inch of water column
The inch of water column, also written in. w.g. for inches of water gauge, is the pressure at the bottom of a one inch column of water, conventionally about 249.09 pascals. It is a conventional unit because water density varies with temperature, and reference temperatures of 39.2 °F, 60 °F and 68 °F are all in circulation with values differing in the fourth digit. It is the standard unit of duct static pressure and gas appliance pressure in North American HVAC.
Watch out: This is a very small pressure, about 1/28 of a psi, and its smallness is the point: a whole air handling system might operate at 2 in w.g. Confusing it with psi overstates duct pressure by a factor of 28. Note also that natural gas appliance manifold pressure is typically 3.5 in w.g. and propane 10 to 11 in w.g., so the unit is what a manometer on a gas valve is actually reading.
| 1 in w.g. | 0.24908891 kPa |
About the inch of water column
Air moves under tiny pressure differences, and the inch of water column exists because no larger unit resolves them usefully. A residential furnace fan might develop 0.5 in w.g. of total static pressure, which is 125 Pa, or about 0.018 psi. Working in psi would require three leading zeros on every number, so the trade uses a unit sized to the job. The instrument matches the unit: an inclined manometer is literally a tube of water, and the reading is the height difference the pressure supports.
Two places it shows up constantly. Duct design specifies friction rate in inches of water per 100 feet of duct, typically around 0.08 to 0.1, and total external static pressure is what determines whether a given blower will actually move the design airflow. Gas piping uses the same unit for manifold pressure, where a natural gas appliance wants 3.5 in w.g. at the burner and a reading well below that points to undersized pipe or a failing regulator. Because it is a gauge measurement referenced to the surrounding room, a negative reading on the return side is entirely normal and simply means the fan is pulling.