Low pressure: city problem or house problem? 10-minute test

Low water pressure is one of those problems that feels like it should have a simple answer. Either the utility is failing to deliver enough water, or something in your house is holding it back. The trouble is that both problems look identical from the shower. You turn the tap, you get a weak stream, and you have no idea whether to call the water department or a plumber. The good news is that a ten minute test can usually tell you which side of the meter the problem is on, and that answer decides everything that comes after.

1. Start with the split test

Pick two fixtures on opposite ends of the house. A hose bib out front and a bathroom sink at the back is a good pair. Run both at the same time and compare the flow. Then shut one off and watch whether the other improves.

Here is what the results mean. If every fixture in the house is weak, including the outdoor spigot closest to where the main line enters, the restriction is probably at or before the meter. If some fixtures are strong and others are weak, the problem is inside your plumbing. If one fixture is weak on its own but fine when nothing else runs, you have a flow problem that shows up only under demand, which is a different diagnosis than a fixture that is always weak.

Write down what you find before you change anything. Homeowners often start adjusting valves and lose track of what the system was doing before they touched it.

2. The outdoor spigot is your best diagnostic tool

The hose bib is usually the first fixture off the main line, so it gets water before the rest of the house does. That makes it the cleanest test of what the supply is actually delivering.

Attach a pressure gauge to the spigot. These cost about fifteen dollars at a hardware store and screw onto a standard threaded bib. With no water running anywhere in the house, open the valve and read the gauge. Then open a couple of indoor fixtures and watch what the gauge does.

Static pressure under 40 psi with everything off points to a supply side problem.
Static pressure between 40 and 80 psi is a normal range for most residential systems.
Pressure that drops more than 10 to 15 psi when fixtures open suggests a restriction or an undersized line.
Pressure above 80 psi is its own problem. That is too high and usually means you need a pressure reducing valve, not more pressure.

If the spigot reads fine and holds steady, your supply is healthy and the issue is downstream of that point.

3. What the numbers actually tell you

Pressure and flow are related but not the same thing. Pressure is the push. Flow is the volume that push delivers. A system can have good pressure and poor flow if something is choking the pipe, and it can have poor pressure and decent flow if the supply itself is weak.

This table covers the common readings and what they usually mean. Treat it as a starting point, not a verdict.

Reading at outdoor spigot	What happens when you open a tap	Likely cause
Below 40 psi, steady	All fixtures weak, no change	Supply side issue at or before the meter
40 to 80 psi, steady	All fixtures weak, no change	Interior restriction such as a clogged aerator, failing valve, or corroded pipe
40 to 80 psi	Pressure drops sharply when a second fixture opens	Undersized main, partially closed valve, or a restriction in the service line
Above 80 psi	Fixtures seem fine or even forceful	No low pressure problem. High pressure needs a reducing valve
Fluctuates up and down	Weakness comes and goes	Supply side fluctuation or a failing pressure reducing valve

If you do not own a gauge, you can still get useful information. Fill a quart container at the spigot and time it. A typical outdoor spigot should fill a quart in roughly five to eight seconds. Much slower than that, and you have a real flow problem worth measuring properly.

4. City problem: what a supply side issue looks like

When the problem is on the utility side, the symptoms are consistent and broad. Every fixture is weak. The weakness does not change much between morning and evening. The outdoor spigot is just as weak as the kitchen sink. Neighbors on the same block often notice the same thing at the same time, which is worth asking about because it saves you a service call.

Common supply side causes include main line work nearby, a failing pressure regulator at the meter, sediment stirred up by construction, or a service line that has partially collapsed or scaled shut over decades. Older galvanized service lines are notorious for this. They corrode from the inside until the opening is a fraction of what it used to be.

If your gauge reads low at the spigot and stays low, call the water utility before you call anyone else. Many will test pressure at the meter for free. If they confirm low delivery on their side, the fix is theirs. If they confirm normal pressure at the meter and your house is still weak, the problem is in the service line between the meter and the house, which is usually the homeowner's responsibility.

5. House problem: what an interior restriction looks like

Interior problems are usually uneven. One bathroom is fine and the other is not. The kitchen sink is strong and the shower is pathetic. Hot water is weak but cold water is fine, or the reverse. That unevenness is the tell.

Work through the likely culprits in order of how cheap they are to check.

Aerators and showerheads. Unscrew the aerator from the faucet and look at the screen. Mineral buildup and debris from recent main line work clog these constantly. Soak them in vinegar or replace them for a few dollars.
Shutoff valves under sinks and behind toilets. These are often gate valves that have not been turned in years. They seize half open and nobody notices until pressure drops. Open and close them a few times to confirm they are fully open.
Clogged cartridge or mixing valve. A failing cartridge in a single handle faucet can restrict flow to that fixture only.
Galvanized or steel interior piping. If your house still has original steel pipes, they narrow over time from corrosion. Weak pressure throughout an older home often traces back to this.
Partially closed main valve or a failing pressure reducing valve. Check the main shutoff inside the house first. A valve that is 90 percent closed instead of fully open will throttle the whole house.
Water softener or whole house filter. A clogged filter cartridge or a softener in bypass or failure mode can cut pressure noticeably. Check the bypass and replace filter media on schedule.

One more interior cause worth knowing about: a slab leak or a hidden leak. A broken pipe under the foundation bleeds pressure before it reaches your fixtures. If your water bill jumped alongside the pressure drop, or you hear water running when everything is off, that changes the diagnosis and the urgency.

6. When to stop testing and call someone

The ten minute test gives you a direction, not a repair. If the gauge says supply side, your next call is the utility. If the gauge says interior, you can handle aerators, showerheads, and shutoff valves yourself in most cases.

Call a plumber when the pressure drop is house wide and the outdoor spigot reads normal, when you suspect the service line or interior piping is corroded, when you see a pressure reading above 80 psi, or when your water bill and your pressure moved in opposite directions at the same time. Those situations need a proper diagnostic, not another round of guessing.

If you have run the test and you are still not sure which side of the meter the problem is on, we will come out and check it. We test pressure at the spigot, at the meter, and at the fixtures, and we tell you plainly whether the fix is yours or the utility's. Call (718) 555-0142 for a free estimate or to schedule an inspection.