There is no silver bullet answer to this question but it is really important to get the basics right and a little research could save lots of money in the long term.
Firstly, the capacity of an air conditioner lies in its ability to cool a room or living space. In the US, capacity is usually measured in British Thermal Units (BTU) or “tons” with the size of air conditioning unit selected, depending on the climate of your location, your house design, facing direction and materials used.
What is a BTU?
A British Thermal Unit or BTU is the basic unit of heat energy used by the HVAC and other industries. A total of 12,000 BTU’s of cooling capacity per hour equals 1 ton of refrigeration.
What is a Ton of Refrigeration?
Before the advent of refrigeration, ice makers commonly used BTU’s to measure their energy needs and estimated 288,000 BTU’s were needed in a 24 hour period to make 2000 pounds or one ton of ice.
Therefore, for every one ton of ice, 12,000 BTU’s (288,000/24=12,000 BTU) per hour are required.
Consequently, the term “ton” became commonly used for refrigeration or air conditioner performance.
So, when we talk about an air conditioner unit rated at 1 ton or 2 or 3 ton, it just means the required amount of energy in BTU to provide that amount of cooling is 12,000 times the rated tonnage. For example, a 3 ton ac unit has 36,000 BTU/hr, a 5 ton ac unit has 60,000 BTU/hr. You might see this rating stamped on the name plate of an air conditioner that you buy, either in tons or in BTU’s.
Now that we understand how air conditioners are rated, let’s consider some other important factors.
3 Main Factors Affecting The Size
Location, location, location. This is an old catch phrase, but it also applies to find the right air conditioner for your house.
As you can see from the map, the United States is divided into around eight different climatic zones, from the hot-humid zone 1 of southern Florida, Puerto Rico, Virgin Islands, Hawaii and Guam to zones 7 and 8 of Alaska.
Zones 1 and 2, are warm, humid regions, taking in all of Florida and stretching west from Georgia to Eldorado, Texas, including south west Arizona.
Zone 3 is still warm, extending westward from Colombia in North Carolina to Tucson, Arizona as well as most of California, however it lacks the humidity of zones 1 or 2.
Zone 4 with a more moderate climate, starts from Virginia in the east to New Mexico, taking in Oregon and Washington State. However, most areas in this zone only achieve an average annual temperature of 55°Fahenreit. Consequently, zone 4 and the remaining areas of the US need an alternative heat source to provide suitable comfort levels.
Each climate zone will have a different impact on the type of system and size of air conditioner to make your home comfortable.
How to Calculate The Proper Size According to Zones
As a general rule, a home in the hot, humid conditions of zone 1 or 2, might need to remove up to 30 BTU of heat for every square foot of floor area. Zone 3 having a lower outside temperature might only need to remove 20-24 BTU per square foot, while the requirements of zones 4 to 8 range between 12 and 22 BTU’s per square foot.
Consider a single level house with a concrete slab floor of area, say 1500 square feet in zone 1.
An air conditioner, based on this size area, in this climate zone, would require a capacity of 3.75 ton (1500 x 30=45,000 45,000/12,000=3.75).
For this example, we would select a 4-ton unit because air conditioner capacities increase in steps of 0.5 ton. Some of this energy will also be required to de-humidify the air in this zone to make the room more comfortable so a little extra capacity will help there.
If your region is zone 3, requiring only 24BTU per square foot, our example would require an air conditioner capacity of just 3 ton (1500 x 24=36,000 36,000/12,000=3).
If you live in any of the remaining zones, an additional heat source such as a heat pump or furnace and air handler, would be required to keep your family comfortable.
You can check detailed sizing calculation formula based on different zones here.
#2 Building Design
While we have considered how air conditioners are rated and considerations of various climatic zones in the US, there are still many factors in your home to influence your decision regarding the best size system for you.
The first thing to consider is the floor area of the space to be cooled. This will have the most influence on the tonnage capacity of the unit required and easily calculated using the method detailed earlier.
Secondly, if the design of your house is a single level building with a concrete slab floor, this may be the most thermally efficient design for air conditioning but always allow extra consideration for kitchen areas or laundry rooms, especially if a clothes dryer is used. These are always a source of heat and place additional load on the air conditioner. In contrast, a multi-story structure will be less efficient, have a greater volume to cool and more opportunity for cool air to leak through walls, doors or windows.
Roof shape, color, material and the direction your house faces can also influence the amount of heat entering your house, as will glass doors and windows and the direction in which they face. Unless these openings can be protected with awnings, blinds, curtains or reflective film, they can become a major source of introduced heat inside your house.
All building materials are graded with an “R” rating indicating respective resistance to heat transfer. A high value of “R” means a greater resistance to heat and for insulating materials, this value is often proportional to the thickness or type of material used.
All of these factors need to be considered and consolidated into what is known as the Manual J Load system. This is an important part of your deliberation with building design factors and materials influencing your selection of a suitable size unit.
The means of transferring cooled air around your house should also be considered. Choosing a ducted system is a great and efficient way to distribute cool air to all parts of your house, but if the ducting is poorly insulated or poorly installed, it can have a negative effect on your comfort level or introduce condensation problems.
Considering all of these factors, it is then possible to determine a suitable tonnage air conditioner for any residence in the USA by following this process and applying known “R” ratings and other building factors to your Manual J Load evaluation.
Importance of Choosing the Right Size Unit
If on completing your estimation process and a unit is selected that is too small, the compressor and other major components will likely run for long periods with high power demand.
This converts into high running costs, increased maintenance and potentially costly repairs.
Considering the life of an air conditioner, these costs will grow into a large amount of money over the life of the unit.
If your selection is too large, the room temperature becomes more difficult to control with frequent cycling of the compressor and other major components. A unit that causes the compressor or condenser fan motor to start and stop frequently, also incurs a high power demand, huge running costs and will probably not provide the constant temperature control required.
As a general rule for residential applications, it is always worth remembering that a unit greater than 5 tons or 60,000 BTU’s per hour would not be expected to be needed. If this is the case, it would be more beneficial to install 2 smaller units enabling operation of different zones or as a backup if one unit needs to be off-line for any reason.
Carefully selecting an air conditioner of the right tonnage will ensure a more continuous cooling temperature range rather than shutting down major components on achieving thermostat settings. A correctly sized unit causes less fluctuations in energy demand, costs less to run and requires fewer repairs.