Find out whether the loss is building-wide or at one fixture. One fixture usually means a clogged aerator or stop. Building-wide loss can come from a partly closed valve, scale in older piping, a softener, a failing pressure regulator or the utility side. A gauge reading settles it.
Is it one fixture or the whole building?
If only one faucet is weak, look at that fixture. Phoenix Water Services lists debris in aerator screens on bath and kitchen sinks among the potential causes of reduced pressure. A clogged aerator or a partly closed stop under that sink is a ten-minute fix.
If every fixture is weak, the cause is upstream of the fixtures. Phoenix's list includes mineral buildup in galvanized plumbing, a water softener in need of maintenance and a partially closed house valve.
What about the utility side?
Phoenix says that pressure varies with demand, and that customers may see reduced pressure when construction in the area affects infrastructure. It also says an acceptable pressure range within the city is 40 to 100 psi, and that issues from the street to the meter are the city's, while issues after the meter belong to the property owner. If neighbors are weak too, call the utility first.
How do you measure it?
EPA's technical sheet on service water pressure describes the check: attach a pressure gauge to a hose bibb or a cold-water faucet, turn the water fully on and read it (the sheet was written for homes, but the check is the same). EPA recommends service pressure between 45 and 60 psi for efficient use and proper system function, and notes that most U.S. plumbing codes require a pressure-regulating valve where pressure exceeds 80 psi. A regulator that has worn out can leave a building with weak pressure even when the street pressure is fine. Do you need one?
What fixes it?
| Cause | Typical fix |
|---|---|
| Clogged aerator or stop | Clean or replace the part |
| Partly closed valve | Reopen and verify |
| Scale in old pipe | Replace the affected piping |
| Failing regulator | Replace or reset the regulator |
| Large demand on a tall building | Booster pump review |
| A hidden leak taking flow | Leak search; start with the meter |
When the problem is a whole-building drop that no valve explains, the plumber's gauge reading at the service entrance decides whether to look outside or inside the building.