What Size AC Do I Need?

Why oversizing backfires

An oversized air conditioner cools the air near the thermostat quickly, satisfies it, and shuts off. That sounds like success. It is not, for three reasons.

It never runs long enough to pull moisture out, so the house feels clammy at the right temperature. It never runs long enough to move conditioned air to the far rooms, so the back bedroom stays warm no matter what you set. And starting is the hardest thing a compressor does — a system that starts and stops constantly wears out years early.

The comfortable system is the one that runs longer at lower intensity, not the one with the most tonnage.

What actually determines the answer

A real load calculation accounts for the specific house, which is why two identical-square-footage homes on the same street can need different equipment.

• —Square footage and ceiling height — volume, not floor area
• —Window area, orientation, and glazing — west-facing glass is a large afternoon load
• —Insulation levels in the attic and walls
• —Air leakage of the building envelope
• —Shade from mature trees, which is significant in Lafayette and Orinda
• —Duct capacity — the limit on how much air you can actually deliver

Why "match the old one" is a weak default

It assumes the previous unit was correctly sized. Frequently it was not — it was sized by the same rule of thumb, or upsized because someone complained about a hot room that was really a duct problem.

It also assumes nothing about the house has changed. New windows, added attic insulation, or a converted garage all move the number, usually downward.

The duct question matters most. If the existing ductwork cannot carry the airflow a larger system needs, a larger system will not deliver it — you will have paid for capacity that is stuck at the air handler. That is why we measure static pressure before quoting a replacement.