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Manual J Calculation

Every contractor on the Approved Installer List will be able to do a Manual J analysis on your home.  We require it because we know what a big difference it can make for your new heating and air conditioning system.  It can also save you money.  Some contractors just swap out the equipment based on the size of your current system (i.e. you have a 4 ton now, replace it with a 4 ton).  But, sometimes the current system is under- or over-sized.  Below is some information about what a Manual J calculation is and why it is so important!


Manual J Load Residential Load Calculation

Every residential structure has a cooling and heating load, which is the amount of cooling or heating required to keep that home at the temperature level preferred by its occupants.

Manual J is the ACCA’s technical manual for conducting load calculations on individual homes. Load calculations use thorough analysis of the thermal and physical characteristics of a home combined with detailed mathematical formulae to determine the amount of cooling a structure needs.

Your HVAC specialist should physically inspect your home, taking measurements, assessing structural components and evaluating the temperature and comfort requirements of you and your family. The load calculation should include consideration of factors such as:

  • Local weather and climate

  • Directional orientation of your home

  • Size and shape of your home

  • Amount of insulation

  • Number and type of windows

  • Landscaping features

  • Energy efficiency of the home

With this information, your HVAC expert can apply the information in Manual J to perform the load calculation. Load calculations and related sophisticated analysis are rarely done by hand.

 

Instead, advanced computer software is used for the mathematical work required for an A/C and heating installation. When the load calculation is completed and your HVAC specialist has analyzed the results, he will have three important pieces of information about your home:

Total Heat Load, which is the amount of heating BTU’s needed to heat the home at a specified temperature.
Sensible cooling load, which is the amount of sensible heat (heat you can feel) that the cooling system should be able to remove during daylight hours of very warm days.
Latent cooling load, which is the amount of humidity the cooling system should be able to remove from your indoor spaces during hot, humid summer days.

With this information in hand, your contractor can tell you what size and capacity air conditioning and heating system will meet your home’s cooling and heating load. This data is critical for properly sizing your new cooling and heating equipment.

When you know what size air conditioner and heater that you need, you’ll be able to choose a system that is big enough to provide plenty of cooling and heating without being too big (which wastes energy and provides reduced dehumidification and comfort) or too small (which results in poor cooling or heating and overstressed equipment).

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