What Size Central Air Conditioning System Do I Need For My House?

There are many factors in your home that affect the efficiency and size of a properly sized central air conditioning system, such as the height of your ceilings, the type and thickness of your insulation, and the amount of windows on the exterior of your home, especially those with direct sunlight.  There is a simple rule of thumb when it comes to calculating how many BTUs (British Thermal Units – the measure used for both heating and air conditioning) your home requires for proper cooling, and then there is a more complex process used by professionals called a Manual J. 

A manual-J calculation is a detailed analysis of your home’s central air conditioning and heating needs.  There are two types of manual-J calculations: a whole house load calculation and a room-by-room load calculation.  A whole house load calculation determines the amount of cooling required for an entire house as a whole, and includes the heat transfer between walls, number of windows and their efficiency rating, type of insulation and how much of it, heat transfer through your concrete slab, the number of people in your house (each person gives off around 250 BTUs per hour), how many sky-lights you have, the amount of lighting you have and what type, what type of ducting and the ducting location (in your attic or inside your home’s insulation), as well as a few other factors.  Typically a whole house load calculation is used when replacing just the equipment; a room-by-room load calculation is used when the equipment and the duct system will be replaced or installed new.   

If your system is too big or too small, it is going to cost you comfort and energy, so choosing the right size is extremely important.  If replacing an existing air conditioning system, do not just replace what is existing.  It may not be what your home requires.  Have a knowledgeable, licensed and experienced air conditioning contractor properly evaluate your home for proper cooling.  In the years since your home was built, you may have done some renovations or added a skylight, which may affect the amount of cooling your home needs now; or the original installer may not have accurately calculated the proper size in the first place.   

There are about 5 climate zones in the United States which determine how to measure the cooling (and heating) requirements for your home.  In our area, it takes about 12,000 BTUs or 1-ton of cooling to air condition approximately 400 to 600 sf of your home.  Keep in mind, there are a lot of variables, however, it is a good rule of thumb for a typical residential home.  An air conditioning contractor should provide an on-sight evaluation of your home before providing you with your specific sizing requirements. 

A system that is too large will cool your house quickly, but you still may not feel comfortable. That's because it will satisfy the temperature setting on your thermostat before it can adequately remove sufficient humidity from the air—which is what makes you feel sticky and uncomfortable in summer. What's more, the stress of short cycling your system will shorten the life of your equipment, create more unnecessary repairs and increase your energy costs. 

So too, a system that is too small will constantly run in an attempt to cool the house, but never satisfy the thermostat.  This will also create stress on your equipment, whereby creating the potential for more breakdowns and increased energy costs.  The laws of science provide very clear rules for effectively cooling your home.  A proper evaluation can provide you the information you need to effectively cool your home and provide you with the most energy efficient way to do so.

Calculating Square Footage - A room's size is measured in square feet; calculating the square footage requires measuring each room that will need to be cooled individually. Multiply the length of the room by the width of the room, and do not include closet space in the calculation. You need to calculate all rooms that will be cooled, including hallways, laundry rooms and bathrooms. Exclude rooms or areas that are not or will not be connected to the system, such as basements and attics. 

Converting Area to BTUs or Tonnage – It is easiest to convert the square footage of your home to tonnage to determine the basic cooling needs of your home.  A good rule of thumb, 400 sf of area needs 1-ton of cooling.  After calculating each room’s square footage, add the square footage together to determine the whole area, then divide the total by 400.  That will provide you with the tonnage needed for your home, which can then be converted into BTUs.

1.0 Ton = (12,000 BTU)

1.5 Ton = (18,000 BTU)

2.0 on = (24,000 BTU)

. on = (30,000 BTU)

3.0 Ton = (36,000 BTU)

3.5 Ton = (42,000 BTU)

4.0 Ton = (48,000 BTU)

5.0 Ton = (60,000 BTU) 

Split Systems – If replacing an existing system, chances are you will keep your existing ductwork and only replace your indoor and outdoor equipment.  However, when having a professional provide you an estimate, you should communicate any problems you have with air flow so that it can be addressed before selecting the equipment and having the work done.  Even if there is no access to your ductwork, the new air conditioning systems on the market help provide solutions to deficient air flow.  The benefits of a having a ducted central air conditioning system are a more even distribution of air and a greater reduction of humidity throughout the house. A properly sized unit will have the least impact to the monthly electric bill than any other cooling options, especially if you choose Energy Star-rated appliances.  

Ductless Units - Ductless units operate similarly to split systems and are often called miniature split systems. The system routes refrigerant through pipes from an outside unit into room-specific air handlers which cool the room without ductwork.  Ductless units provide more efficient cooling for specific areas where they are installed as opposed to the whole dwelling.  They are also a great alternative for homes where ductwork cannot be installed. 

The cheapest way to cool your house is prevention — keeping your house from getting hot in the first place.  If possible, windows should be shaded on the outside by trees, overhangs, awnings or exterior shades to keep the sun's rays off the window. Interior curtains and blinds don't work as well since the solar heat is already inside the room. Let in cool air over night or in the early morning to cool down your house. Then, close the windows during the day to keep the heat out. Make sure your home is well-insulated and caulked.  During very high outdoor temperatures, do not shut the air conditioning system off when you leave the house.  Rather raise the thermostat a few degrees so that when you return, the air conditioning system only needs to cool, not dehumidify.  This will allow the house to cool off more quickly.

Gregg Gaal, President - Gregg Mechanical Corp.

(718) 761-2300   www.greggmechanical.com