not necessarily. You also need to consider the Sensible Heat Ratio (SHR), which describes the moisture-removal capability of air conditioning systems. A SHR on HVAC equipment of 0.7 means that 70% of the air conditioning unit’s load is devoted to cooling, and 30% to removing humidity. It is critical that the HVAC contractor accurately estimate the humidity load (also referred to as latent load).

Outdoor air, coming in through poorly sealed windows and doors, open fireplace dampers, and bath and kitchen vents, causes most of the moisture load in your home. Even the simple act of opening and closing exterior doors adds humidity to your home. In addition, plants, bathing, cooking, cleaning, combustion, standing water in commodes and drains, and even breathing add to indoor humidity.

A proper Manual J load calculation yields both the latent (moisture) load and the temperature-based or sensible load. Typical dehumidification is performed by systems that use the same basic mechanics as air conditioners, and often air conditioners alone dehumidify the space. To keep humidity within comfort ranges, the building’s thermostats should have humidity sensors. If they do not, you can specify separate systems that can control humidity separately from the HVAC system. However, both comfort and energy efficiency will be better when humidity control is integrated with temperature control.

Mechanical dehumidification is not the most energy-effective means of dehumidifying, but it is the most common because it uses standard technologies. Mechanical dehumidifiers over-cool incoming air below the dew point (the point where it can no longer hold all the water vapor that was in solution). As a result, the water condenses on the cooling coils. Afterwards, the cold dry air is heated back up again to the desired temperature and/or mixed with untreated air to provide air at the desired temperature and humidity to occupied spaces. It is important that the HVAC contractor verify that the unit chosen can remove the calculated latent load from the structure. Keep in mind that with air conditioners, which operate based on room temperature, humidity may not be controlled directly, and any humidity control is a by-product of temperature control.

How does it all add up?

This is where it gets a bit complex. The SEER, the SHR, and the system tonnage must be in balance so difficulties don’t occur with indoor air quality. Systems without an adequate SHR, or with inaccurate tonnage, cool without removing moisture.

An over-sized air conditioner will cool your home too quickly to remove moisture very effectively. This results in a home that is cool but clammy. If systems are not providing sufficient dehumidification, the typical owner response is to lower the thermostat setting. Since every degree the thermostat is lowered actually increases cooling bills up to 8%, systems that have nominally high efficiencies but inadequate dehumidification may suffer from higher than expected cooling bills.

What are some short-term solutions to improve the efficiency of my existing system?

The U.S. Department of Energy suggests the following:

  • Set your thermostat at 78°F or higher. If your setting is currently more than one degree lower than this, consider adjusting the set point one degree higher at a time over a period of weeks and even months. This allows your body a chance to adjust to the new setting.
  • Use bath and kitchen fans sparingly when the air conditioner is operating.
  • Inspect and clean both the indoor and outdoor coils. The indoor coil in your air conditioner acts as a magnet for dust because it is constantly wetted during the cooling season. Dirt buildup on the indoor coil is the single most common cause of poor efficiency. The outdoor coil must also be checked periodically for dirt buildup and cleaned if necessary.
  • Check the refrigerant charge. The circulating fluid in your air conditioner is a special refrigerant gas put in when the system is installed. If the system is overcharged or undercharged with refrigerant, it will not work properly. You will need a service contractor to check the fluid and adjust it appropriately.
  • Reduce the cooling load by using cost-effective conservation measures. For example, effectively shade east and west windows. When possible, delay heat-generating activities, such as dishwashing, until the evening or early morning on hot days.
  • Over most of the cooling season, keep the house closed tight during the day. Don’t let in unwanted heat and humidity.
  • Try not to use a dehumidifier at the same time your air conditioner is operating. The dehumidifier will increase the cooling load and force the air conditioner to work harder.

In addition:

  • Consider installing ceiling fans to circulate the air more effectively. The improved circulation will make you feel cooler.
  • Install a programmable thermostat. Using a programmable thermostat, you can schedule the time blocks when the heating or air-conditioning system operates. As a result, you can set the air conditioner to more economical settings—such as higher temperatures when you are away from home. Choose one that can store and repeat multiple daily settings, so that you can have both a workday and a weekend timetable. A manual override feature is a great convenience, allowing you to override current settings without affecting the rest of the program. Before purchasing, make certain the thermostat is designed to operate with your system.
  • Consider ENERGY STAR® certified heating and cooling equipment. ENERGY STAR® certified central air conditioners have higher seasonal energy efficiency ratio (SEER) ratings, making them over 15% more efficient than conventional models.

When investing in an air conditioning system, what considerations should I keep in mind?

AC component Locations

Central HVAC systems have a component called an air handling unit or AHU (often referred to simply as the “air handler”). If you have the option, choose a conditioned space for placement of the air handler. The advantages of placing the AHU in conditioned space include the following: it is in a more favorable environment; a central location can minimize duct lengths and optimize air flow; there is easier access for maintenance; and any air leaks occur in conditioned space.

Another often ill-considered area of installation concerns the placement of the outside unit (condenser). Manufacturers’ recommendations for proper clearance distances should be followed to the letter to ensure there is no blockage of air flow from the unit. Also, do not vent a clothes dryer within 10 feet of the outdoor unit as dryer lint will cling to the condensing coil, lowering both the system’s efficiency and service life.

Keep in mind that the major components of the system, such as the air handling unit and the condenser, are joined together for the first time at your home. The efficiency and reliability of the entire system are directly related to the care and quality of the work that goes into the planning and installation of the complete system, including the thermostat and duct system.


Digital thermostats (the ones with digital displays) are typically better at maintaining a constant temperature and a constant temperature differential compared to analog thermostats (the old fashioned kind with numbers on a dial). For a long time there were two types of analog thermostats being manufactured – the mercury bulb thermostats and the bi-metal thermostats. The mercury thermostats were more expensive and typically a bit more accurate IF they were properly leveled on the wall. These are now being phased out because they contain mercury. The digital models contain electronic sensors, i.e. thermistors, which respond faster to changes in temperature. Most of the digital models are also equipped with an electronic time delay mechanism to prevent the thermostat from accidentally turning the A/C system off and back on too quickly, which is very hard on the outside condenser unit.

Some of the digital models are also available in programmable versions. These models typically allow you to specify up to four times each 24 hour period you would like the thermostat to automatically change the temperature setting. Most programmable thermostats are either digital, electromechanical, or some mixture of the two. Digital thermostats offer the most features in terms of multiple setback settings, overrides, and adjustments for daylight savings time, but may be difficult for some people to program. Electromechanical systems often involve pegs or sliding bars and are relatively simple to program.

When programming your thermostat, consider when you normally go to sleep and wake up. Also consider the schedules of everyone in the household. If there is a time during the day when the house is unoccupied for four hours or more, it makes sense to adjust the temperature during those periods. The location of your thermostat can affect its performance and efficiency. Read the manufacturer’s installation instructions to prevent “ghost readings” or unnecessary furnace or air conditioner cycling. To operate properly, a thermostat must be on an interior wall away from direct sunlight, drafts, doorways, skylights, and windows. It should be located where natural room air currents–warm air rising, cool air sinking–occur. Furnishings will block natural air movement, so do not place items in front of or below your thermostat. Also make sure your thermostat is conveniently located for programming. Watch the ENERGY STAR® video for some helpful information in choosing and setting up a programmable thermostat.

The leading edge of thermostat technology is the “smart thermostat”. Smart thermostats provide the same functions as programmable thermostats but also have the ability to control home temperature from an Internet connected device, have monitoring systems to track energy use in real time, and can include occupancy detection sensors to allow a system to program itself. A smart thermostat is more expensive than a regular programmable thermostat but may fit the lifestyle of the homeowner that forgets to set a regular programmable thermostat. For more information see programmable thermostats.

What about Ductless, Mini-Split Systems?

Ductless, mini split-system air-conditioners and heat pumps (mini splits) have numerous potential applications in residential buildings. Common applications include installations in multifamily housing, room additions, and small apartments, where extending or installing distribution ductwork (for a central air-conditioner or heating system) is not feasible. Like central systems, mini splits have two main components: an outdoor compressor/condenser and an indoor air-handling unit. A conduit, which houses the power cable, refrigerant tubing, suction tubing, and a condensate drain, links the outdoor and indoor units. Many models can have as many as four indoor air-handling units (multi-splits) connected to one outdoor unit. The number of multi-splits depends on how much heating or cooling is required for the building or each zone (which in turn is affected by how well the building is insulated and air sealed). Each of the zones has its own thermostat, so you only need to condition occupied spaces.

Ductless mini-split systems are easier to install than some other types of space conditioning systems. For example, the hook-up between the outdoor and indoor units generally requires only a three-inch hole through a wall for the conduit. Most manufacturers of this type of system can provide a variety of lengths of connecting conduits, and, if necessary, you can locate the outdoor unit as far away as 50 feet from the indoor air handler. This makes it possible to cool rooms on the front side of a house, but locate the compressor/condenser in a more advantageous or inconspicuous place outside of the building.

Mini splits have no ducts, so they avoid the energy losses associated with the ductwork of central forced air systems. Duct losses can account for more than 30% of energy consumption for space conditioning, especially if the ducts are in an unconditioned space such as an attic. In comparison to other add-on systems, mini splits offer more interior design flexibility. The indoor air handlers can be suspended from a ceiling, mounted flush into a drop ceiling, or hung on a wall. Floor-standing models are also available. Most indoor units are about seven inches deep and have sleek, high tech-looking jackets. Many also offer a remote control to make it easier to turn the system on and off when it’s positioned high on a wall or suspended from a ceiling.

The cost of installing mini splits can be higher than some systems, although lower operating costs and, if available, rebates or other financial incentives can help offset the initial expense. The installer must correctly size each indoor unit and determine the best location for installation. Oversized or incorrectly located air handlers can result in short cycling, which wastes energy and does not provide proper temperature or humidity control. Too large a system is more expensive to buy and operate. Some homeowners may not like the appearance of the indoor part of the system. While less obtrusive than a window room air conditioner, these units don’t have the built-in look of a central system. Check with local heating and cooling contractors to find out how common these systems are in your area and who has experience installing and servicing them. Be sure to choose an ENERGY STAR® certified unit and hire an installer familiar with the product and its installation.

Questions the HVAC contractor should ask you

The HVAC contractor should ask you the following questions to help properly conduct a comfort analysis and system design for your family and home:

  • Would you like to change anything about your current air conditioning and/or heating system?
  • What do you like most about your present system?
  • What benefits do you expect from your new system?
  • Does your existing system heat and cool your home to your satisfaction?
  • Are there rooms that are too hot or too cold?
  • What temperature is your thermostat set on during the summer? Winter?
  • Do you have a scheduled lifestyle that encourages adjusting the thermostat frequently, or for stretches of time when you know the home will not be occupied?
  • Do you set the thermostat at different temperatures for the hours that you’re awake and the hours you’re asleep?
  • What types of heating or cooling problems have you experienced?
  • Have you had any problems with condensate drainage?
  • What is your average summer electric bill?
  • Who performs your regular energy-savings check-ups?
  • How often are the air filters changed and what type are you currently using?
  • How long do you plan on residing in this home?
  • Do you plan to remodel or expand your floor plan in the future?
  • Have you made any changes to your home since the existing air conditioning and/or heating system was originally installed?
  • How many people reside in your home?
  • Does anyone residing in your home have allergies?
  • Do you understand ratings like SEER and SHR?
  • Do you understand how HVAC systems work or, more specifically, do you understand how the system I’m recommending for your home works?

You should also realize that many of the same questions listed above should be asked when determining what HVAC system should go in a new home as the building plans are being drawn.

In addition:

  • Be sure your contractor is licensed, well trained, and experienced. Ask to see a valid contractor’s license (in Florida you can check to see if they are licensed by referring to the Florida DBPR website – click on “Verify a License,” then conduct your search), proof of coverage for workers compensation, and certificate of insurance coverage for liability and property damage. (Note: For definitions/descriptions of the different kinds of licenses, visit the construction industry licensing board’s website) Ask for proof that your contractor is certified to handle refrigerant in cooling systems. Also, inquire about references and membership in contractor associations.
  • Request a calculation of your savings. Heating and cooling equipment comes with three price tags: the cost to buy the equipment, the cost to repair and maintain, and the cost to operate. Your contractor should be able to calculate your utility bill savings and total lifetime costs.
  • Request a load calculation. Ask your contractor to calculate equipment size using computer software or professional guidelines. This will require taking measurements in your house and asking questions. Don’t use a contractor who wants to size your system solely on the square footage of your house or by the size of the system being replaced.
  • Inspect ducts. Ask your contractor to inspect your ducts for leaks, incomplete connections, and compatibility with the rest of your system. Evaluate your system’s performance. Ideally, your contractor should use diagnostic equipment, and, if necessary, fix leaks using a UL-rated quality duct sealant. In some cases, proper duct repairs may include actual duct modifications to ensure proper supply and return airflow.
  • Consider a house pressurization test. If you have any kind of fuel burning equipment (gas, wood, kerosene, propane, oil) in your home, test your house and appliances for “back drafting.” Back drafting occurs when the fumes from the combustion process are pulled back into the home, threatening the health and safety of occupants.
  • Replace both indoor and outdoor units. If you’re replacing an air conditioner or heat pump, be sure to replace both indoor and outdoor units for maximum efficiency and reliability if the units do not match in age, refrigerant, or efficiency.
  • Obtain a written contract. Always obtain a written contract or proposal before allowing your contractor to install a new system. Be sure to ask about warranties for labor and parts.
  • Weigh the costs. Remember that the lowest price may not always be the best price. Carefully evaluate a contractor’s proposal to ensure you get the equipment and service that best meets your needs. Paying slightly more now may get you better equipment and service, and save you money in the years to come due to lower costs of ownership. When looking for upfront financial incentives related to renewable energy and efficiency, visit the Database of State Incentives for Renewables & Efficiency (DSIRE) website and enter your zip code. You can also search, by zip code, for special offers and rebates as related to ENERGY STAR® certified air conditioners.
  • Install for easy maintenance. Make sure the inside coil can be reached for annual cleaning. The air filter(s) should also be easily accessible for cleaning and changing when dirty. Check filter(s) monthly during peak season.

ENERGY STAR® Product Finder

The table(s) below offer a real-time list of ENERGY STAR® Certified products related to this fact sheet. Using the vertical and horizontal scroll bars, you can look for specific manufacturer brand names, model numbers, and compare a variety of product specifications and energy performance metrics. Individual columns can be filtered using the column specific “Menu” icons adjacent to their “Information” icons. The entire dataset can be searched using the “Magnifying Glass” icon in the dark grey bar at the top of the table.

Certified Non-AHRI Central Air Conditioner Equipment and Air Source Heat Pump

Powered by Socrata

Certified Geothermal Heat Pumps

Powered by Socrata

Certified Room Air Conditioners

Powered by Socrata

References and Resources


[1]This document is FCS3262, one of an Energy Efficient Homes series of the Department of Family, Youth and Community Sciences, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. This material was initially prepared in June 2008 with the support of the Department of Environmental Protection, Florida Energy Office, which is now the Florida Department of Agriculture and Consumer Services, Office of Energy. Revised versions were prepared June 2012 and June 2015 with the support of the Florida Energy Systems Consortium. Any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the sponsoring organizations. Please visit the EDIS website.

[2]Wendell A. Porter, lecturer and P.E., Department of Agricultural and Biological Engineering, Craig Miller, senior associate in, Program for Resource Efficient Communities, and Hyun-Jeong Lee, former assistant professor, Department of Family, Youth and Community Sciences, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.

Leave a Reply