How do they determine what size Air Conditioner you need?

What are some things that you should require from your Air Conditioning Contractor before you sign a contract? Make sure he is Licensed and Bonded, if you do not know him ask for references and then check them out. Ask him what size unit he is proposing and how he arrived at that tonnage. A good Air Conditioning Contractor will always run a load calculation on your home first, this removes any guess work and gives both of you peace of mind.

The load calculation, based on Manuel J, will take many things into consideration, such as, how much insulation you have in your ceiling and walls, if it is on pier and beam they will also take the floor insulation into consideration and whether it is under pinned or not. The height of your ceilings, the square footage of your home, the type windows you have, the size of your windows and which direction they are facing. You exterior doors, size, type and direction they are facing and the type of exterior siding on your home.

Load calculations also take in consideration how much infiltration your house has. they will need to know how many people are in your family and if you entertain guests, if so how many on the average. The contractor should also ask you what you expect out of your A/C system. All these figures determine how much heat gain, for Air Conditioning purposes and how much heat loss, for heating purposes your house has in terms of BTU's. They should also run a load calculation on each individual room to determine how much air is required for that particular area.

in Air Conditioning terms 12,000 BTU;s is equivalent to one ton of air conditioning, so if you have a 48,000 btu heat gain then you would need a 4 ton unit. A 4-ton Air Conditioning unit should have an Air handler that will produce a minimum of 400 cfm of air per ton, or 1600 cfm. Depending on the brand, some will move 1800-2000 cfm while others will move 1600 cfm. This will determine how you size your duct system!

The Duct system is the "Heart of any Air Conditioning System", without a properly sized duct system it doesn't matter what type unit you have you will never be satisfied.Usually residential systems are based on .08 static pressure, example: a 6" duct will move 100 cfm of air, 7" will move 150 cfm & 12" will move 600 cfm etc.

Sizing the return air chamber is of critical importance, you should have at least 1 sq. ft. of Return Area per ton of Air Conditioning, example: with a 4-ton system multiply 144sq. inches x 4-tons and you will have the necessary area of R/A grill opening for a 4-ton system, so you will need 576 sq. in. of return air grill. That could be one-12x48" grill or 2- 12x24" R/A grills. If you set your system up like this you will have a quiet system that will operate at maximum efficiency.