BTU/(h·ft²)
BTU per hour square foot
Intensityexact by definition
The BTU per hour square foot is the customary North American unit of heat flux, about 3.1546 watts per square metre. Full midday sun of 1000 W/m² is about 317 BTU/(h·ft²), a radiant floor delivers 25 to 40, and fire engineering treats about 4400 as the flux at which unprotected wood ignites. The conversion is exact as a fraction, the international BTU and the foot both being exactly defined, though the decimal does not terminate.
Watch out: Do not confuse this with the U-factor in BTU/(h·ft²·°F), which is a heat flux per degree of temperature difference and a different quantity entirely. Dropping the degree from the denominator turns a conductance into a flux.
| 1 BTU/(h·ft²) | 3.1545907 W/m² |
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