Choosing your next air conditioning system isn't just a matter of picking the best features. Size also has a lot to do with how well your new system cools your home throughout its working life. Buying a system that's too big or too small for your home to handle can have plenty of unintended and rather expensive consequences.
Wrong Size Equals Poor Performance
Many contractors and homeowners wrongly assume that "bigger is better" when it comes to a new A/C system. In fact, most buyers are so afraid of coming up short when it comes to cooling capacity that they overcompensate by buying a larger unit. However, there are many problems that come with buying an oversized A/C system:
- Moisture problems – An oversized A/C unit can cool your home quickly, but it doesn't have enough time to dehumidify the air. As a result, your conditioned space will feel damp, clammy and generally uncomfortable.
- Short cycling – Oversized A/C units are also prone to short cycling, a condition where the compressor starts and stops rapidly in an attempt to keep temperatures at set levels instead of gradually bringing temperatures down by running for an extended period. This can actually shorten your A/C's lifespan by speeding up wear and tear on the compressor and other components.
- Energy waste – When you have more cooling capacity than you need, you're bound to waste unnecessary energy. Installing an oversized A/C system could mean higher-than-normal utility bills in the near future.
On the other hand, an undersized A/C unit may have to run continuously to bring temperatures down to comfortable levels, resulting in higher energy costs, higher maintenance costs and a shorter lifespan for your A/C equipment.
How to Choose the Right Size
Now that you know the potential consequences of oversizing or undersizing your A/C system, it's time to determine the best size for your home. The simplest way of doing just that involves using the "rule of thumb": you'll need 1 ton of cooling capacity for every 500 square feet of living space. You can calculate your home's square footage by multiplying the length of the room to be cooled by its width. If the room happens to be triangularly shaped, take the result and divide it by 2.
While "rule of thumb" calculations offer a good starting point, your contractor can get a much more accurate picture of your home's cooling needs by performing a Manual J load calculation. Developed by the Air Conditioning Contractors of America (ACCA), Manual J takes a number of important variables into account that are often left out of rule of thumb calculations, including moisture content, heat gain and loss, house orientation and ventilation setup.
What Tons Have to Do with Size
Most contractors and technicians measure A/C system cooling capacity in tons. This practice dates back to the days when ice was used for cooling and refrigeration. So how does this relate to a modern system's cooling capacity? The answer sounds a bit complicated at first, but it becomes crystal clear later on.
Take that proverbial ton of ice, for instance. It takes 143 British thermal units (BTUs) of heat energy to melt a pound of ice. If you want to melt 1 ton (2,000 pounds) of ice, it'll take 286,000 BTUs to get the job done. If you want to melt that ice uniformly over a 24-hour period, you'll need approximately 12,000 BTUs of heat energy per hour.
In essence, a "1-ton" A/C system offers 12,000 BTUs of cooling performance per hour. This is why contractors and technicians often refer to A/C systems as "1.5-ton" or "3-ton" systems. They're really referring to the system's equivalent capacity in BTUs.
After performing a Manual J load calculation, your air conditioning contractor will suggest the best A/C cooling capacity for your home, either in tons or BTUs. With the above information in mind, you'll be able to choose the best A/C system without worrying about buying too much or too little cooling performance.