Proper AC Sizing

Why an Oversized Air Conditioner or Heat Pump Is Bad News

proper ac sizingAn oversized air conditioner or heat pump doesn't mean cool comfort for you. It means higher first costs, higher electricity bills, and a home that's uncomfortable to be in.

Oversized air conditioners run up your utility bill. An oversized unit short-cycles (runs for shorter periods of time than it should), and uses more electricity than a properly sized unit due to the start and stop cycles. Air conditioners operate more efficiently the longer they run continuously, and should be sized so they don't turn on and off frequently on hot days. A properly sized system will run continuously to maintain 75° in your home when it is 95° outdoors.

An oversized air conditioner can cover up other problems. Ducts that are leaky or have restricted air flow, improper refrigerant charge, and dirty coils and filters can make an air conditioner work much harder than it should. An oversized unit may be able to put out cold air even when these problems exist. But these conditions can mean costly breakdowns and the spread of pollutants into your living space.

Oversized air conditioners cost more to buy and don't last as long. Don't pay extra for an air conditioner that's too big for your house. The larger system will cost you hundreds of dollars more and give you fewer years of service - and bad service besides.

An oversized air conditioner makes your house clammy. A unit that's too big can't dehumidify the air effectively because it runs for only short periods at a time. The indoor coil never gets cold enough to do its job of removing moisture from the air. An oversized unit will leave your home cold and clammy, especially in humid climates like ours.

Oversized air conditioners are noisy. The bigger the air conditioner the more air rushes through the ducts and grilles. Often the grilles are not designed for the larger unit, so the noise will get even louder as air tries to force its way through the small openings.

There is no happy medium with an oversized unit. It delivers noisy blasts of frigid air, then shuts off while moisture builds. On hot days, it will do this repeatedly, making the home more and more uncomfortable as the day passes.

Tips for Buying the Right Size Air Conditioner

  • Don't use a contractor who wants to size your unit solely on the square footage of your house or the system that was originally installed. Contractors should calculate how much cooling a home needs according to guidelines such as the Air Conditioning Contractor's of America's Manual J. To gather the information necessary, a contractor should spend about an hour poking around your house, taking measurements in each room and asking questions. He or she needs to measure floors, ceilings, and walls-including all the windows and the crawl space. Some other factors that go into the load calculation formula include the amount of insulation you have in your walls, floor and ceiling, indoor and outdoor temperatures, number of occupants, shading, siding, and roof color.

  • Request a copy of the cooling load calculation (or computer printout) before the work begins. Most HVAC Contractors will not release a copy of the document to you until you have committed in writing to have them do the work. The reason for this is that they are concerned the homeowner will give a copy to the low budget competitor who can't or won't do the load calculation but will use the original HVAC Contarctor's knowledge and hard work to properly size the equipment. Question the contractor if the calculations do not include at least all the items mentioned above, or if they require an explanation.

  • Don't be tempted by the lowest bid. You're not buying a TV or refrigerator that is built in a factory with tight quality control from beginning to end. The craftsmanship and knowledge of the installers is paramount to a properly installed and functioning system. Be willing to pay for the knowledge and the time the contractor must spend to do the right job.

  • Check for duct leaks. Be sure air flow is not restricted by ducts that are disconnected, crushed, or too small for the system. Ideally, the contractor should use diagnostic equipment to find leaks and then fix them with quality duct sealant (not duct tape). It doesn't make a lot of sense to buy a larger air conditioner to cool your attic or crawl space!

  • Buy a high efficiency unit. Central air conditioners and heat pumps are rated by SEER, or Seasonal Energy Efficiency Ratio. New systems are required by federal law to have a SEER of 13.00 or higher. Even though it will cost more, it is usually worth it to buy at least a 14.00 SEER unit because over time that initial investment will be paid back with energy savings. An easy way to identify 13.00 SEER and higher-rated units is to look for the EPA Energy Star logo. If you are replacing an existing air conditioner or heat pump, you must replace the whole unit, including the inside coil and the blower fan, to achieve the rated efficiency.

  • Install for ease of maintenance. Make sure the inside coil can be easily accessed for cleaning. The contractor may have to install an access panel, depending on the model. The coil should be inspected annually and cleaned as necessary but at least once every two years. The air filter should be located where it is easy to remove. Check it every month and change or clean it whenever it is dirty.

  • Place the outside unit on the north or east side of the house, out of direct sunlight unless you are near the ocean. If you are in close proximity to the ocean place the outdoor unit on the south or west side of the home to help protect it from the harsh coastal environment. Leave plenty of room for free airflow on all sides, and at least four feet at the top. Keep the area free of debris and shrubbery.

When the dog days of summer set in, you want your air conditioner to help you keep your cool.

But studies show that one-third to one-half of home air conditioners don't work the way they should because they are oversized!

Contractors generally size air conditioners at least a half-ton larger than necessary, and often oversize by a ton or more. This way they can avoid callbacks from customers because a bigger unit will almost always put out cold air.

And, besides, the bigger, the better.


Not in this case.

Bring Your Air Conditioning Costs Down to Size

You'll pay hundreds of dollars more for an oversized air conditioner. Then you'll pay higher electricity bills to run it because it is so inefficient.

But you'll lose your cool when you find out what a high price you're paying for being uncomfortable all summer.

Investing a little more in a good contractor who will take the time to properly size and install your system is a very wise investment, both for your comfort and for your pocketbook.

For even greater energy savings, ask for an Energy Star certified air conditioner or heat pump!

This brochure was reproduced using information from Home Energy magazine. No endorsement of Outer Banks Heating and Cooling by Home Energy Magazine is implied. This information is provided as a service to the consumer.