Definition
British Thermal Unit — the standard measure of heating or cooling energy. The higher the BTU rating, the more heating or cooling a system can deliver.
Why this matters for your home
One BTU is the energy needed to raise a pound of water one degree Fahrenheit — a tiny amount, which is why equipment is rated in thousands of BTUs per hour. A furnace might be 60,000–100,000 BTU/hr of input, while air conditioners are measured in tons, where one ton equals 12,000 BTU/hr.
Bigger isn't better. An oversized system blasts to temperature, shuts off, and short-cycles, leaving rooms uneven and humidity high. We size by a Manual J load calculation rather than a square-foot rule of thumb, so your equipment matches how your specific home actually gains and loses heat.