How is air conditioner size measured?
Cooling capacity is measured in tons or BTUs per hour. One ton equals 12,000 BTUs per hour. The goal is a system that matches how much heat your house actually gains on a hot day.
Why square footage is not enough
Rules of thumb based on square feet ignore most of what makes one house hotter than another. Two homes of the same size can need very different systems.
What a load calculation looks at
A proper sizing starts with a load calculation. The industry-standard method for homes is called Manual J. It accounts for:
- Insulation in the walls and attic
- Windows: how many, how big, which way they face and how much shade they get
- Air leakage
- Ceiling heights and the layout of the house
- How many people live there
- The condition and size of the ductwork
What happens if an AC is oversized?
- Short cycling. It cools the air quickly and shuts off before running long enough to work efficiently
- Clammy air. Short cycles do not remove much humidity
- Uneven rooms. Some rooms get cold while others stay warm
- More wear. Frequent starts are hard on the equipment
- Noise and airflow problems, especially if the ducts were not sized for it
What happens if it is undersized?
An undersized system runs constantly and still can’t keep up on the hottest days.
Do not just copy the old unit
The old system may never have been sized correctly, and the house may have changed since with new windows, insulation or an addition. Variable-speed equipment can adjust its output and handle a wider range of conditions, but it still needs to be sized for the home.
Jordan assesses the home on site before recommending a size, and checks whether the ductwork can carry the airflow. See air conditioner installation.
Talk it through with Jordan
Jordan personally handles every call and every quote. Tell him what is going on, and he will tell you plainly what it needs.
Call (650) 650-2653 — that’s 650-650-COLD. Monday–Friday, 7am–5pm.