Common HVAC Sizing Myths Homeowners Should Ignore

Sizing an air conditioner is math. It is physics. It is not an art, and it is not a guessing game. Yet, these rule of thumb myths persist.

Myth 1: "Bigger is Better"

The Truth: In HVAC, bigger is deadlier. An oversized system acts like a drag racer in a school zone: it speeds up, hits the target temp in 5 minutes, and slams on the brakes. This leads to humidity problems, mold, and broken compressors.

Myth 2: "Old House = 500 sq ft per Ton"

The Truth: This rule from the 1970s assumes you have zero insulation and single-pane windows. If you have done any upgrades (new windows, attic insulation), this rule will oversize you by 50%.

Myth 3: "I'll Replace it 'Like for Like'"

The Truth: "If the old one was a 4-Ton, the new one should be a 4-Ton."

This assumes the original builder sized it correctly (spoiler: they didn't). It also ignores any energy improvements made to the house over the last 15 years.

Myth 4: "Manual J is just for Permits"

The Truth: What is Manual J? It is the manufacturer's required operating specification. Installing without it voids many warranties.