HVAC · Industry standard
BTU & Tonnage
Also called: BTU · British Thermal Unit · Ton (HVAC)
BTU (British Thermal Unit) measures heating or cooling output; one BTU is the heat needed to raise one pound of water 1°F. In HVAC, a ton equals 12,000 BTU per hour of cooling. A 3-ton AC delivers 36,000 BTU/hr. Austin homes typically need 2.5 to 5 tons depending on size, orientation, and insulation.
Definition
BTU and tonnage are the two ways the HVAC trade talks about capacity. BTU/hr is the engineering unit — heating or cooling output per hour. Tonnage is the shorthand for residential cooling sizes, where 1 ton = 12,000 BTU/hr. The term comes from the cooling power of one ton of melting ice over 24 hours.
Residential AC and heat-pump sizes in Austin almost always fall in half-ton increments: 2 ton (24,000 BTU/hr), 2.5 ton (30,000 BTU/hr), 3 ton (36,000 BTU/hr), 3.5 ton (42,000 BTU/hr), 4 ton (48,000 BTU/hr), 5 ton (60,000 BTU/hr).
A common rough-cut number is 600 sq ft per ton in a typical Austin build, but the only reliable way to size is a Manual J load calculation — square footage alone overlooks insulation level, window orientation, duct loss, and infiltration rate, all of which swing the right tonnage by half a ton or more.
Why it matters in Austin
Oversizing is the most common installation mistake in Austin. A 5-ton AC dropped into a home that only needs 3.5 tons will short-cycle on mild days, never run long enough to wring humidity out of the air, and leave the house clammy at 76°F instead of crisp at 73°F. Undersizing has the opposite problem — the unit runs continuously through summer afternoons and still can't keep up.
The right tonnage is the difference between a comfortable home and an expensive, humidity-soaked one. We Manual J every install — it's not optional and it's not an upcharge.
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