How to Repair Bad Water Pressure
Improve weak water flow rate at sinks, tubs, showers, or sprinklers or in tall buildings

  • CLOGGED SUPPLY PIPES, REPAIR - CONTENTS: How to Diagnose Bad Water Pressure due to clogged water supply piping in a building. What does it mean if cold water pressure is good but hot water pressure is bad? Bad water pressure repair alternatives, which method to use; Where to replace clogged building piping to improve water pressure
  • POST a QUESTION or READ FAQs about how to de-clog or de-scale building water piping
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Repair methods for improving poor water pressure traced to clogs, pipe diameter, or clogged water supply piping.

This article describes How to Cure Bad Water Pressure Due to Clogged Water Supply Pipes. The process of diagnosis and the costs of the repair are explained.

What makes us think that a water pressure (really more accurately water-flow-rate) problem is due to clogged piping? A quick diagnosis that suggests that bad water pressure at a fixture or in a building is due to pipe clogging is the observation that when you first turn on the water, in that first instant, pressure is pretty good, but falls off almost immediately to a much slower level.

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Guide to Repair of Clogged Water Supply Piping and Poor Water Pressure & Flow

Clogging pipe illustrationWater pressure falls off very quickly:

[Click to enlarge any image]

When a water supply pipe pressure has been traced to supply pipe clogging there are several repair approaches that we can try. Here are some clogged piping repair attempts along with comments about how likely it is that they’ll help.

The sketch of rust clogging of galvanized steel pipe shown here, courtesy of Carson Dunlop, shows how the inner diameter of a water supply pipe is reduced by build-up inside the pipe walls. This build-up may be due to rust, mineral deposits, or other debris in the pipe.

The reduction in pipe diameter means a corresponding reduction in the flow rate seen at building plumbing fixtures. When you first turn on a faucet water pressure may be normal or "high" but as soon as water starts to flow the pressure and flow fall off to a noticeably lower level. This is the classic diagnosis of clogged water piping. But we don't know, yet, how extensive the clogging is - a single elbow, a clogged faucet strainer, or an entire length of clogged water piping can all produce this effect.

The suggestions which follow presume you’ve already followed our diagnostic suggestions to get an idea of the type of water supply flow problem that is present.

That’s important because some water pressure problems are due to water flow problems caused by a water pump, water pump pressure control, water tank, and on municipal water supply systems, flow problems may be due to problems with the water pressure regulator, water supply piping from a municipal supply, or even water meter and main water valve problems.

Watch out: people generally use the term water pressure to talk about the force or volume of water delivered at a faucet or plumbing fixture. Properly this is water flow rate; water pressure in the system is only one factor that determines the water flow rate at fixtures. Clogged pipes, partly closed valves, length of piping runs, number of ends or elbows are other factors that reduce the actual flow rate or "pressure" observed at a plumbing fixture.

Here we discuss how to correct bad water pressure due to apparent clogs in water supply piping. First we need to do some further diagnosis to determine if the poor water flow problem is at every fixture in the building or just at some of them.

How to Correct Poor Water Pressure & Flow at One or More Plumbing Fixtures When Some Fixtures Flow Well

Water pressure versus flow rate (C) Carson Dunlop Associates 2015 used with permissionAlthough people commonly speak of "bad water pressure" when referring to weak water flow at the tap, water flow rate is not really the same as "water pressure", though the two are related. Higher pressure will give us improved water flow rate at a fixture, but other factors such as pipe diameter, pipe length, the number of bends and elbows, and frictional loss as water runs through the system have a big effect on perceived water pressure at the tub, tap or shower.

Carson Dunlop's graph at left reviews the relationship between water pressure and water flow rate for 1/2" diameter and 3/4" diameter water supply piping. You will notice that the pressure drop when using 3/4" diameter piping is significantly less than when using 1/2" piping.

The graph also shows that for a given starting water pressure (static pressure), the water delivery pressure at any plumbing fixture will drop as the flow rate increases at that fixture. That's why the shower seems to operate at higher pressure than the tub spout: the flow rate through the small perforated openings in the shower head is usually less than through the big-mouthed tub spout.

[Click to enlarge any image]

In discussing the difficulty of explaining pressure and flow Al comments:

It is tricky to understand that flow and pressure at a hose or faucet are inversely proportional. One of the ways I like to explain it is to think of a garden hose with no faucet attached. When you turn the tap on, the water comes out without much pressure, but it fills a bucket quickly. If you put your thumb over the hose and block most of the opening, the water comes out with much more pressure and you can spray somebody 10 feet away. However, it would take a long time to fill a bucket.

A bathtub is another good example. It takes a long time to fill a bathtub using the shower head. The tub faucet is much faster. On the other hand, the showerhead delivers a lot more pressure and is better for rinsing shampoo out of your hair. - A. Carson to D Friedman, private email, 14 May 2015

How to Diagnose & Fix Individual Clogged Faucet Strainers or Valves

Static water pressure (C) Carson Dunlop Associates Toronto OntarioStatic vs. Active or Dynamic Water Pressure

Let's also clear up the idea of what information you are getting when you measure water pressure.

Static water pressure is a pressure measurement obtained (using a pressure gauge) when no water is being run in the plumbing system. Static water pressure readings give us a starting point in understanding water pressure and flow rate in a building. In vertical pipes the water pressure is approximately 0.43 psi lower for every one foot of elevation.

We use the word dynamic water pressure for pressure readings when water is running though this simplified view gets some flack from some hydraulics engineers who have a more special use of that term.

See WATER PRESSURE MEASUREMENT if you need to measure your building's water pressure.

Carson Dunlop Associates' drawing at above- left explains that when no water is running pressure measured anywhere in the plumbing system will be the same. That's static water pressure.

Building height vs water pressure (C) Carson Dunlop Associates Home Inspections Toronto OntarioBuilding Height Effects on Water Pressure

In vertical pipes the water pressure is approximately 0.43 psi lower for every one foot of elevation when no water is flowing, and building height affects water pressure and flow at higher fixtures when water is flowing as well. So in tall buildings whose water supply is fed from street level or below, the starting water pressure can be quite important. Larger pipes may help improve water flow at fixtures but in buildings over three stories you may need a booster pump system as well.

Carson Dunlop's sketch at left illustrates a related fact that can help diagnose water pressure problems.

In sum, before looking for specific water pressure problems in the building we need to check the basics:

  1. What is the static water pressure?
  2. What is the building height?
  3. What are the sizes and condition of the water supply piping and what about its length and number of bends.

If poor flow is at just one fixture it’s a good bet that we can trace the flow problem to a local issue that we should be able to repair without more extensive plumbing surgery.

Slow or weak water pressure at the sink - check for debris or mineral clogging of the sink faucet strainer

If the slow fixture is a sink, first check the strainer – debris-clogged strainers can simply be un-screwed. If the pressure is suddenly wonderful you need a new or cleaned strainer, that’s all.

Slow water fill at a toilet - check or replace the toilet tank fill valve

If the slow fixture is a toilet, often the toilet tank fill valve itself becomes clogged with debris; it may be possible to remove, clean, and reinstall the valve but often it’s easier to just install a new one. We sometimes also find clogging at the small shutoff valve or water supply riser to the toilet.

Slow water flow at a shower - check for debris or mineral clogging in the shower head or flow restrictor

If the slow fixture is a shower head, debris or mineral clogging can be diagnosed by removing the shower head and, as with the sink strainer, look for good water flow. If we find that the problem was the shower head, just replace it. Some shower head models claim to be resistant to mineral clogging by using rubber or other flexible components to form the spray.

Bad plumbing valves or bad fixture supply risers: Plumbing valves or shutoff valves to individual fixtures or valves controlling sections of water supply piping can also become clogged with debris, minerals, or even a broken faucet washer.

Procedure for Fixing Clogged Faucet Strainers or Valves

Clogged Building Water Piping

If you have good water pressure at all fixtures in some building areas but not any fixtures at other building areas, you may have a clog in building piping (such as due to mineral deposits, rust, or even excessive solder in new work), or mineral and debris clogged strainers at faucets and shower heads (check these first).

Watch out: especially if plumbing fixtures in different building areas are of different ages (say you added a bathroom after the building was built), bad water pressure at all fixtures in a given building area could still be due to clogs at fixture faucet strainers or valves. But if you check or clean or replace valves and water pressure is still poor in some but not all building areas, we suspect a problem with the piping serving the poor-performing areas: mineral clogs, solder or debris clogs, etc.

See WATER PIPE CLOG DIAGNOSIS - focused on clogged building water supply piping

See WATER PRESSURE LOSS DIAGNOSIS & REPAIR - addressing all causes of inadequate water pressure & flow

How to Improve Hot Water Pressure / Flow

Poor hot water flow but good cold water flow at one or more plumbing fixtures usually diagnoses either a blockage at the water heating equipment itself or mineral clogging in the water heating system piping. Heat causes mineral precipitation out of hot flowing water faster than out of cold water, that’s why often mineral clogs appear on hot water components of a plumbing supply system first.

When only hot water supply, pressure, or flow are poor while cold water pressure is good, then we take a different approach, focusing on hot water supply equipment, piping & controls.

Note: Building water pressure that is poor at all fixtures at both hot and cold water taps may be due to clogged building cold water supply piping. Other links at the end of this article provide guides for diagnosis and repair of other causes of bad water pressure throughout a building.

How to Fix Bad Water Pressure Throughout a Building or in Large Areas:

Repairs when ALL Hot Water, all Cold, Water Fixtures, or all Both Hot and Cold Fixtures have Bad Water Pressure & Flow

How to diagnose bad water pressure in the whole building or in large areas

Where an entire building or large building areas have bad water pressure, check to see if there is clogging at a valve, pipe section, or pipe elbow close to the water source. On occasion we’ve found that a building always had bad water pressure after a plumbing “repair” had been performed near the water tank. The installer had pushed too much solder into a pipe elbow, blocking water passage through that fitting. It was discovered by making successive cuts in the supply piping beginning back near the source to check for good water flow.

Clogged water filters cause bad water flow throughout a building and can cause water pump problems (if a private well system is installed. Water pump short cycling is discussed further

How to diagnose Bad water pressure in just some areas

Where a single or one or two plumbing fittings are causing poor water pressure, due to poor installation (solder blobs), damage (broken faucet washer or other debris clogging an elbow or valve), the fix is to find and replace the culprits.

Clogged water pipe diagnosis & repair guides

What to Do to Improve Water Pressure Where Long Sections of Piping are Clogged or Too-Small in Diameter

sketch of water pipe diameter effectsClarification: don't confuse "water pressure" with "water flow rate". Most people think "water pressure" in the sense of how forcefully water seems to exit at a faucet or at the end of your garden hose. What you are seeing is some water flow rate in gallons per minute (gpm). If we can get a flow rate at the outlet of a faucet of 4gpm when we used to have just 2 gpm, most people call that "better water pressure" but it's really water flow rate.

[Click to enlarge any image]

An accurate measurement of water pressure in a system is taken when no water is running, by hooking a pressure gauge up to any supply point in the system (say the end of your garden hose) and turning on the water.

For homes on a private well you probably have a (possibly reasonably accurate) pressure gauge right at the water tank. That's what we call "static water pressure" - when nothing is running, since running water somewhere will change both the pressure we measure in the system and the flow rate we'll observe when we open a second faucet.

Local piping clogs can cause bad water pressure in entire sections of hot, cold, or both hot and cold water supply pipes and fixtures in a building. Local water supply clogs due to minerals in the water may clog entire lengths of piping so badly that the only repair is replacement of the water supply pipes.

Note: the most restrictive pipe in the the water supply system determines the maximum flow rate for any water usage fixture downstream from that restriction.

[Click to enlarge any image] In case it's not clear in Carson Dunlop Associates' illustration at above left, the formula for area of a circle is Area = pi x r2

Certainly if we find that this is what’s needed, there are two approaches that can protect the investment in improved water flow. These can both be done or  you may choose just one of them.

Improving water flow by larger pipes (C) Carson Dunlop Associates

We took this second approach on a building where we found that the hot water line clogged with a combination of minerals and iron.

On cutting open the old hot water supply pipe we saw that its interior was blocked to a mere pinhole of remaining water passage. But the cold water line was basically clear. So we replaced the hot water line, including straight runs and all elbows, with a ¾” copper pipe. The repair has continued to function well for nearly 30 years.

Contact us know if you have other questions or repair ideas or if this material leaves you with a question or suggestion.

When Increasing Water Flow Rate or Velocity or GPM, Watch out for Water Hammer & Water Scrubbing Damage

Watch out: as we discuss at WATER HAMMER NOISE DIAGNOSE & CURE, if the velocity of water in your piping system is too fast, when faucets or other controls STOP that water flow you may hear a horrible hammering or banging in the piping system. Water hammer is more than a horrible noise, it can damage equipment, cause potentially dangerous leaks at temperature/pressure relief valves, and may even cause a divorce.

A second problem with very high water velocity rates through building piping is scrubbing or wearing away of the pipe interior: a problem that occurs at very high water speeds (measured in feet per second) in piping systems. Generally the maximum safe water velocity or speed in residential water piping systems is 7 feet per second, and for 2-inch or larger pipes you should not exceed 5 feet per second. Scrubbing is more of a worry in metal piping than in the smoother (less friction) plastic piping systems. Scrubbing and corrosion are common sources of pinhole leaks in water supply piping.


Continue reading at CLOGGED SUPPLY PIPES, DIAGNOSIS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see PEX BRASS CONNECTOR LEAKS where we discuss corrosion build-up in brass elbows & tees that can clog PEX piping systems.

Or see WATER QUANTITY IMPROVEMENT - steps to increase well yield or other means to improve water flow into the well itself

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