Btu (I.T.)
The British thermal unit (International Table), Btu (IT), is about 1,055 joules, roughly the energy needed to warm one pound of water by one degree Fahrenheit. In the United States air conditioners and furnaces are rated in Btu per hour.
Btu (thermoc.)
The Btu (thermochemical) variant aligns with the thermochemical calorie and is defined as the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit at a constant pressure of one atmosphere and is equivalent to approximately 1,054.350 Joules.