A Guide to Air Conditioner Sizing

Air conditioning sizing is an important factor to discuss when getting a new air conditioner. The majority of HVAC contractors make use of rule of thumb for deciding the size of a cooling equipment in the Bay Area and beyond. This rule is generally based on the conditioner floor area. 1 ton of air conditioning capacity is said to be adequate for every 400 to 600 square feet of floor area as per the rule.

Unfortunately, the rule doesn’t account for insulation levels or airtightness in a house. It doesn’t even consider the specific features in different houses that may impact the cooling needs. It simply scales the system to the house size. Keep reading to learn the right way of air conditioning sizing.

Load Calculation vs. Rule of Thumb

Higher numbers, when talking about square feet per ton, usually indicates lower air conditioner capacity and cooling loads. Air conditioner capacities and cooling loads are given in the amount of heat lost or gained per unit time. You would need to divide the cooling load by 12,000 to convert it to tons or BTU per hour. Next, the capacity needs to be divided by the floor area

Most newer homes have a square feet/ton that is higher than 400 – 600. Getting 1500 square feet/ ton is not unusual. This means the contractor installing a 500 sq. ft. per ton air conditioner is using something that is three times larger. The rule of thumb will require an air conditioner with a 6-ton capacity in a 3000 square foot house. In reality, the house may only require a 2 – 3 ton unit.

A better rule of thumb, especially for newer houses, is using 1,000 square feet per ton. This should be used as a quick check. This only applies to above-grade parts of a home. You should use 2,000 square feet per ton or higher for basements in newer homes because of additional insulation and airtightness.

Key Sizing Points for Air Conditioners

These are a few key points to consider while sizing your air conditioner:

• Very few houses, such as older homes, will benefit from using the rule of thumb of 400 – 600 square feet per ton. You will almost always end up with an oversized air conditioner if you size a unit to that metric. In some cases, you will end up paying more for a grossly oversized system which would hurt even more during high gas price times and when inflation is running far above normal.
• Cooling loads as per the ACCA Manual J protocol come out to be much lower as compared to the rule of thumb numbers. This means that the numbers attained through square feet per ton are on the higher side.
• Actual sizing of air conditioners required is usually lower than the rule of thumb.

Another important factor to consider while sizing an air conditioner is to use actual capacity instead of nominal capacity. With that said, you should know that it is better to have a system that is slightly oversized as compared to something that is undersized. Undersized systems may not be able to cool your home as effectively.

This is not to say that oversized systems don’t have their disadvantages. They may short-cycle often causing wear and tear to parts. It’s recommended that you work with an accomplished and reliable HVAC contractor that knows how to correctly size a system by accounting for all factors.