You will find charts online that say a 1,500 square foot house needs a 2.5 or 3 ton air conditioner. For a quick guess, fine. For buying a system you will live with for 15 years, it is not enough.
Why square footage misleads
Two houses of the same size in Los Angeles can need very different systems. A shaded Glendale bungalow with small windows and a Studio City ranch with a wall of west-facing glass behave nothing alike on an August afternoon. Ceiling height, insulation, attic temperature, window type and how many people live there all change the load.
What a load calculation looks at
A Manual J calculation goes room by room. It takes the size and direction of every window, the wall and ceiling insulation, the local design temperature, air leakage and the heat from people and appliances. The result tells you how much cooling and heating the house needs, and how that splits between rooms.
The problem with oversizing
Many older systems in LA are too big. An oversized AC cools the air quickly, shuts off, and starts again a few minutes later. That short cycling leaves some rooms cold and others warm, removes less humidity, makes more noise and wears out parts sooner. Bigger is not safer.
What about undersizing?
An undersized system runs all afternoon during a heat wave and still falls behind. In the Valley, where summer heat waves push past 100°F, we size for the real load and often recommend variable-speed equipment, which can run gently most of the day and ramp up when needed.
Sizing and your ducts
The right size only helps if the ducts can carry the air. Undersized or leaky ducts make even a perfectly sized system underperform. That is why we test ducts as part of a replacement.
- Ask every contractor if the quote is based on a load calculation.
- Be wary of a quote that simply matches the size of your old unit.
- Ask how the ducts were checked.
