Sizing Your AC System

Properly Sizing your AC System

Older Air Conditioning and Heating systems (10 years+) are usually unreliable and much less efficient than a modern system.

When it’s time for a replacement, selecting one of the proper size is crucial to getting the best efficiency, comfort, and lowest maintenance and operating costs throughout the life of the system. The most common sizing mistake is Oversizing.

What Oversizing Means to you:

  • Makes the new system more expensive to install
  • Forces the system to operate inefficiently
  • Breaks down more often, and cost more to operate.
  • Oversized heating equipment often creates uncomfortable temperature swings in the house.
  • Oversized air conditioners & 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.

Incorrect Sizing Methods

It is the installer’s responsability 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 a not a correct sizing method. 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.

Most Older Systems are Oversized

Before the era of tightly constructed homes, it was not uncommon to install furnaces and air conditioning systems 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 Air Conditioning and Heating 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 unit. 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 packages or worksheets that can simplify the calculation procedure. You should make sure that the procedure used by the contractor follows Manual J.

If ducts are part of the installation, they should be sized using the ACCA’s Manual D, “Residential Duct Design.” The ACCA also offers a comprehensive guide for choosing home heating and cooling systems, called Manual S, “Residential Equipment Selection.”

A Special Case: Sizing Steam Heating Systems

One exception to the above is in steam heating systems. For these systems, the boiler should be sized to match the radiators. However, there is still room for energy savings. First of all, the original boiler may be oversized for the radiators, so the contractor shouldn’t just order the same capacity boiler, but instead should match the boiler to the radiators. Second, if you’ve increased the energy efficiency of your home, it may have more radiators than it needs.

It may be possible to remove radiators in the core of the house and shift the others around, replacing larger radiators with smaller ones. Since radiators are modular, it is theoretically possible to downsize a radiator by removing sections; in practice, this is usually difficult to do without damaging them. In many parts of the country, used radiators are available cheaply, so you can potentially buy small radiators to replace large radiators; if you do so, be prepared to replace the shutoff valves as well, since they often won’t match. Newly manufactured steam radiators are available as well.

In any case, you should work with a heating and cooling professional when downsizing your system. Your house’s heating needs should be calculated using Manual J, and your radiators should be downsized appropriately. Match the new boiler to the remaining radiators. Note that balancing steam heating systems is more an art than a science; ideally, you will find a heating professional with experience in steam heating systems.

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