Why does an inspector operate fixtures on an upper floor as well as a lower one?

Every foot of height costs pressure — roughly half a pound per square inch per foot of rise — so the fixtures highest in the house see the lowest pressure the system delivers. A supply limitation that is invisible on the ground floor, whether from corroded galvanised piping, an undersized service, a partially closed valve or a failing pressure regulator, shows up first upstairs. Running fixtures on both levels, and running several simultaneously, is how the inspector puts the system under enough demand to reveal it: a shower upstairs that drops noticeably when a lower-floor fixture is opened is the classic demonstration. The same test also finds the drainage side of the problem, since a slow upper-floor drain or a venting fault often only appears under the load of an upper fixture discharging into the stack.

Pressure falls with height, so upper-floor fixtures see the least pressure and reveal supply limitations that lower fixtures would not show.

Static pressure decreases with elevation, so upper-floor fixtures operate at the lowest system pressure. Running multiple fixtures simultaneously loads the supply and reveals restrictions from corrosion, undersizing or regulator faults.

Height Costs Pressure. The top floor is where a weak supply confesses.

Note the piping material while testing. Galvanised supply piping corroding internally is a common cause and the material is visible at the meter or water heater.

• •Testing fixtures one at a time and missing the limitation
• •Only testing the ground floor
• •Not noting supply piping material alongside the flow finding