The Size of HVAC Units

Oversized

• An oversized HVAC unit may heat and cool the home faster than a unit that is the correct size for the house, but the oversized unit also has drawbacks. Running the larger unit increases the homeowner’s energy costs, and wastes electricity and natural gas. The larger HVAC unit also makes more noise, creating more inconvenience for homeowners. Oversized units also short cycle, or turn on and off more frequently instead of running for more prolonged periods of time. As the oversized HVAC system for a house short cycles, it runs less efficiently, which can also increase the humidity inside the house, leading to mold or mildew growth and condensation on the walls and ceilings, which may create water damage.

Local Climate

• The local climate where the HVAC system resides partly determines the size of the unit. An installer should look at the average temperatures for both winter and summer to determine the size of the HVAC system for the house. If the installer uses the highest high temperatures and the lowest low temperatures in a year’s time, the HVAC system will be too big for the house.

House Features

• Your home's features also affect just how large of an HVAC system is needed to properly heat and cool it. How your house sits in relation to the movement of the sun affects how the sun heats up your house during the different seasons. The size of your house’s exterior walls, as well as the size and type of windows installed in your house affect how well your house stays warm in the winter and cool in the summer. If your house has rooms that are joined by open doorways, this affects air flow. A house with vaulted ceilings has more air that needs to be heated and cooled, so it needs a larger HVAC system.

Sizing Systems

• An HVAC contractor may use one of several methods to calculate the proper size of an HVAC system for your house. Many organizations, such as the United States Department of Energy, agree that the Manual J and Manual D methods provide the most reliable and accurate results. Both Manual J and Manual D methods require the installer to input numerous points of data about your house in a software program. The program weighs the different factors and then recommends an optimally sized HVAC system. Other sizing methods such as calculating size based on square footage alone do not take all of the factors affecting the heating and cooling of a house into consideration.