How to Size an Air Conditioner (and Why Bigger Is Not Better)

When Seattle homeowners add air conditioning for the first time, there is a natural instinct to go bigger "just in case." It is one of the most common and most expensive mistakes in HVAC. Here is why sizing matters and how it is done properly.

What happens when an AC is too big

• It cools the air quickly and shuts off before it has time to remove humidity, leaving the house clammy.
• Constant starting and stopping, called short cycling, wears out the compressor and contactor early.
• Temperature swings between cycles make rooms feel uneven.
• You pay more up front for capacity you never use.

What happens when it is too small

An undersized unit runs constantly on the hottest days and never quite reaches the setpoint. It is less common than oversizing, but it is what happens when an installer guesses low or when a home has been expanded without updating the system.

How proper sizing works

Sizing should be based on a load calculation, commonly called a Manual J. It accounts for square footage, ceiling height, insulation levels, window area and orientation, air leakage, the number of occupants, and the local climate. The output is the cooling capacity your home actually needs, expressed in BTUs or tons.

Rules of thumb like "one ton per 500 square feet" are a starting point at best. Two homes of the same size can need very different systems depending on their windows and insulation.

Ductwork matters too

A correctly sized unit attached to undersized or leaky ducts will still underperform. A good installer checks static pressure and duct condition and will tell you if the ducts need attention before the new system goes in.

What to expect from a good installer

• Measurements of your home, not just a glance at the old unit's nameplate
• A written load calculation and a recommended capacity
• Options at different efficiency levels, with honest tradeoffs
• A clear plan for ductwork, electrical, and permits