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Size and Occupancy: Determining Your HVAC Needs

HVAC Needs

The two major contributors to your homeandrsquo;s HVAC needs include its size and how many people occupy the house on a routine basis. Within those two parameters, a cluster of variables influence how large the system should be for dependable comfort and energy efficient operation.


HVAC contractors use software called Manual J to run the sizing calculation. It takes into account the factors that influence how large the system needs to be based on the cubic footage of the structure that include its:

  • Insulation levels.A home with enough insulation in the attic and exterior walls resists temperature change for longer periods. The U.S. Department of Energy recommends that homes in northern California have at least 10 inches in the attic, and 20 inches for the optimal level. Insulation blocks heat transfer. In the summer, it keeps the cooler air indoors, and in the winter, it retains heat inside. A poorly insulated wall will gain and lose heat quickly.
  • Air infiltration rates.The most accurate way to assess air leakage in a home is by having an energy audit. HVAC and auditing professionals use calibrated equipment to measure infiltration that they factor into the Manual J calculation.
  • Windows and their efficiency.If your home has single-pane windows mounted in aluminum frames, your HVAC needs skyrocket. Neither glass nor aluminum has any ability to resist temperature change.


Family size and their ages make a fundamental difference in the size of the system you need because of body heat. More people indoors translates to more heat. How often you use heat-producing appliances also makes a difference, like cooking and doing laundry.

Some people prefer it cooler in the summer and warmer in the winter, while others stick with moderate settings to save energy. Which you prefer makes a difference in the size of the system you need.

Identifying your homeandrsquo;s HVAC needs is a complicated and necessary exercise. Visit our website to learn more about most HVAC topics, or give us a call at (408) 868-5500. Valley Heating, Cooling, Electrical services homeowners in Santa Cruz and the South Bay Peninsula.

Our goal is to help educate our customers in the South Bay Peninsula and Santa Cruz areas of California about energy and home comfort issues (specific to HVAC systems).

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