Low water pressure diagnosis and repair field guide for plumbers

Low water pressure and the diagnostic approach

Low water pressure is weak force at the fixture, the water dribbling where it should push. The job is not to make it stronger by reflex. It is to find why it went weak and fix that, because the wrong fix wastes a day and the pressure stays low. A plumber who replaces a showerhead on a whole-house problem has fixed nothing and billed for it.

The approach is a series of cuts that each cut the suspect list roughly in half. First, is this a pressure problem or a flow problem, because they have different causes and different fixes. Then, is it the whole house or one fixture, which separates a supply or PRV problem from a clog at a single outlet. Then, did it come on suddenly or build over time, which separates a break or a valve from scale and corrosion. Then, is it hot, cold, or both, because hot-only points straight at the water heater. Four questions and you have usually boxed the cause into a corner before you open a single fitting.

Pressure and flow lean on the supply behind them, so two companion calculations sit under this whole subject. Whether the building pressure is being cut too far or boosted is the booster and PRV problem. Whether the pipe was ever sized to carry the demand is the supply-sizing problem. Both are covered in their own guides, and a low-pressure call often ends by pointing at one of them.

Pressure vs flow: the distinction that decides the diagnosis

Pressure and flow are not the same thing, and confusing them sends a diagnosis down the wrong road on day one. Pressure is the force pushing the water, measured in psi, and the truest reading of it is static pressure, the psi with nothing running. Flow is the volume that actually comes out, measured in gallons per minute, and it is what a customer sees and calls pressure even when the pressure is fine.

Here is the part that separates a diagnostician from a parts-changer. A clog cuts flow, not pressure. A scaled aerator, a restricted cartridge, a half-closed stop, a pinched line, all of them choke the volume while the static psi behind them can read perfectly normal. So a fixture that trickles can sit on a system that gauges 65 psi, and the answer is at the fixture, not the supply. The relationship runs one way: low pressure always drags flow down with it, but low flow does not prove low pressure.

Carry that asymmetry into every call. If the static pressure measures fine and the fixture is still weak, you have a flow restriction, and you go looking for the choke point. If the static pressure measures low, you have a real pressure problem, and that lives in the PRV, a partly-closed valve, the supply, or the source. Run the wrong errand and you will replace a perfectly good fixture on a supply problem, or repipe a house when one aerator was plugged.

The first questions before you touch a tool

Three questions on the doorstep narrow the fault more than the first hour of poking ever will. Ask them, listen, and you walk to the right part of the house instead of starting at the meter every time.

Whole house or one fixture? One weak fixture with the rest of the house strong is a local restriction, an aerator, a cartridge, a stop, or a supply line, and you can ignore the whole supply side. Every fixture weak at once is a whole-house cause, the PRV, a main or meter valve, scaled pipe, a leak, or the source. This single question splits the suspect list cleanly down the middle.

Sudden or gradual? A sudden drop is a thing that moved or broke. A PRV that failed, a valve someone closed, a line that froze or split, a meter the utility throttled. A gradual fade over months or years is something accumulating, scale closing down old galvanized pipe from the inside or corrosion narrowing the bore. The onset tells you whether to hunt for an event or for buildup. Then, hot, cold, or both? Hot-only weakness points at the water heater and its nipples and sediment, not at the supply, and that one answer can end the call before you reach the basement.

How do you measure water pressure?

Thread a pressure gauge onto an outside hose bibb, shut off everything drawing water in the building, and read the static psi. That reading, taken with nothing running, is the number the whole diagnosis turns on, and it is the step most people skip in their hurry to start replacing things. Measure first. You cannot tell a pressure problem from a flow problem by feel at the faucet.

Normal residential static pressure runs roughly 40 to 80 psi, with most comfortable systems landing in the 45 to 60 range. Below about 40 the fixtures start to feel weak, and 80 is the code ceiling above which a pressure reducing valve is required, covered in the booster and PRV guide. A cheap gauge from the supply house with a hose-thread fitting reads it in seconds, and a gauge with a lazy hand that holds the peak will catch the overnight high if you leave it on.

Take the reading where it tells you the most. A gauge at the hose bibb near the meter reads the supply coming in. A gauge at the water heater drain or a laundry connection reads what the house sees after the PRV. If the bibb near the meter reads 70 and a fixture upstairs trickles, the pressure is fine and you have a flow restriction. If the bibb itself reads 30, the problem is upstream of the house and you go to the PRV, the main valve, and the meter next.

When only one fixture is weak

One weak fixture on an otherwise strong house is almost always a flow restriction right at that fixture, and the causes rank by how often they bite. Start at the cheap end and work in, because the cheap end is where the answer usually is.

The aerator is the number-one easy fix, the screen at the tip of the faucet packed with scale and grit until barely a thread of water gets through. The showerhead clogs the same way at its face. Inside a single-handle faucet, the cartridge or the mixing valve can scale up or partly fail and choke one fixture, and on a tub-shower a worn pressure-balancing valve can starve the hot or cold side. The angle stop under the sink or behind the toilet may be only part open, a quarter turn from where someone left it, which throttles that one fixture and nothing else. And a supply line can be kinked behind the vanity or crushed where a cabinet was shoved back against it.

Test the way the fault sorts itself. If hot and cold are both weak at the fixture, suspect the aerator, the stop, or the line, the things common to both. If only one temperature is weak, suspect the cartridge, the valve, or that one supply stop. Pull the aerator and run the faucet open: if the flow jumps with the aerator out, you found it in thirty seconds and never had to chase the supply.

Cleaning the aerator and showerhead

The aerator and the showerhead are the first things to clean on any weak fixture, because they catch everything the water carries and they are free to check. Unscrew the aerator from the spout, and most thread off by hand or with tape-wrapped pliers so you do not chew the flats. Inside you will find the screen and the insert caked with mineral scale, sand, or pipe debris, and on a fresh repipe or a water-main repair you will find a slug of grit that the work knocked loose.

Soak the parts in white vinegar to dissolve the scale, commonly thirty minutes to an hour for a light deposit and longer for a hard one, then brush the screen clean and flush it under the tap. A showerhead gets the same treatment, soaked or bagged with vinegar over the face overnight if it is bad. Rinse everything before it goes back so you are not feeding the grit you just loosened straight back into the screen.

When you reassemble, run the fixture wide open for a few seconds with the aerator still off to blow any remaining debris clear, then thread the clean aerator back on. If cleaning brings the flow back, the diagnosis is done and it cost nothing. If a freshly cleaned aerator clogs again within weeks, that is a different message: something upstream is shedding scale or rust into the line, and now you look at the pipe and the water heater rather than the fixture.

When the whole house is weak

Every fixture weak at once moves the cause off the fixtures and onto the supply that feeds them all. The suspect list here is short, and the sudden-versus-gradual answer from the doorstep already tells you which half to look at first.

A failed pressure reducing valve is the most common whole-house cause, and it usually shows up suddenly when the valve sticks down and throttles the whole building. A main shutoff or the meter valve left only part open does the same thing and is the fastest one to rule out, because it is free to check. Old galvanized pipe scaled nearly shut from the inside starves the whole house, but it does it gradually over years, so it fits the slow-fade story, not the sudden one. A hidden leak large enough to bleed off pressure pulls the system down, and the meter test finds it. And the source itself can be the problem, a city main running low at peak demand or on high ground, or a well system that is not keeping up.

Work them in order of effort. Check the main and meter valves are fully open, then test before and after the PRV, then look at the pipe age and material, then run a meter test for a leak, and only then call the supply itself low. Most whole-house calls end at the PRV or a half-closed valve, and the ones that do not are usually old galvanized or a soft source.

The partly-closed valve nobody checked

Before you condemn a PRV or a pipe, put your hand on every valve in the path and confirm it is all the way open. This is the dumbest cause and the one that burns the most pride when you find it after an hour of harder work. A main shutoff, a meter valve, a building isolation valve, or a fixture stop left part open throttles the flow exactly like a clog, and the static pressure downstream of it reads low because the valve is eating the pressure across itself.

It happens more than anyone admits. A valve gets closed for a repair and reopened by feel, not all the way. A multi-turn gate valve looks open with the handle off the stop but the gate is half down inside. A quarter-turn ball valve gets bumped to forty-five degrees behind an appliance. The water authority works the curb stop or the meter valve and leaves it shy of full open. Any of those throttles the whole house or one fixture and reads as a pressure problem.

The check is one minute. Trace the path from the meter in, and open every valve fully, then back a quarter turn off the hard stop so it does not seize. If the pressure or flow comes back, you are done, and you did not need a single part. It is worth doing first precisely because it costs nothing and rules out an embarrassing miss.

Why a failed PRV is the common culprit

A pressure reducing valve sits near the point of entry and holds the building pressure below the street pressure and under the 80 psi code ceiling. When it fails, it can stick high and let the pressure climb, or stick low and choke the whole building, and the low-side failure is the one that shows up on a low-pressure call. The valve closes down further than it should and holds the house at a trickle no matter how good the street pressure is behind it.

Test it with the gauge on both sides. Read the pressure ahead of the PRV, on the street side, and behind it, on the house side, and compare. Strong street pressure with weak house pressure across the valve is a PRV that has failed low or scaled up internally. Many PRVs have an adjusting screw, so before you condemn it, try adjusting the setpoint up while you watch the downstream gauge: if it responds and holds a higher pressure cleanly, it just drifted. If it will not come up, hunts, or creeps back, it needs a rebuild kit or replacement.

The PRV is also a wearing part with a strainer that fouls, so a piece of debris on the seat can hold it part closed. The full picture of PRV failure, the creep, the strainer service, the closed-system expansion tank that goes with it, and the setpoint and sizing, lives in the booster and PRV guide. On a low-pressure call, the PRV is the first whole-house suspect after you confirm the valves are open, because it is the part built specifically to control the pressure and the part most likely to fail doing it.

Scaled-shut galvanized pipe

Old galvanized steel pipe closes itself off from the inside, and it is the classic gradual whole-house cause in a house built before copper and plastic took over. The zinc coating wears, the steel corrodes, and decades of rust and mineral scale build up on the inside wall until a 3/4 in pipe carries water like a 3/8 in pipe. The bore shrinks slowly, the pressure and flow fade over years, and the owner gets used to it a little at a time until a new fixture or a guest makes it obvious.

The tells are specific. The house is old and the visible pipe is steel, threaded, often rusty at the fittings where the wall is thinnest. Horizontal runs scale worse than verticals. The hot side often runs weaker than the cold, because heat speeds the scaling. And the pressure measured at the meter reads fine while the pressure at a fixture reads low, which says the restriction is in the house piping, not the supply. Cut a section of suspect pipe and look at the bore: if the opening is choked down to a pencil, you have your answer in your hand.

There is no cleaning this back. Once galvanized has scaled to where it starves the house, the fix is a repipe in copper or PEX, in part or in whole depending on how far it has gone. Replacing one segment and leaving the rest just moves the bottleneck downstream. Size the repipe to the demand off the supply-sizing guide, because the original galvanized was often undersized to begin with and you have a chance to fix both problems at once.

Hot water only: the water heater

Weak hot water with strong cold water points at the water heater and the fittings around it, not at the supply, and it is the fastest diagnosis on the list because the cold side proves the supply is fine. If the cold runs full and only the hot trickles, you do not need to look at the meter, the PRV, or the main. The restriction is in the hot path, and the hot path starts at the heater.

A few things choke the hot side there. The heat-trap nipples at the top of the tank, the short fittings with internal flaps or balls that stop convection, can scale up or jam and strangle the outlet. Sediment, the calcium and magnesium that drop out as water heats, piles up in the bottom of the tank and can foul the outlet and the dip tube. On the cold inlet, the dip tube can break down and shed plastic that lodges in fixtures downstream. And any isolation valve on the hot side that is part closed does the same as anywhere else.

Walk it from the tank out. Confirm the hot isolation valve is full open, then check the heat-trap nipples, which thread out and can be cleaned or swapped. Flushing the tank clears the sediment and is worth doing anyway for the life of the heater. If the whole-house hot is weak after that, suspect the nipples or a failed dip tube. If only one fixture's hot is weak, you are back to that fixture's cartridge or stop, not the heater.

Undersized supply and too many fixtures running

Some low-pressure complaints are not low pressure at all. They are low flow under load, the shower fading the moment someone flushes or starts the dishwasher, while a single fixture alone runs fine. That is the signature of an undersized supply or a service that cannot carry the simultaneous demand. The static pressure reads normal at rest, but the pipe is too small to deliver the volume when several fixtures pull at once, so the residual pressure collapses on the longest run.

Recognize it by the pattern. The trouble appears with simultaneous use, not at a single tap. Each fixture passes its own test in isolation and they fail together. The static reading is fine, but the residual, the pressure with water moving, drops hard under load. That is a sizing problem in the pipe or a choke at the meter or a partly-scaled main artery, and no amount of fixture cleaning touches it.

This is where the diagnosis hands off to the supply-sizing guide. Confirm whether the service, the main, and the busy branches were ever sized to the building's diversified demand, and check the meter is not the choke. Sometimes the cure is upsizing a starved segment or the meter; sometimes the supply is fine and the real limit is the source pressure, which is the booster question. Measure the residual at the worst fixture during real simultaneous use before you decide which it is.

Low municipal supply and peak demand

Sometimes the house is fine and the street is the problem. A municipal main can run low because the building sits on high ground in the pressure district, because the utility runs the whole district lean, or because the pressure sags at peak demand when the neighborhood draws hardest in the morning and evening. The pipe is right, the fixtures are clean, and the pressure coming in is just not there.

Confirm it at the source. Measure the static pressure at the hose bibb nearest the meter at different times of day, including the busy morning hour, and you will catch a supply that reads acceptable at noon and sags at 7 a.m. If the pressure at the meter itself is low and the house piping is sound, the limit is upstream of you. Call the water utility, because they can tell you the district pressure, check the meter and the curb stop, and confirm whether they have throttled or whether a main project is suppressing the area.

If the supply is genuinely and chronically low, no PRV adjustment and no repipe fixes it, because there is no pressure to redistribute. That is the booster case. The booster and PRV guide covers when a pump is the right answer and how it is sized, and it makes the point that a booster cannot cure an undersized service feeding it. Prove the supply pressure is the limit, with readings at the meter under peak, before you reach for a pump.

When the source is genuinely low: the booster

Where the supply pressure itself is too low to serve the building, the fix is a booster pump that adds the pressure the street cannot give. This is the right answer only after you have proven the source is the limit, not the pipe, the PRV, or a clog, because a pump installed on the wrong problem just makes a weak system loud.

The booster takes suction near the point of entry and holds a discharge pressure setpoint, and on anything but the smallest system the trade runs variable-speed so it tracks demand and does not short-cycle. The sizing, the suction protection that keeps the pump from pulling a vacuum on the main, the break tank some water authorities require, and the high-rise zoning all live in the booster and PRV guide. On a diagnosis call, the booster is the destination you arrive at when the readings say the source is low and the supply behind it was sized right, not the first tool you reach for.

Well systems: pressure switch, tank, and screen

On a well, the pressure is made on site, so low pressure has its own short list that a city service does not. The pressure switch turns the pump on at a cut-in and off at a cut-out, commonly a 40/60 setting, and a switch out of adjustment or with pitted contacts can run the band low or fail to bring the pump up. Confirm the switch is cutting in and out where it should before you blame anything else.

The pressure tank is the next suspect. A waterlogged tank with a lost air charge makes the pump short-cycle and the pressure surge and sag, and the cure is checking the pre-charge, commonly set about 2 psi below the cut-in, with the system drained. Beyond the switch and the tank, a well can lose pressure from a failing pump, a dropping water table, a clogged foot valve or intake screen, or a fouled sediment filter or pressure tank tee scaled with iron and mineral. Check the simple electrical and tank items first, then move to the screen and the filter, then to the pump and the well itself, which is the expensive end.

Hidden leaks and the meter test

A leak large enough to bleed off pressure shows up as a whole-house weakness that nothing at the fixtures explains, and the meter is how you catch it. Water that escapes underground in the service line, under the slab, or inside a wall pulls the system down and can also show as an unexplained jump on the bill, a wet spot, or the sound of running water with everything shut.

Run the meter test. Shut off every fixture and appliance that draws water, then watch the meter. A meter that keeps creeping with everything off is registering a leak somewhere between it and the last closed fixture. Many meters have a small low-flow indicator, a dial or a triangle, that spins on a leak too small to see on the main register. Note the reading, leave it an hour with nothing used, and read again: any movement is water going somewhere it should not.

There is also a pressure-decay version. Pressurize the system, close the supply at the main, and watch the gauge. On a tight system the pressure holds. A gauge that bleeds down with the supply closed says the system is leaking, and the rate of decay hints at the size. Isolate sections, the irrigation, the service, the hot loop, to narrow where it is before you start opening walls or ground.

Thermal expansion: the related high-pressure problem

Thermal expansion is the other side of the pressure coin, and it is worth a word on a low-pressure call because the two get confused. When a PRV or a check valve closes the building into a one-way system, the water the heater expands has nowhere to go and the pressure spikes, which is a high-pressure problem, the opposite of what you came for. It shows up as a relief valve weeping or a system that bangs, not as weak fixtures.

It matters here because installing a PRV to fix high pressure creates the closed system that then needs an expansion tank, and a missing or wrong-charged expansion tank is a common after-effect of pressure work. If a low-pressure complaint turns out to be a failed PRV and you replace it, you may be making a closed system that now needs the tank. The expansion tank, its pre-charge, and the closed-system rules are covered in the booster and PRV guide. Mention it on the ticket so the pressure fix does not hand the owner a relief-valve drip a month later.

The systematic workflow

Run every low-pressure call the same way and you stop guessing. The order moves from free and fast to expensive and slow, and most calls end early.

First, ask the three questions: whole house or one fixture, sudden or gradual, hot, cold, or both. Second, measure the static pressure at the hose bibb with everything off, and decide whether you have a pressure problem or a flow problem. Third, isolate: if it is one fixture, work that fixture; if it is the whole house, work the supply. Fourth, check the easy things before the hard ones, the aerator and the supply stop on a fixture, the main and meter valves on the house, then the PRV, then the pipe, the supply, and the source.

The discipline is in the order, not the steps. A plumber who measures before opening anything never repipes a house over a plugged aerator and never replaces an aerator on a failed PRV. The reading tells you which branch of the tree to climb, and the whole-house-versus-fixture answer tells you which side of the house to stand on. Work it top down and the cause falls out instead of getting chased.

• Ask: whole house or one fixture, sudden or gradual, hot, cold, or both.
• Measure static pressure at a hose bibb with everything off; normal is roughly 40 to 80 psi.
• Decide pressure problem or flow problem from the static reading.
• If one fixture, check the aerator, then the cartridge or valve, then the supply stop and line.
• If whole house, confirm the main and meter valves are full open, then test before and after the PRV.
• If still low, check pipe age and material, run a meter test for a leak, and read the source pressure under peak.
• Fix the confirmed cause, then flush the debris and clean the aerators before you call it done.

Flushing debris after the repair

Any repair that opens the system sheds debris into the lines, and that debris lands in the aerators and screens downstream, so the last step of a pressure job is to flush it out. A repipe, a PRV change, a main repair, a water heater swap, all of them knock scale and cut chips loose, and a customer who had strong pressure restored will call back the next day about a weak faucet that is just a screen full of fresh grit.

Flush before you reinstall the screens. Pull the aerators and the showerheads, open the fixtures wide, and run cold and hot at the high-volume taps like the tub spout and the laundry to push the debris out where it can leave the system instead of catching at a small screen. Run it until the water clears. Then clean or replace the aerators and put them back, because the ones you pulled likely caught the first slug already.

On a new water heater, flush the lines before you blame the heater for weak hot water, because the work often loosens scale on the hot side. The flush is cheap insurance against a callback, and it turns a repair that looks done into one that is.

Commercial and high-rise: the top floors go first

In a tall building, low pressure shows up at the top first, because the upper floors live at the thin end of the pressure budget. Every foot of height costs about 0.43 psi of static, so a fixture high in the building has the least pressure to start with and is the first to go weak when the source sags or demand peaks. A complaint that the top floors are weak while the lower floors are fine is the high-rise signature, and it is a delivery problem, not a fixture one.

Diagnose it by floor. Read the pressure at the top of the affected zone and compare it to the lower floors and to the booster discharge. If the building runs a booster and pressure zones, a weak top floor can be a booster that lost a pump or fell off its setpoint, a zone PRV failed low, or a riser that cannot carry the peak. The zoning, the booster staging, and the suction rules are the booster and PRV guide's territory. On the diagnostic side, the lesson is to read pressure where it is weakest, which on a high-rise is the top of the zone at peak demand, not the gauge in the mechanical room.

Data center and facility water

On a data center or a large facility, low domestic or makeup-water pressure reads differently because the big water load is not the restrooms. It is the makeup for evaporative or adiabatic cooling and humidification, a near-continuous demand that can dwarf the fixture load and that does not diversify the way intermittent fixtures do. A weak reading on a makeup line at peak cooling demand is a sizing or supply problem on that line, not a fixture clog.

Diagnose with the load in mind. Read the pressure at the makeup connection while the mechanical demand is high, not at a quiet hour, because the line can gauge fine at rest and sag when the cooling system pulls. Confirm the service and the makeup branch were sized for the continuous mechanical load added on top of the diversified fixture load, which is the supply-sizing guide's large-facility case, and confirm the backflow protection on the makeup line is not the choke. The fix is usually upsizing the starved segment or boosting a genuinely low source, decided the same way as any building, by measuring before replacing.

What to document

A low-pressure call that gets fixed without a record is a call that comes back as a mystery, because the next person has no idea what was measured or ruled out. The record is what turns a guess into a diagnosis and what lets the next tech start where you stopped instead of from scratch.

Capture the static pressure you measured and where you read it, the answers to the three triage questions, what you ruled out and what you confirmed, the cause you found, and the fix. If you adjusted or replaced a PRV, record the new setpoint. If you found a partly-closed valve, say so plainly, because it tells the next person to check the obvious first. The table below pairs a symptom with its likely cause and the check that confirms it, which is the shape the documentation should take.

Common mistakes

• Replacing fixtures or pipe without ever measuring the actual static pressure first.
• Ignoring the pressure-versus-flow distinction and chasing pressure when a clog cut the flow.
• Overlooking a clogged aerator or showerhead, the cheapest and most common fix on a single weak fixture.
• Missing a partly-closed main, meter, or supply valve that throttles the system like a clog.
• Missing a PRV that failed low or scaled up, the most common whole-house cause after a closed valve.
• Not recognizing scaled-shut galvanized pipe behind a slow, gradual whole-house fade in an old house.
• Replacing fixtures when the real cause is an undersized supply, a low municipal source, or a well problem.
• Calling a hot-only problem a supply problem instead of looking at the water heater nipples and sediment.
• Skipping the meter test, so a pressure-bleeding hidden leak gets chased as a supply problem.
• Leaving the system full of repair debris that clogs the aerators and brings the customer right back.

Standards and references

The plumbing code sets the pressure framework the diagnosis works inside. The Uniform Plumbing Code and the International Plumbing Code both treat the building distribution as needing enough pressure to serve the fixtures and cap the static at 80 psi, above which a pressure reducing valve is required, the UPC carrying it at 608.2 and the IPC at 604.8 in recent editions. The codes also set the minimum residual flowing pressure each fixture needs, which is the floor a weak fixture has fallen below. The exact section and table numbers shift between code cycles, so confirm them against the edition the jurisdiction has adopted and any local amendments before citing one.

The common 40 to 80 psi working range and the 45 to 60 psi comfort band are field figures and code-anchored limits, not a single mandated setpoint, so treat them as the window to land in rather than a number to hit exactly. The fixture and PRV manufacturers control the rest: the residual pressure and flow a given fixture needs, the setpoint range and rebuild parts for a PRV, the heat-trap and dip-tube details on a water heater, and the cut-in and cut-out and pre-charge on a well system. Where a manufacturer's figure is tighter than the rule of thumb, the manufacturer governs.

The water utility is the other governing party on anything upstream of the building. It sets the guaranteed service pressure, owns the meter and the curb stop, and is who you call when the static reads low at the meter. Cite the standard that controls the point, hedge the psi ranges to the adopted code and the manufacturer, and let the project documents and the utility override any rule of thumb in this guide. Above all, measure before you replace, separate pressure from flow, and check the PRV.

Units, terms, and conversions

Low-pressure work mixes a couple of units and a handful of terms that get used loosely, and the same idea reads differently across a gauge, a fixture sheet, and a code table.

Pressure is psi in the field, with kPa and bar on metric and manufacturer sources, and 80 psi is about 552 kPa or 5.5 bar. Flow is gallons per minute, gpm, with liters per minute on metric drawings. Static pressure is the psi with no water moving, the number you measure first; residual pressure is what is left at a fixture while water flows. Elevation converts to pressure at about 0.43 psi per foot of height, or read the other way, one psi lifts water about 2.31 ft. Low pressure is a force problem in psi; low flow is a volume problem in gpm, and the two are not the same complaint.

Static pressure

The pressure in the system with no water flowing, in psi, the first thing to measure on a low-pressure call

Residual pressure

The flowing pressure left at a fixture while water is running, what proves the far fixture is served

Flow rate

The volume delivered over time, in gpm, what a clog cuts while the static pressure can stay normal

PRV (pressure reducing valve)

A spring-and-diaphragm valve that holds building pressure below the street pressure; can fail low and choke the whole house

Aerator

The screen at the tip of a faucet that mixes air into the stream and catches scale and debris; the number-one single-fixture restriction

Heat-trap nipple

A fitting at the top of a water heater that stops convective heat loss and can scale or jam, choking the hot side

Galvanized scale

Internal rust and mineral buildup that narrows old steel pipe over years, the classic gradual whole-house cause

Pressure switch (well)

The control that starts and stops a well pump at a cut-in and cut-out, commonly 40/60 psi, with the tank pre-charged just below cut-in

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