What size AC does a home need?

A San Diego home needs an AC size set by 3 inputs: floor area, insulation and sun exposure. AC size is measured in tons, and 1 ton equals 12,000 BTU per hour.

Key takeaways

Measure the AC in tons

1 ton equals 12,000 BTU per hour.

Compare the 3 size inputs

floor area, insulation and sun exposure change the size.

Avoid an oversized or undersized AC

each causes a different problem in run time, temperature and humidity.

Request a written estimate

a Manual J load calculation confirms the size.

AC size estimate request

How is AC size measured?

AC size is measured in tons and BTU per hour, and 1 ton equals 12,000 BTU per hour. There are 2 units.

Tons

A ton of AC capacity equals 12,000 BTU per hour. The unit comes from ice cooling: 1 ton of ice melting over 24 hours absorbs 12,000 BTU per hour.

BTU per hour

A BTU is the heat needed to raise 1 pound of water by 1 degree Fahrenheit. An AC rated at 12,000 BTU per hour removes 12,000 BTU of heat from the air in 1 hour.

What happens when an AC is too big or too small?

The table lists how an oversized AC and an undersized AC differ in 5 areas. Short cycling is covered in Why Is My AC Not Cooling?

Problem	Oversized AC	Undersized AC
Run time	Short cycles	Long run time, with the AC running most of the day
Temperature swings	The temperature rises and falls between short cycles	The home does not reach the set temperature on the hottest days
Humidity control	Less moisture is removed because each cycle is short	Moisture is removed during the long run, and the home stays warm
Wear on the compressor	Frequent starts and stops	Continuous operation
Energy use	Extra energy goes to repeated startups	Extra energy goes to continuous operation

How does a Manual J load calculation work?

A Manual J load calculation is the room-by-room heat gain calculation that sets the AC size. The Air Conditioning Contractors of America publishes Manual J, 8th Edition (ANSI/ACCA 2 Manual J-2016), as the national ANSI-recognized standard for sizing HVAC equipment loads for single-family homes.

A Manual J calculation uses 5 inputs:

Measure the floor area

The floor area sets the volume of air to cool.

Record the insulation

The insulation in the walls, the attic and the floors sets the heat gain.

Count the windows

The size, the orientation and the glass type of the windows set the solar heat gain.

Test the air leakage

The air leakage sets how much outdoor air enters the home.

Count the occupants

The occupants add heat inside the home.

What is SEER2?

According to U.S. Department of Energy research from the Office of Energy Efficiency and Renewable Energy, October 5, 2022, SEER2 is the seasonal energy efficiency ratio of an AC measured under the revised test procedure effective January 1, 2023. A higher SEER2 rating changes the equipment price and the operating cost.

How much does an AC cost by size?

The cost factors of AC size are explained on How Much Does HVAC Cost in San Diego?

Schedule San Diego AC Installation Service Today

No matter the HVAC issue, Sandiego HVAC Services has the solution for San Diego residents. Call +1 (858) 392-9754 or send the service request form with name, phone, ZIP or neighborhood and system type. See also AC Installation and Replacement in San Diego, Ductless Mini-Split Installation in San Diego and How Much Does HVAC Cost in San Diego?