When installing a new AC system, you may be able to base the needed size on the system that was in place before. But if the system isn't sufficient for the house or if there wasn't one in place originally (which can happen in moderate climates, especially with old houses), you may be wondering how to estimate the size of system you'll need. A professional will be able to give you educated advice on precisely which system is best for hvac installation, and they'll likely use these exactly three factors to help calculate which system and size to install.

1. Climate

The average temperatures of the area you live in will affect the amount of cooling needed on average per square foot. In areas where summer temperatures are usually fierce, you'll need a unit that can work harder than the one you'd need if you were in a milder climate. In addition, you'll need to factor in the climate if you're considering an HVAC unit that will provide both cooling in the summer and heating in the winter, so you can balance the heating and cooling capabilities of your system. HVAC professionals use a zone chart to help calculate what changes should be made to the system size based on your climate.

2. House size (sq ft and ceilings)

The square footage of your house is the most basic way to estimate needed system size. You can get a rough estimate just by taking the number of square feet in your house and multiplying by 25 to obtain the approximate number of BTU needed. Then, divide the BTU by 12,000 and round down for your extremely rough estimate of the number of "tons" your new unit should be rated for. This is the base you'll use, which will be modified by the other factors discussed in this article. Climate is one of these factors. Another is ceiling height; if your ceilings are more than a normal ceiling (say more than eight feet or so), you'll need extra cooling power for that extra space, so you'll need to adjust by increasing the size of your system.

3. Occupancy

The number of people who are in the house can change the amount of heating and cooling needed. Each additional person adds a little extra heat, raising the base temperature in summer, which means that if your house has a large expected occupancy you'll need to figure that in as well. 

These three factors are some of the main ways to calculate and fine-tune the size of the AC system you'll need. Your professional HVAC contractor will help you determine the best system for your particular situation.