House & Home

Load Calculation is Step 1 for an A/C Install

Posted March 11, 2015
Updated March 30, 2015

When it's boiling hot outside, it's time to turn down the temperature inside. But anyone who has called an air conditioning repairman lately for a freon boost knows prices are up after the federal government banned production of the popular R-22 refrigerant.

Bigger is not always better, especially when it comes time for you to install new central air conditioning. Choose a system that is just the right size for your home and its particular characteristics by using a Manual J load calculation, which follows the guidelines of the ACCA (Air Conditioning Contractors of America Association), also known as the Indoor Environment & Energy Efficiency Association. This calculation will take into account both the architecture and the location of your house, as well as the optimal indoor temperature to keep you and your family comfortable during the summer months.

How to Accurately Calculate the Appropriate Central A/C System for Your Home?

FACT: In most states, a load calculation is required by building code before you install or replace central A/C; however, this is rarely checked up on. As a result, unless you're dealing with a very reliable HVAC contractor, the installer may skip this step and rely instead on a general rule of thumb in choosing the size of the equipment. Although in past years, a ratio of 1 ton of cooling equipment to every 400 feet of space was the standard, with today's better sealed and insulated houses, that has since been changed to 1 ton of cooling equipment per 600 feet. However, this formula may still be an overestimation -- the ratio may be closer to 1 ton: 1000 feet. Load calculation is a detailed, precise method of calculating the size of cooling system that is ideal for your home. This calculation used to have to be performed with painstaking care using pencil and paper; nowadays, specialized computer software makes it a much easier task.

Which Factors Play a Part in Load Calculation?

Every home is different. For this reason, A/C load calculation does not take into consideration only your property's total square footage. Other important factors include:

  • Your local climate and geographic latitude
  • The house's layout and age
  • Its orientation (that is, which direction it faces)
  • The number and location of windows, as well as their u-values and coverings
  • Your roof's material and condition
  • Any shade provided by overhangs or nearby trees
  • Appliances and light fixtures inside the home and how their degree of energy efficiency
  • The number of human and animal occupants (who all raise the ambient temperature with the body heat that they give off).

If you already have HVAC ductwork in place, its layout and efficiency will be noted. Any "green" updates you have made to help with energy conservation, such as insulation and sealing, will also be part of the calculation. Finally, you will need to decide on your desired indoor summer temperature.

What are the Advantages of Correctly Sized A/C?

There are many advantages to performing an accurate load calculation. First, it costs less to purchase and install or replace air conditioning that is correctly sized for your needs. Second, you will spend less on running the system. In addition, an overly large system may tend to short cycle (turn on, cool your home quickly, then turn off). This results in uneven cooling and reduced comfort levels and tends to wear out the A/C more rapidly. Short cycling may also prevent your HVAC's dehumidification feature from kicking in, leaving your home with uncomfortably high relative humidity in the warmer months.

Laura Firszt writes for

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