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Understanding How to Size an Air Conditioner System

Bigger Ac isn't always better

An improperly sized air conditioning system can lead to all sorts of problems for homeowners that may range from excessive indoor humidity to inflated utility bills. The US Department of Energy recommends replacing systems that are older than 10 years, so it’s important that homeowners understand the sizing requirements that go along with a new AC installation. In order to receive the highest level of efficiency and preserve the lifetime value of the system, there are a number of factors buyers should consider.

Why bigger isn’t always better

In Florida, humidity can make life outside nearly unbearable. On our hottest days, it’s as if stepping outside is equivalent to being wrapped in a warm, wet blanket. One of the primary benefits a properly sized AC unit will provide is to draw the humidity out of the air in your home. However, an oversized unit will not cycle on long enough to pull the humidity from the air and your home will begin to feel damp and clammy. Homeowners who have received an oversized AC unit may notice extra condensation building on their doors and windows.

The Manual J calculation

Your home is susceptible to a number of different factors that will impact your system’s ability to cool or heat efficiently. The Air Conditioning Contractors of America (ACCA) has developed Manual J to help HVAC professionals properly size an AC system and account for a variety of issues. A contractor who understands the Manual J, or residential load, calculation will account for insulation levels; orientation of the home; size, type, and location of windows; number of occupants in the home; local climate; and estimate air infiltration and leakage.

Buyer beware: Inaccurate methods for sizing

A unit that is improperly sized is usually the result of a poorly trained installer or one who looking to increase their own profit margins. Beware of these misguided methods for sizing an air conditioner system:

  1. The contractor only looks at the existing system and recommends replacing the unit with the same size or something larger.
  2. Prepared charts that use over simplified calculations and generalizations. Your home is unique, so make sure the contractor takes all of these features into account.
  3. You are only asked for your home’s square footage. This method only takes into account the size of the home and does not consider any of the other factors impacting your home’s comfort (i.e. windows, insulation, orientation, etc.).

Our team of air conditioning technicians, installers, and professionals understand the best methods and practices for sizing your new AC system. For questions regarding the proper size of your next system, call our Port St. Lucie team at (772) 398-0023 or our Martin County office at (772) 220-1496.


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