Manual J is the name for a specific protocol (often called "Heat Load Calculation" or "Cooling Load Calculation") used to determine how much heating / cooling a home needs to stay cool and dry in the summer and warm in the winter. This load calculation process was developed by engineers in the heating and air conditioning industry and has been used for decades to accurately size heating and air-conditioning equipment. After completing this load calculation process, one can choose a properly sized piece of machinery to satisfy the load.

Why is a properly sized HVAC system important?

• Efficiency: You wouldn't buy a dump truck just to drive around town, right? That would be larger than necessary and be less efficient than a car that fit the task. The same goes for heating and cooling equipment. If your machinery is larger than necessary, it means that you are spending money to operate larger fans and larger compressors. The goal is to buy one that is sized "just right."
   
• Comfort: Contrary to popular belief, bigger is NOT always better. Over-sized air conditioners are not able to dehumidify the air inside the home. Most air conditioning units need to run for 7-12 minutes before their coils get cool enough to cause condensation (this is what dehumidifies the air in your home). If the system is oversized, the thermostat will be satisfied before the unit is able to remove any significant amount of moisture. This creates cold, clamy conditions. Properly sized air conditioners should produce relative humidity levels of ~50% in the cooling season. If your relative humidity is larger than this, you may have an oversized system.
  
  Heating season comfort is also impacted by over-sized machinery. Especially with heat pumps. Why you ask? Because heat pumps produce supply air that is roughly 100 degrees, compared to furnaces that produce supply air that is 125-135 degrees. This means that a heat pump has to move more air around the house in order to deliver a fixed amout of heat. If a heat pump is oversized, this means that there will be more air than necessary moving about the house, creating drafts, and causing comfort complaints. Heat pumps should be sized based on the cooling load of the house (to ensure proper dehumidification), with the rest of the heating needs delivered by the "auxillary" heat - typically electric strip heaters.
   
• Air quality: If your system is over-sized and therefore not properly dehumidifying, you'll have moisture problems. This may show up as strange smells, mold or mildew growing in areas that don't receive much air flow, excessive presence of dust mites, etc. In much of the nation, humidity control is the main purpose of air conditioning systems. In order for them to control this humidity, they MUST be sized properly.