Inches of Water (inH₂O)

A fine-grained pressure unit for very low pressures, used in gas supply, flue draught and ventilation work.

Where feet of water suit mains water pressure, inches of water suit the far gentler pressures of gas and air. British gas fitters still test domestic supply in inches of water gauge, and ventilation engineers use it to specify flue draught and duct resistance. The unit is measured with a simple U-tube manometer, making it as verifiable on site today as it was in the Victorian gasworks.