What unit is used to measure gas pressure in HVAC systems, as commonly specified for operating ranges?

• A. PSI
• B. Pascal
• C. Inches of water column
• D. Bar

Answer: (C)

Gas pressures in HVAC systems are measured in inches of water column because the pressures involved are very small, and this unit provides a practical, intuitive scale. The idea is that a column of water of a certain height exerts the same pressure as the gas at that point, so readings stay within easy-to-read ranges on gauges and manometers. For context, 1 inch of water column is about 0.036 psi (roughly 249 Pa), so typical operating ranges for gas appliances—only a few inches of water column—translate to small, manageable numbers. Using psi would yield tiny fractions that are harder to read in the field, while pascals aren’t the common go-to in HVAC practice, and bars are far too large to capture these small pressures. That’s why inches of water column is the standard unit used to specify gas pressure operating ranges.