Why running pressure is lower than static pressure, and why smaller bores and higher flows lose more of it.
Static pressure and running (dynamic) pressure
Static pressure is what a gauge reads with every outlet shut and nothing moving — it comes from the head of water alone. As soon as a tap opens and water starts to flow, friction between the moving water and the pipe wall, and through fittings and bends, uses up some of that pressure. The reading while water is actually flowing is sometimes called the running or dynamic pressure, and it is always lower than the static figure.
Why friction loss grows so quickly
Friction loss rises sharply as water moves faster. Forcing the same flow through a smaller bore raises the velocity, because flow rate equals velocity multiplied by the cross-sectional area — shrink the area and the water must move faster to get the same volume through. That higher velocity brings a much bigger friction loss: for a fixed length of pipe, resistance to flow rises steeply as the bore shrinks, so even a small reduction in pipe size makes a large difference to what survives as pressure at the outlet.
What this means on site
- A good static pressure reading does not guarantee a strong flow: once water moves, friction losses can use up much of what looked available.
- Opening several outlets at once increases the combined flow through any shared pipework, which raises velocity and friction loss there and weakens what each outlet receives.
- A long run, a small bore, or extra bends and fittings all add friction loss, on top of whatever static head the system has.