Air conditioning professionals mathematically determine the proper unit size. In the heat of the summer, one thing's sure to keep you cool and comfortable: a properly sized air conditioner. But buying an A/C unit can overwhelm you as you see all the available sizes, features and brands. The unit size is the most critical. An undersized unit will never cool your home enough, and an oversize unit is nearly as bad. Square Footage Estimations The simple way to calculate a home's air conditioner size is to use a rule of thumb. According to this common but somewhat inaccurate method, you need 1 ton of air cooling capacity for every so many square feet of living space. While there's some dispute over the exact amount, an often-used amount is 600 square feet. A 2.5-ton unit, then, theoretically would fit a 1,500-square-foot home. Sizing Inaccuracies One problem with estimating air conditioning size based on square footage is that every home is different. An average home in Florida, for instance, needs more cooling power than if it was situated in Michigan.

Given that even Energy Star provides a square-feet-to-BTU chart (with 12,000 BTUs equating to 1 ton), the estimation is valid for a homeowner to use -- but illegal for contractors. Never compare your home to a neighbor's similar-sized house. Manual J Calculations To obtain a precise calculation, you must consider your home's unique factors: What region is it in? How good is the insulation? What direction does the house face? Some websites use online calculators to enable you to perform a closer estimate. An HVAC professional can use the Manual J -- an engineering guide that precisely pinpoints the size you need. References Florida Solar Energy Center: How Contractors Really Size Air Conditioning SystemsEnergy Star: Properly Sized Room ACCentral Air Conditioner Prices: Cost Comparison of AC Brands, Sizes, Repair Costs, & High Efficiency Central AC Systems Photo Credits Jupiterimages/liquidlibrary/Getty Images Suggest a Correction Network with Industry Professionals Science and Technology for the Built Environment

Supplier Directory & Sponsored White Papers Citation and Abstract Services Publication Updates and Errata Standards, Research & Technology Standards Forms & Procedures Purchase Standards & Guidelines Resilience Activities in ASHRAE Advanced Energy Design Guides HVAC Design and Operation Training 2017 Las Vegas Winter Conference Courses ASHRAE Courses at Industry Events Self-Directed or Group Learning Rules of the Board ASHRAE Associate Society Alliance Young Engineers in ASHRAEResources & Publications | Choosing the Right System for Your Home Buyers Beware: Correctly Sized Air Conditioners Save Money! When you buy a new air conditioning or heating unit to replace your old one -- or if a completely new system is being installed in your new home -- the installer needs to know how big or, more importantly, how small the unit should be. In order to know exactly how big (or small) it should be, the contractor needs to calculate how much cooling and heating capacity it must have to keep the occupants comfortable.

This is called the load of the home. The load of the home is partly dependant upon the home’s square footage. However, a true load calculation goes beyond that. window ac unit for saleAn accurate load determination includes building construction; top 10 air conditioner unitsorientation to the sun; ac unit does not turn on“R” value of the insulation; number, size, and placement of rooms; number, size, and placement of windows and doors; types of windows and doors (thermal efficiency); number and arrangement of floors; Residential load calculations make use of mathematical formulas that take all these variables into consideration. They have been computerized, so they’re not as time-consuming as they were in the past. Determining the load by using rules of thumb almost always leads to an over-sized heating and cooling system, resulting in an increased initial cost, increased monthly utility bills, increased maintenance, and shortened equipment life because the equipment cycles off and on too frequently.

Who Does the Load Calculation? The contractor you choose to install your new system should be able to perform these calculations. You may ask him for this service. If he doesn’t want to do it, or says it isn’t necessary, you may want to look for a different contractor. In some areas, the local utility will perform a load calculation – check with your local utility. How Big Is Big? Cooling size is given in tons of cooling; 1 ton = 12,000 Btu/h. 'SEER' (Seasonal Energy Efficiency Ratio) indicates the efficiency of the air conditioning unit, not its capacity. Why it’s Important to Have the Right Sized System Having the wrong-size heating-cooling system can lead to multiple problems. Over-sizing your cooling system is not a good thing. Without getting into too much detail, your cooling load consists of two parts: We’ve all heard the saying, “It’s not the heat; A cooling system that is too big cools down the temperature very quickly, but it does not run long enough to remove sufficient moisture or “humidity” from the air.

The result is you feel cool but clammy. Many people then turn down the thermostat to make the air conditioner run longer, increasing the utility bill. In some cases, the insufficient moisture removal resulting from over-sized cooling equipment can lead to mold growth and other types of moisture-related damage. A correctly sized cooling system runs long enough to “wring” moisture out of the air. This allows you to be comfortable at a slightly higher thermostat setting and allows you to save money on your utility bill. It also takes care of the “mugginess” without making you feel too cold. Air conditioning systems are sized in “tons,” which is a measure of the rate at which they deliver cooling, not how much they weigh. One ton of cooling is equal to 12,000 Btu/h (British Thermal Units per hour), the rate of cooling required to freeze one ton of ice at 32°F in one day. It is better to have a system that supplies a little less than the required tonnage of cooling, rather than a system that supplies more than the required tonnage of cooling.