Compare Central Air-Conditioning Units and Size Ductless Air Conditioners

Central air-conditioning units can bring a new level of comfort to your home. If you’re looking for a new AC unit, you’ll find energy efficient models that will cost less on your utility bill every month. Buying the biggest and most efficient model usually isn’t the right move because those arent the only two factors to consider when you compare central air-conditioning units.

How to Compare Central Air-Conditioning Units

The best way to shop is to determine how long it will take to recover the amount you spend on the AC in energy savings. Price indoor and outdoor components when you shop for central air-conditioners so you know future replacement costs. The compressor and the condenser coil are outdoor components, and the evaporator coil is an indoor component.

At some point, you will need to replace some of these parts. If possible, you will want to replace all the parts at the same time. Manufacturers design their parts to work together, and using new parts combined with old parts could mean you don’t get as much efficiency.

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Look at the SEER, or Seasonal Energy Efficiency Rating, of each air-conditioner and the Energy Guide rating listed on the yellow tag on the appliance to compare energy costs. Compare similar models with the same features you want. The Consortium for Energy Efficiency see Resources will also help you compare energy efficient models.

Add the total annual savings to determine how quickly you will recover the cost of a new AC unit. If you are replacing an old AC unit, use the yellow Energy Guide rating to determine the average annual energy costs for the new air conditioner. Then compare it to your current utility bill.

Determine your expected monthly savings, then divide that into the total cost of the air conditioner. Some high-efficiency air-conditioning units cost so much that you will never see a return on your money. Hire a licensed HVAC contractor to measure the size of your house, how many levels it has, the quality of your windows and your insulation, and your local energy rates to determine which size air-conditioner is right for your house.

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Ask about installation costs up front, what contractors include in the installation price and if the price varies depending on the model of AC unit. For example, ask if the price includes new wiring or pouring a new concrete slab outside for the unit to sit on. Whether you have existing ductwork will change the price significantly.

How to Size Ductless Air Conditioners

A large room needs a higher-BTU system for proper cooling than a smaller one. Measure the length and width of all rooms that you will connect to the ductless system. Multiply the two measurements to find the area of each room.

Do not include closets when calculating the area of a room. If the room is not perfectly rectangular, measure the length and width of any alcoves separately, multiply these measurements and add them to the first measurement. Convert the square footage to total BTUs.

Rooms with an area of 100 to 150 square feet call for 5,000 BTUs. Rooms of 150 to 250 square feet call for 6,000 BTUs. Rooms of 250 to 350 square feet need 7,000 BTUs.

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Multiply rooms larger than 350 square feet by 25 to determine the approximate BTUs for the room. Add 4,000 BTUs for kitchens and 1,000 BTUs for bathrooms. Add the total BTUs to determine the whole house BTUs.

Split-systems are mostly ranked by total BTUs. Some units are ranked by tons. Divide the total BTUs by 12,000 to calculate the tonnage for the house.


Ductless air conditioners, or mini-split AC units, use similar technology to that of standard central heat and air systems. The principal difference is that refrigerant is piped directly into fan units inside the room through small pipes, making duct work unnecessary. Sizing a ductless system requires measuring the square footage of a room and converting the measurement to British thermaul units BTUs, a measure of energy.

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