The Importance of Properly Sizing Heating and Air Conditioning Equipment

Incentives Available! For many homeowners, selecting and investing in a new heating and/or air conditioning system can be an overwhelming experience. There are many choices to be made which a thorough heating & air conditioning contractor will help you to understand such as the type of system, the efficiency level and accessory options. And while proper sizing of equipment is not one of the choices that you will make, it is one of the most crucial yet often overlooked, aspects of designing a suitable system for your home. The information provided in this page will give you a basic understanding of what to expect from your contractor.You might be surprised to know that although the HVAC industry has a standardized process and calculation for properly sizing equipment, many contractors use inaccurate methods such as sizing based on the square footage of your home, the size of your previous equipment or worse, rules of thumb. Improper sizing is one of the most common mistakes made and will lower the efficiency of your equipment and shorten its life expectancy. The following is an excerpt from the article "Sizing Heating and Cooling Systems" (2005) by the U.S. Department of Energy.Sizing Heating and Cooling Systems

Sizing Heating and Cooling Systems

Older space 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. 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 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 (see dehumidifying heat pipes as one solution to this problem).

Incorrect Sizing Methods

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 furnace 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 a not a correct sizing method and not in your best interests! 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.

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 two to four times the necessary capacity. Since many people have added new windows, caulking, weather-stripping, and insulation to their homes, going by the nameplate is likely to result in an oversized system. Making improvements such as these to reduce heat loss in the winter and heat gain in the summer should allow you to install a smaller systems while still being comfortable, as well as saving large amounts of energy.

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

Correct system sizing requires considering many factors other than simply reading the nameplate of the existing gas furnace, heat pump or air conditioner. Key factors for correctly sizing a heating and cooling system include the following:
  • The local climate
  • Size, shape, and orientation of the house
  • Insulation levels
  • Window area, location, and type
  • Air infiltration rates
  • The number and ages of occupants
  • Occupant comfort preferences
  • The types and efficiencies of lights and major home appliances (which give off heat).
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. Manual J, "Residential Load Calculation," published by the Air Conditioning Contractors of America (ACCA), is the recommended method for use in the United States. There are also many user-friendly computer software Sizing Heating & Cooling Systemspackages or worksheets that can simplify the calculation procedure. You should make sure that the procedure used by the contractor follows Manual J... U.S. Department of Energy, Energy Efficiency and Renewable Energy, Sizing Heating and Cooling Systems, 2005.For this article and more useful information please visit the U.S. Department of Energy online.

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