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Sizing Your HVAC System

Getting the right size air conditioner makes a huge difference in your home’s energy efficiency.

Older space conditioning systems (more than 10 years old) are often unreliable and much less efficient than a modern system. When it’s time to replace existing equipment, choosing one of the correct size (heating and/or cooling output) is critical to getting the best efficiency, comfort, and lowest maintenance and operating costs over the life of the new system. Some national surveys have determined that well over half of all HVAC contractors do not size heating and cooling systems correctly.

The most common sizing mistake is in oversizing. This not only makes the new system more expensive to install, but also forces it to operate inefficiently, break down more often, and cost more to operate. Oversized heating equipment also creates uncomfortable and large temperature swings throughout the house.

In addition, oversized air conditioners and heat pumps do not run long enough to dehumidify the air. If your AC or heat pump unit is too big, your house will get too cold very quickly. Because it cools your house so fast, it doesn’t circulate all the air, and therefore doesn’t get the moisture out of the air. The result is that you'll feel not just cold, but cold and clammy most of the time. This can also result in unhealthy mold growth in many air-conditioned houses.

Correctly sizing the HVAC System not only improves equipment performance, but can provide a healthier and safer atmosphere throughout the entire house. The following values are standard measurements the HVAC industry uses to identify equipment output. Heating and cooling equipment is rated by how many British Thermal Units (BTUs) of heat they remove or add per hour. They may also be rated in tons, where one ton equals 12,000 BTUs per hour.

Incorrect Sizing Methods

Neither "rules of thumb" nor your previous unit are sufficient to determine the unit that's right for your home; professional guidance is needed.

Correct Sizing Methods

Correct system sizing requires the consideration of many factors other than simply reading the nameplate of the existing unit. Homeowners should insist that contractors use a correct sizing calculation before signing a contract.

Steps to a good sizing

Many factors affect a home’s heating or cooling requirement. Your contractors must be diligent to determine it correctly, and you must be diligent to get proof that they did their homework.