How Large of a Central Air Unit Do I Need?

Whether you're thinking about buying a brand new central air conditioning unit or replacing your old one, there are a lot of things to consider, such as its efficiency and what kind of cooling capacity it should have. If you get a unit that is too small for the space you're trying to cool it will struggle and eventually freeze over. If you choose a unit that is too large for the space you're trying to cool, it will turn on and off too frequently and may not control humidity effectively enough. The size of your new central air conditioning unit is the single most important factor when you're looking for a new one.

The function of a central air conditioning unit is to cool an entire space evenly and effectively. Central air systems cool houses using a split system, with components located both indoors and outdoors. Bulky, noisy components of the central air conditioning system that produce heat, like the compressor and the condenser coil, are located outdoors, while the evaporator coil is located indoors. Two refrigeration lines and a low-voltage ready cable connect the indoor and outdoor components. The furnace's blower moves warm air over special coils and distributes chilled air indoors. If the building already has a forced air baseboard or radiant floor heat, the evaporator coil is located there. If not, the evaporator coil is mounted in a dedicated blower unit. Many buildings already have the proper duct work that is needed for central air conditioning systems. If not, central air is still possible without having to install special ducts. This is done often in Europe, and now many American companies provide trained contractors who can do this as well.

Most central air conditioning units manufactured today are either multispeed or scroll. Multispeed central air units can cool at a lower speed 80 percent of the time. Scroll central air units are new. They generate much less friction, which means they experience much less wear and tear, so that they last longer.

Air conditioning units are much more efficient today than they were just five or 10 years ago. Central air conditioning units are now being produced in accordance with a federal mandate that says they need to be as efficient as possible. The condenser units on central air conditioning systems are given Seasonal Energy Efficiency rations (SEER). The higher the SEER number, the more efficient the unit is. Central air units are sized in tons. As a general rule, you will need one ton per 500 square feet of space that you need to cool, but this varies. According to Flex.net, higher SEER number units cool fewer BTUs (British thermal units) per ton, so old methods of determining what size central air conditioning unit you need don't work anymore. It's best to have an HVAC professional come and figure out what size and efficiency unit you will need.

When an HVAC expert comes to your home to give you an estimated cost for installing your central air system, he will probably take a lot of notes and use a special software program to figure out what size, SEER number and quality of central air conditioning unit you need. This software is available to buy online at the HVAC website www.hvaccomputer.com, and even promotes a free trial. According to "Popular Mechanics," if an HVAC specialist gives you a cost estimate for a central air conditioning system based on the square footage of your space, she is probably assuming that you have standard 8-foot ceilings, that less than 20 percent of the windows in the building are on a south-facing wall and that they have blinds or drapes on them, and that the walls are properly insulated. Some of the factors an HVAC specialist might consider when he comes to the property are the age of the building, the number and quality of the windows, how well the building is insulated, the number of stories in the building, the size and layout of the building and local energy rates and regulations. You should also consider your electricity costs and your climate. All of these factors will help determine how much you will use your central air conditioning system. The more you will use it, the higher efficiency you will need.

The most important thing to consider when you're buying a new central air conditioning system is that the more you run a unit, the higher efficiency you will need. Higher efficiency units cost more up front (you can expect to pay about $200 for each step up in SEER number), but will pay off in the long run if you use them often. There are two quality levels per SEER number. Lower quality models are still fairly efficient, but higher quality models run quieter and last much longer. Many dealers also offer a better warranty with the higher quality units. The EPA is phasing out Freon in favor of R4-10A, which requires pressure up to 50 percent greater than the pressure Freon requires. This might make a brand new central air conditioning system more expensive, but it will also be more energy efficient, meaning that it will cost less to run. Maintenance is critical on and central air conditioning unit. Make sure that you are having it checked, especially when something doesn't sound or feel quite right. The cost of having someone check and maybe do small repairs is much lower than replacing the unit altogether.