Manual J. – What is it and How is it Used to Size Your Columbus AL Air Conditioning System?

Published on: July 20, 2012

Columbus, AL is known for its extreme weather conditions, much like the rest of the southern United States. Unfortunately, Columbus, AL also is not located in a region with very stringent efficiency ratings for their air conditioning units. This means that without the proper information, you can end up purchasing a system that is much too large for your living space. This means more money spent on your AC service and air conditioner maintenance calls, as well as a less comfortable living space.


Although there are no federal regulations detailing what kind of air conditioning unit a particular home should get for their living space, the AC industry itself has formulated a manual for these types of calculations. This manual is known as Manual J., which has long been used by consumers to properly size the unit that their family’s home needs. It is also a great way to compare notes with salesmen and technicians that you will have to communicate with in order to have thorough air conditioner maintenance performed.


Below are some of the more important inclusions of Manual J and how it can help you decide as a consumer and a homeowner. It provides three main statistics that will help you choose the amount of energy that you need to cool and heat your house.


The heating load – The heating load of your living space as calculated by Manual J will give you the amount of energy that is required to heat your house in the most difficult circumstances. Manual J calculates the amount of heat that your house needs on the coldest day of the year without help from the sun.

The cooling load – There are actually two types of cooling loads: the sensible cooling load and the latent cooling load. The sensible cooling load measures the amount of heat that your system can remove on the warmest day of the year.

The latent cooling load – This measures the amount of heat that your system can remove from your living space amidst the absolute worst case scenario. This latent cooling load combined with the sensible cooling load and the heating load gives you the information that you need to select the right size system for your living space.

Selecting the right size system in this manner will save you an incredible amount of money on AC service calls. The reason that a system that is too big can cost you more money will be detailed below.



HVAC systems that are too big for the living space do not allow the air to properly dehumidify. That air will then condensate and cause all kinds of health problems for the inhabitants of the living space as well as the air conditioning system. The condensation, when combined with humidity, will result in mold and mildew growth. It will erode the components of your air conditioning system more readily and cause its life to substantially shorten.



If the system is too large for a living space, it is like driving a Hummer in New York City, which is ultimately a waste of gas, energy, and space. In this scenario, the cost certainly does not outweigh the benefit received.

Most people have a tendency to overcompensate when it comes to assessing their own heating and cooling needs. They want to have systems that immediately cause the living space to become comfortable, and in this type of impatient, unscientific assessment, they choose systems that are too powerful.

This causes a great deal of inefficiency when it comes to heating and cooling living spaces. Manual J also gives statistics for the amount of energy that it takes to cool and heat the living space as a total area and from room to room. These statistics are known as the “block load” and the “room by room load.”

The block load and the room by room load should be used to determine the ductwork size that will most efficiently deliver air into different rooms of the living space. It will also help a homeowner decide how to best divide the airflow throughout the house. Small rooms do not need as much airflow as bigger rooms, and determining this scientifically through Manual J calculations can save a homeowner a great deal of money.