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Sizing Heating and Cooling Systems

Heating and cooling systems that are more than 10 years old are often unreliable and less efficient than a modern system. When it is time to replace your system, choosing a unit with the correct heating and/or cooling output is critical. This is because equipment that is properly sized for your home means that you are getting the most efficient and comfortable units. These newer units will also have lower operating costs over the lifetime of the system, and will require less maintenance. National surveys have determined that more than half of all HVAC contractors are not properly sizing heating and cooling systems. However, Bay Area Services uses precise calculations to determine the properly sized equipment for your Wisconsin home.

The most common mistake when sizing a heating or cooling system is oversizing. This means that not only is your new system is more expensive to install, but is also being forced to operate inefficiently, and this could cause it to break down more often and cost more to operate. Oversized heating equipment can also create large and uncomfortable temperature differences throughout your home. Oversized air conditioners (and heat pumps) are not running long enough to dehumidify the air, and this results in a clammy feeling throughout your home, as well as the chance of unhealthy mold growth.

Incorrect Sizing Methods

While it is the consultant’s job to perform the correct sizing calculation for the building, many other company’s salespeople are only checking the nameplate (the label on the unit that has the BTU per hour output) of the existing system, then they sell you the same unit, or a larger one when it’s not needed. Another improper, but common, sizing method is using “rules of thumb” which are based on the size of your home, and use a chart that accounts for a variety of factors. While these methods may be able to provide you with a first, and tentative estimate; they are not the correct sizing methods and should never be used to size your system.

Why Most Older Systems are Oversized

Before the era of tightly constructed homes, it was not uncommon to install furnaces and air conditioners that had 2-4 times the necessary capacity. Nowadays, many people have added new windows, caulking, weather-stripping, and insulation to their homes, and so going by the nameplate of a system when considering replacement is likely to result in an oversized unit. Making improvements to your home can help to reduce heat loss in the winter and heat gain during the summer, and it should allow you to install smaller, more energy efficient systems while still maintaining your desired comfort level.

Manual J and Manual D: The Correct Way to Size a System

Correctly sizing a system for your home requires considering many factors other than reading the nameplate of your existing unit. Other important factors to take into consideration include:

  • The local climate
  • The size, shape, and orientation (direction) of your home
  • Insulation levels
  • The location, size, and type of windows in your home
  • Air infiltration rates
  • The number and age of the occupants of the home
  • Your own personal comfort preferences
  • The types and efficiency of lights and major appliances in your home because they give off heat

Homeowners should insist that their contractors use a correct sizing calculation before they sign a contract. This service is often offered to customers by gas and electric utilities, major heating equipment manufacturers, and conscientious heating and cooling contractors. The recommended sizing method for use in the United States is Manual J “Residential Load Calculation” which was published by the Air Conditioning Contractors of America (ACCA). There are many user-friendly computer software packages, or worksheets that can help to simplify the calculation procedure. Bay Area Services performs a Manual J heat loss and heat gain calculation on every home that our consultants visit when considering replacement of the existing heating or cooling equipment.

When installing new ductwork, or making ductwork modifications in your home, sizing should be done using the ACCA’s Manual D “Residential Duct Design”. Our consultants at Bay Area Services use this method whenever ductwork is being modified to allow for enhanced airflow and maximum efficiency.