What is Manual J Load Calculation?

Getting the right sized air conditioner and heater can be a bit difficult but there are methods that can help you determine the right size for your flat or house. The most famous, the most accurate and the best method so far is Manual J Load calculation. This method is required by the law.

The Manual J load calculation determines the amount of heating and cooling the home requires to keep you and your family warm during the winter and cool during the summer. When sizing your air conditioner, Manual J calculates how much heat is gained through the windows, walls, building materials, basement, crawl space, etc. When considering the perfect size air conditioner for your home the manual J calculation considers different factors, and some of them are following:


  • Number of people living in the house. It is common knowledge that our bodies give off heat. So the number of people living in the residence affect the size of the air conditioner.
  • High, vaulted ceilings – air flow volume is measured by cubic feet not square feet. The height of your ceilings affect the size of the air conditioner not just the square footage.
  • The quality and amount of insulation you have in your walls and attic affect the performance of the air conditioner.
  • The number, the type, the age of windows and doors, as well as the direction they are facing affect how much heat will be absorbed from the
  • How often you use your appliances like stove, microwave, dryer and other heat-generating appliances affect the calculation.
  • Spaces with computers, gaming system and large TVs generate more heat, so home offices or game rooms must be considered in the calculation.
  • If your duplex is on the corner of the building, you may require a larger air conditioner than your neighbor in the middle space. The home in
    the middle space will enjoy the conditioned air from their neighbors so they will need the smaller air conditioner.
  • Whether your house is built on a crawl space, basement or a slab will affect the size of your air conditioner.
  • The building materials your house is made of make a difference. Brick absorbs heat while other materials reflect heat.
  • Location of duct work – It is easier for your air conditioner to push cool air through ducts in your crawl space versus in your attic.
  • The amount of shade from trees and shrubs reduce the heat load and affect the calculation.
  • Direction of the sun

This is not where the list ends, but you get the idea. Air conditioners are designed to remove heat, and this calculation is the perfect way to determine how much heat your home absorbs. Which determines the amount of cooling your home needs to fight high temperatures. It’s important to invest the time in the Manual J calculation, it will help you decide on the correct size of your air conditioner and not to mention saving a lot of time and money.

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