A draft gauge on a furnace indicates the draft in what measurement?

A draft gauge on a furnace measures the draft using a water column, typically expressed in millimeters of water column. This measurement represents the difference in pressure created by the draft within the furnace compared to atmospheric pressure. The use of water column measurements is standard in assessing the efficiency of combustion and indicates how effectively the furnace is pulling air in and expelling exhaust gases.

In practical terms, the water column measurement is favored because it relates to the physical properties of the system being monitored, making it easier to measure small differences in pressure that can significantly affect combustion efficiency. It directly correlates to the performance of the furnace, helping ensure safe and efficient operation, which is critical for heating applications.

Other measurement types, such as millimeters of mercury or atmospheric pressure, are not typically used for draft gauges in this context because they reflect different standards or scales that do not relate directly to the draft conditions within a furnace. Inches of water column is also a common measurement, yet in the context provided, the specific mention of millimeters aligns with common practices in various regions where metric measurements are standard.