HVAC draft readings are typically expressed in inches of water column.

• A. Psi
• B. Pascals
• C. Inches of water column
• D. Bars

Answer: (C)

Draft readings in HVAC are expressed in inches of water column because the pressures involved are very small and the height of a water column provides a convenient, intuitive measure for these differential pressures. The inch of water column (in. w.c.) directly relates to the pressure needed to support a column of water, making it easy to read on gauges and to compare values across manuals and codes. In practice, technicians measure the slight negative or positive pressure differences created by burners, fans, and venting systems with meters calibrated in inches of water, so readings like a few tenths to a few hundredths of an inch are meaningful and consistent.

Other units like psi or bars describe much larger pressures and aren’t as practical for the small draft values typical in combustion and venting systems. Pascals are a precise metric unit, but HVAC work traditionally uses inches of water for quick, everyday reference, with clear conversions available if needed. To put it in perspective, 1 inch of water column is about 0.036 psi, highlighting how much coarser those other units are for these small pressures.