Residual water pressure is defined as the water pressure in the line when water is flowing, as opposed to static pressure, which is the pressure of the water when it is not flowing. In cases of domestic demand (how water gets into our home), water pressure is never static because the system is always being drawn on from somewhere.
Calculating Water Pressure
Water pressure is the force of water pushing on a unit area measured in pounds per square inch. To calculate water pressure, you divide the force by the area on which the force is applied. Remember force is the calculation of the mass of an object times its acceleration.
Incorporating Drag and Pipe Friction
Drag and pipe friction reduces residual water pressure. This is the same as any other material traveling by another. Think about dragging your feet across the floor. It requires more effort to drag your feet across the floor than it does to lift them through the air. This is because there is less friction and drag in the air than on the solid floor.
Small Pipes
Most domestic water supply systems use gravity to maintain water pressure and deliver water to homes. If residual water pressure is low, it could be caused by a low volume of water in the system, pipes being clogged and pipes that are too small. Pipes that are too small reduce the amount of pressure that can travel through the system. Long periods of small pipes in a system will reduce the volume of water passing through, making water pressure drop regardless of the strength of supply pressure.
Elevation
Another determination of residual water pressure is elevation. It is a given that it would take more pressure to run water uphill, and the further away from the pressure source you are, the more your elevation will affect your water pressure.