Getting Your Air Conditioning Right

In order to know the exact Btu heat-loss/heat-gain of your house, a Manual J load calculation must be performed. All rooms, windows, doors, materials, and local climate have a say in the final size of your system. The most common sizing mistake is over-sizing. 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 often creates uncomfortable and large temperature swings in the house. Oversized air conditioners (and heat pumps) do not run long enough to dehumidify the air, which results in the “clammy” feeling and unhealthy mold growth in many air-conditioned houses.

Older central air conditioning systems (more than 10 years old) are often unreliable and much less efficient than a modern system. When it’s time for a new replacement, 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.

It is the installer/contractor’s job to perform the correct sizing calculation for the building. However, many installers only check the “nameplate” (the label on the unit that has the Btu per hour output among other things) of the existing system and sell you one just like it, or even worse, one that’s larger. This is not a correct sizing method and not in your best interest! Other methods include simple “rules of thumb” based on the size of your home or using a chart that accounts for a variety of factors. While these methods might provide a first estimate, they should not be used to size your system. Homeowners should insist that contractors use a correct sizing calculation before signing a contract. This service is often offered at little or no cost to homeowners by gas and electric utilities, major heating equipment manufacturers, and conscientious heating and air conditioning contractors.