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What Is Manual J?
Posted: April 2006

what is ‘manual  j’  ?

By Jim Berry 

     Residential air conditioning systems are available in a variety of capacities, commonly referred to as ‘tons.’ You’ve probably heard of an air conditioning system referred to as a three-, four-  or  five-ton system. This term has nothing to do with the actual weight of the equipment, but rather, a measurement that determines the amount of heat that needs to be removed from a particular building. We call that air conditioning.

     The calculation used to determine the correct capacity for your home is known as a Manual J Load Calculation. This is the only way to determine, with certainty, how much air conditioning a specific home will require. If there is any question as to the capacity of the equipment your home will require, a Manual J Load calculation should definitely be performed.

     Contractors now input information such as square footage, ceiling height, wall and ceiling insulation, window type, orientation to the sun, geographical location, lighting wattage, people loads, and a variety of other information into a computer program to determine a home’s required load.

     After the correct information has been entered, the Manual J program calculates the size of the system required for your home. Every home has specific information that can affect the outcome. For example, your neighbor who lives across the street, with the same model home, may need a different capacity system than needed for your home due to a different orientation to the sun.

     If the air conditioner installed in your home is undersized, it will need to operate for much longer periods than a properly sized unit. A system that is too large is also a problem since the unit will cycle on and off too quickly, which will shorten its life.

     So, before allowing anyone to replace an existing system (if the capacity is in question) or install a completely new system, make sure a Manual J Load Calculation has been completed, and ask to see the results. You can then be confident a properly sized system has been selected for your home. More importantly, if anyone asks you to sign a contract before properly determining the correct capacity for your home, it is my recommendation that you politely but firmly ask him or her to leave immediately.

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