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Manuals J, D, and S: What You Should Know


If you're in the market for a new HVAC system, it's important to accurately size your new system to fit your home. Beware HVAC consultants who want to calculate the size of your system based on square footage alone, or who simply plan to replace your old system with the same size. The fact is, many HVAC systems are not the right size for the homes they serve. That's where Manuals J, D, and S come into play.

Cooling/Heating Loads are Key

To accurately estimate the size HVAC equipment your home requires, your consultant will need to know the home's cooling and heating loads, or how much energy it takes to heat or cool your home. Many things affect cooling and heating loads, including the number of occupants in the home, number and size of windows, orientation of the home, insulation levels and other factors in addition to square footage. A consultant should come to your home and compile detailed records of all this information.

The fact is, if you don't calculate the size of your HVAC equipment accurately, you're in for trouble. A system sized too large will short cycle, or turn on and off continuously without reaching temperature set points, leaving you uncomfortable and wearing out equipment before its time. A system sized too small will strain to do its job, and again, wear out parts prematurely.

Calculating With Manuals J, D and S

Once the consultant has all the data, it should be fed into HVAC industry software Manuals J, D, and S. Here's what each is designed to calculate:

  • Manual J: Heating and cooling loads
  • Manual D: Size and design of ductwork
  • Manual S: Size of equipment needed, based on cooling and heating loads

Since loads, ductwork and size of equipment all have an impact on your system's efficiency, all should be considered. Using Manuals J, D, and S will provide the most accurate estimates possible for sizing your new HVAC system correctly.

To learn more about Manuals J, D, and S, contact Valley Heating, Cooling Electrical. We serve South Bay Peninsula and Santa Cruz.

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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