What does a BTU measure in terms of heating?

A BTU, or British Thermal Unit, specifically measures the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. This unit is essential in the context of heating, as it quantifies energy usage in heating systems, such as gas furnaces. By understanding BTUs, one can assess the capacity and efficiency of heating equipment. This metric allows for better comparisons and decisions regarding heating systems for a given space or application, helping ensure that a furnace can adequately meet the heating demands of a home or building.

The other choices pertain to different aspects of heating and gas systems but do not accurately define what a BTU represents. For example, pressure of gas relates to the operation of gas systems but is not about measuring heat. Quantifying airflow concerns the movement of air within a space, which is crucial for ventilation but does not relate to heat measurement. Temperature of air indicates how warm or cool the air is at a given moment but does not encompass the concept of energy required for heating. Therefore, the definition tied to BTU as the heat energy needed to raise the temperature of water is the correct interpretation.