How to Measure Water Pressure & Flow in a Building

How do we measure water pressure and flow accurately?
- How to make or buy a portable water pressure gauge.
- Causes of variation in building water pressure and water flow rate for both municipal water and well water systems
- What are static water pressure & dynamic water pressure?
- Diagnosing a Bad Water Pressure Regulator at buildings connected to a municipal water supply
WATER PRESSURE VARIATION CAUSES - separate article
WATER PRESSURE TOO HIGH: DANGERS - separate article
Questions & Answers about how to accurately measure water pressure & water flow rate in buildings
References

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How to measure municipal or well water water pressure at a building: this article describes how to measure water pressure and water flow in buildings in order to diagnose bad pressure or bad water flow. The process of water pressure problem diagnosis and the costs of the repair are explained. We explain the difference between measuring static water pressure (nothing running) and dynamic water pressure. We explain the difference between water pressure, and water flow rate, and we describe how to measure water flow rate in a building.

How to Measure Building Water Pressure: Definitions of Static Building Water Pressure & Dynamic Water Pressure

Also see WATER PRESSURE REDUCER / REGULATOR for a discussion of how we reduce building water pressure to a safe level and how we assure uniform building water pressure and flow using a pressure reducing valve or pressure regulator. See WATER TANK PRESSURE GAUGE and WATER PRESSURE GAUGE ACCURACY for a discussion of built-in water pressure gauges. Readers whose building is served by a private pump and well system should see WATER PUMP PRESSURE CONTROL SWITCH.

Distinguishing between static water pressure, dynamic water pressure, and water flow rate can help diagnose water problems in a building. Here we explain these concepts and we describe how to measure water pressure and flow at a property where either municipal water supply or a private well and pump water supply is in use.

Definition of Static Water Pressure

Static water pressure is the pressure shown anywhere on the water supply piping system when no plumbing fixtures are running.

Typically on a municipal water supply the static water pressure in the building will be 30-60 psi, depending only on the setting of the water pressure regulator - the regulator determines static water pressure in the building.

On a private well and pump water supply system water pressure varies between 20-40 psi or 30-50 psi depending on the equipment installed and the pump pressure control switch settings. (See WATER PUMP PRESSURE CONTROL ADJUSTMENT).

Our photo (left) shows a simple and inexpensive home-made water pressure measuring gauge that we attached to the cold water faucet for a washing machine hookup. Building suppliers also sell water pressure gauges with the same fittings to attach the gauge to a hose or faucet hookup (see our page top photo).

To measure static water pressure, attach a pressure gauge anywhere in the building on water supply piping. Make sure that no plumbing fixtures are running, and if the building is served by a well and pump system, make sure that you have run water until the pump starts running, then turn water off. The pressure read on the gauge with all fixtures "off" is the maximum static water pressure at the building.

Definition of Dynamic Water Pressure

Dynamic water pressure is the pressure shown anywhere on the water supply piping system when one or more plumbing fixtures is drawing water. You can see that the more fixtures that are running, the lower the dynamic water pressure will be.

On a municipal water supply system the dynamic water pressure and flow seen in a building will drop to a number lower than the static water pressure but will normally remain steady when you turn on one or more plumbing fixtures.

On a pump and well water supply system the dynamic water pressure and flow seen in the building will drop to a lower number than the static water pressure, and will also vary as the pump cycles on and off.

Example of variation in building water pressure:

Incoming building water pressure is 80 psi at the street-side of the pressure regulator.
The pressure regulator is set to 60 psi and installed on 1/2" diameter building water supply piping
The building has four apartments and in each apartment simultaneously people are running multiple plumbing fixtures, drawing water out of the system so fast that ...
The dynamic building water pressure drops to 20 psi when all of these fixtures are operating. Even though the pressure regulator is set to 60 psi, when water flow in the building exceeds the capacity of the incoming water main to deliver water at that pressure, the in-building water pressure will drop to a lower number. A solution to this problem may be the installation of a WATER PRESSURE BOOSTER PUMP.

On a pump and well water supply system when multiple plumbing fixtures are operating, the dynamic water pressure will fall to a lower number and will hover there once the quantity of water being drawn causes the pump to turn on and stay on - that is, when you are running water fast enough that the pump cannot "get ahead" of the water draw out rate.

How to Measure Municipal Water Pressure

Static municipal water pressure may vary at different times of the day depending on what pressure is being delivered by the municipal supplier. In some communities municipal water pressure varies little while in others the variation can be significant.

To measure municipal water pressure in a building, use a water pressure test gauge like the home made water pressure test gauge shown above or the store-bought water pressure gauge shown at page top.

Install the water pressure gauge at a convenient outside hose bibb or to the drain connection on a water heater, or at a washing machine hot or cold water hose connection point.

If the building water pressure gauge reading is below 60 psi, leave the gauge in place for two days, checking it frequently for different pressure readings, because water pressure may vary by time of day or by the plumbing fixtures in use in the building.

An advantage of the store-bought Watts water pressure test gauge over the home-made version is that the special Watts test gauge includes a high-point red indicator needle that will record the highest water pressure sensed by the gauge during the test period, even if you did not happen to be looking at the gauge when that condition occurred.

Since your water pressure measurement itself could be inaccurate, see WATER PRESSURE GAUGE ACCURACY where we explain causes of false high water pressure readings and false low water pressure readings. For pressure gauge repair or replacement see WATER TANK PRESSURE GAUGE.

How to Interpret Your Municipal or City Water Pressure & Flow Measurements

If your dynamic water pressure is too low on a municipal water supply system you can boost building water pressure by installing a pressure booster pump and water tank. See WATER PRESSURE BOOSTER PUMP. Also see our discussion of parallel water pressure reducing valves found at WATER PRESSURE REDUCER / REGULATOR.

If your municipal or city water pressure is always too low, perhaps below 30 psi, see WATER PRESSURE LOSS DIAGNOSIS & REPAIR.

For more help figuring out why your municipal water pressure is too weak, see MUNICIPAL WATER PRESSURE DIAGNOSIS and then see MUNICIPAL WATER PRESSURE IMPROVEMENTS. Also see WATER PRESSURE VARIATION CAUSES.

Watch out: If the building water pressure gauge reading is ever found at 80 psi or higher, you will want to install a water pressure regulator at the point where water supply enters the building. Watts produces a Watts Governor 80™ used for this purpose, but other manufacturers also produce a wide variety of water pressure regulators.

If your building already has a water pressure gauge installed, it may be defective or it may be set too high. If your static water pressure is too high on a municipal supply, see WATER PRESSURE REDUCER / REGULATOR. Also see
WATER PUMP PRESSURE CONTROL ADJUSTMENT

At HOT WATER PRESSURE EXPANSION RATE we discuss how we measure water pressure and how temperature changes affect water pressure in a closed water heater tank or heating boiler.

How to Measure Private Pump & Well Water Pressure

Static well water pressure depends on the setting of the pump controls. You can try reading the water pressure shown on the pressure gauge that is usually installed at the pressure tank or near the pump pressure control switch.

Watch out: dirt or debris in the small diameter pressure switch mounting pipe at your water pressure tank, or a failing pressure gauge itself can give inaccurate water pressure readings. Particularly if your water pressure gauge reading does not rise and fall smoothly as the water pump turns on and off, you may want to replace the gauge and/or make an independent water pressure reading using the water pressure test gauge we have described just above.

How to Interpret Your Private Well Water Pressure & Flow Measurements

If your well water pressure is always too low, perhaps below 30 psi, see WATER PRESSURE LOSS DIAGNOSIS & REPAIR. Also see WATER PRESSURE VARIATION CAUSES.

If your dynamic well water pressure is too low on a private pump and well system you may be able to boost water pressure by adjusting the pump pressure control switch. See WATER PUMP PRESSURE CONTROL ADJUSTMENT. Also, because poor water flow rate from a private well system is often due to a limitation of the well's flow rate, see WATER PRESSURE LOSS DIAGNOSIS & REPAIR.

Watch out: If your well water pressure is too high, see WATER PUMP CONTROLS & SWITCHES and see WATER TANK SAFETY.

Diagnosis of private water well problems divides roughly into these areas

WATER PUMP CONTROLS & SWITCHES
WATER PUMP PROBLEM DIAGNOSTIC TABLE
WATER TANK REPAIRS

WELLS CISTERNS & SPRINGS

How to Measure Water Flow Rate in a Building - Bucket & Stopwatch

Our photo (left) shows water running into a five-gallon plastic bucket. If this is the only fixture running water in the building we can time the number of seconds or minutes needed to fill the bucket.

For example, if the time required to fill the five gallon bucket is one minute, then the water flow rate at this plumbing fixture is 5-gallons per minute or 5 gpm.

One can purchase "flow meters" that connect to various plumbing fixtures to pretend to make this measurement, but remember that we are measuring the flow rate at the particular fixture - to obtain a number that does not necessarily describe the water flow rate capability of the water supply system.

The measurement of water flow rate at a particular plumbing fixture does not accurately measure the true water flow rate of the plumbing system because:

Different flow rates will be found at individual fixtures depending on the water flow restriction of the fixture itself, its' faucet strainer or even a disk that has been inserted into the faucet or shower head (for example) to deliberately restrict the water flow rate.
Water piping supplying a particular fixture may restrict flow rate depending on the length and diameter of piping as well as possible hidden errors in the plumbing system such as a partial pipe blockage by solder at a copper joint, or blockage in the water supply piping if it is clogged by minerals or rust.
Water flow on a well pump and tank system will vary during the measurement period depending on just when the water pressure drop turns the well pump on or off.

On a pump and well system when we turned on water at just the kitchen sink (Dynamic Water Pressure photo above) the flow rate dropped slowly until the pump turned on. Then the water pressure rose slowly until the pump turned off. Water pressure varied between 38 psi (pump off) and 25 psi (pump on).

When we turned on water at a bath tub faucet (photo just above) water pressure dropped to about 28 psi and stayed there as the well pump ran continuously, delivering water to the building at that rate. You can see our pressure gauge reading 28 psi.

Frequently Asked Questions (FAQs) about how to measure buiding water pressure

Question: Does closing a water supply pipe stop valve part way reduce water pressure in the building?

this may sound silly but by reducing the flow via the stop tap do you reduce the pressure - Allan

Reply: No. Here is the difference between water flow rate and water pressure

Does closing a stop valve reduce water pressure? Well yes if you mean flow rate and no if you mean static water pressure in the system.

No honest question is silly - thanks for asking.

Properly speaking, no, reducing the water flow rate does not reduce the STATIC pressure in the system in that the water pressure is determined by the water pump cut-off pressure setting on the pressure control switch if you have a private well, or by the street pressure and /or local water regulator pressure if you have a municipal water supply.

So if you are not running water - it's turned off - and you measure the water pressure using the methods we describe above, the water pressure will be the same regardless of whether or not a stop valve is partly closed.

But lots of people use the term "water pressure" to mean the DYNAMIC water pressure - how fast water comes out of the faucet. We call that the flow rate.

And yes, if you partly close a stop tap or valve, you will get slower water flow at any faucet or plumbing fixture that is downstream from that valve. That's because we're now supplying water through a smaller opening, so at the same starting pressure or static pressure, the volume of water that flows through that valve will be less per minute if the valve is partly closed than it would be if the valve were fully open.

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

Watts, 815 Chestnut Street, North Andover, MA, USA 01845-6098, web search 09/18/2010 Watts Regulator Corporation, 815 Chestnut Street, North Andover, MA, USA 01845-6098, provides pressure and temperature relief valves, water pressure test gauges, water pressure regulators, backflow preventers, check valves, and other plumbing and heating controls and supplies. Website: http://www.watts.com/
- Watts Backflow preventers - 978-688-1811
- Watts Control valves - 713-943-0688 for example Watts pressure reducing valves, original source: http://www.watts.com/pages/learnAbout/reducingValves.asp?catId=64
- Watts Drainage products - 828-288-2179
- Watts Potable water PEX plumbing - 978-688-1811
- Watts Water safety controls - 978-688-1811
- Watts Water quality & conditioning products - 352-465-2000

Engineering toolbox properties of water - http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html editor.engineeringtoolbox@gmail.com web search 09/16/2010
SI Metric.co.uk provides tables and constants for the properties of water - web search 09/16/2010 original source: http://www.simetric.co.uk/si_water.htm
Wikipedia on the Density of water at 1 atmosphere, web search 09/16/2010, original source: http://en.wikipedia.org/wiki/Density

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