What BTU Means and Why Sizing by Square Footage Gets It Wrong

What BTU Means

A BTU (British Thermal Unit) is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. HVAC equipment is rated in BTUs per hour: the amount of heat an air conditioner can remove from your home, or a furnace can add, in one hour.

Tons. Air conditioners and heat pumps are often sized in tons. One ton equals 12,000 BTUs per hour, so a 3-ton AC is rated at about 36,000 BTUs per hour. The term dates back to the use of ice.

Furnace input vs. output. A furnace label may list input BTUs (fuel burned) and output BTUs (heat delivered). The difference is efficiency. An 80,000 BTU furnace at 80% AFUE delivers about 64,000 BTUs of heat. When comparing quotes, compare output.

Why Square Footage Gets It Wrong

The common rule of thumb is roughly 1 ton of cooling per 500 square feet. It is quick, but it treats every house as if it gains and loses heat the same way. Houses do not.

Picture two 2,000-square-foot homes in St. Louis County. One is a 1950s brick ranch with original windows and a thin layer of attic insulation. The other is newer, tightly sealed, with low-e windows and a well-insulated attic. Same square footage, very different heat gain and loss, and they may need very different equipment.

The same problem shows up when an installer simply matches the old unit. If you have added insulation, replaced windows, or finished a basement since the last system went in, the old size may no longer fit. It may never have been right in the first place.

What a Load Calculation Looks At

The industry method is a load calculation, commonly called a Manual J. It estimates how much heat your specific home gains in summer and loses in winter, based on:

• Insulation levels in walls, attic, and floors
• Window size, type, and the direction each one faces
• Air leakage around doors, windows, and attic openings
• Ceiling height, shading and local climate
• Where your ductwork runs (an unconditioned attic adds load)
• Number of occupants and heat-producing appliances

The result is a BTU target for heating and cooling. Equipment is then chosen to match it. Two companion steps follow. Manual S selects equipment whose actual capacity matches the load, and Manual D designs ductwork that can deliver the right amount of air to each room. A correctly sized unit on undersized ducts can still underperform.

Why Bigger Is Not Better

An oversized AC cools the air quickly, shuts off, then starts again, a pattern called short-cycling. Air conditioners remove humidity only while they run long enough, so short cycles can leave a home cold but clammy, which is common in humid St. Louis summers. Frequent starts also add wear, and the larger unit costs more upfront.

• An oversized furnace heats in quick bursts, which can cause uneven rooms and temperature swings.
• An undersized system may run nonstop and fall behind on the hottest and coldest days.
• Heat pumps add a wrinkle because a single unit handles both cooling and heating. Sizing has to balance St. Louis summers and winters, and a backup heat source may be part of the design.

One note: a correctly sized AC may run for long stretches on a 100-degree afternoon. That is normal and part of how it controls humidity.

What This Means When You Replace a System

If you are getting quotes, ask each contractor how they sized the equipment. A clear answer describes what they measured in your home, not just the square footage or the old unit’s label. For more questions worth asking, see Questions to Ask Before You Sign an HVAC Replacement Quote or learn about HVAC installation in St. Louis.