Alan McIntosh  HVAC Consulting Services

Sizing Air Conditioning Systems

Air conditioning cooling equipment is rated by the number of British Thermal Units (BTU) of heat they can remove per hour (BTUH). Another common rating term of air conditioning size is the "Ton", which is 12,000 BTU per hour.

What size should your air conditioning system be?

The size of your air conditioning system depends on many factors:

• Locality outdoor and indoor design temperatures.
• Orientation of the home.
• Quantity of conditioned floor area and volume of space.
• Building materials.
• Window or glass areas.
• Conduction of building materials and assemblies.
• Ceiling heights.
• Insulation.
• Air leakage into the home from outside.
• Fresh air requirements.
• Duct loss.
• Number of occupants.
• Equipment and appliance loads.
• Use of space.

An air conditioning system efficiency, performance, durability and initial cost depend upon matching its size to the above factors. Be aware that an oversized air conditioning system  may not provide the best heating or cooling. Buying and oversized air conditioning system may penalize you in the following ways:

• Initially cost more to buy.
• May use more electricity and create higher demands on electrical generation and delivery systems.
• The larger-than-necessary air conditioning system will cycle on and off more frequently, reducing its overall efficiency. 
• Frequent cycling makes for more temperature fluctuation and results in a less comfortable spaces.
• Frequent cycling wears out the compressor  and electrical parts more rapidly.

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Last modified: 03/23/11