Does size really matter when it comes to an AC unit? Absolutely! Here’s why:
- If your AC system is too small, it will run constantly and will always be playing catch-up to your thermostat setting. Since modern energy-efficient AC units are meant to run slowly over time and are not designed for rapid cooling (like the old units from the 80s), they need to run constantly to maintain the desired temperature if the house is too big for the unit. This effectively kills their energy efficiency and runs up your electric bill.
- If your AC system is too big, it will cool your house very rapidly and then turn off, and repeat this cycle again and again. The best analogy for this is if you are hot and decide to open the freezer door to enjoy a blast of cold air; then you close the freezer door and within a short time you feel hot again; so you open the freezer door again… and so on. Repeated cycling is hard on the unit, it drives up your electric bill, and doesn’t maintain a consistently pleasant air temperature.
- If your AC system is just the right size for your home, you get steady, efficient cooling, a consistent ambient temperature, and low energy bills.
The system’s suitability for the home is easily determined upon inspection.
Determining the right size of AC unit for a home is based on the square footage of the house but also considers variables including:
- Ceiling heights
- Thickness of insulation (walls and ceiling, but also windows)
- Climate; how hot it typically gets during the cooling season (or year ‘round)
- Whether the house is in full sun, partial sun, or shade
- Exposure (especially relevant with large south-facing windows)
- Slab foundation, crawlspace or basement
- The color of the roof
AC systems come in different sizes, measured in tonnage. This is not based on weight, but reflects the unit’s cooling ability. One ton = the capacity to cool 12,000 BTUs per hour (a BTU is used to measure the amount of energy required to heat or cool one pound of water by one degree F).
Most residential AC units range from 1.5 ton to 6 ton, in ½ ton increments. In general, unless your home is extremely poorly insulated, or it’s a very, very large home, you will not need a unit larger than 6 tons.
Ultimately the purpose of the correct sized unit is to allow the unit to run through its complete cycles efficiently, as it was designed to do, without working too hard or continuously cycling on and off.