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WATER PUMPS, TANKS, TESTS, WELLS, REPAIRS
WATER CONSERVATION MEASURES
WATER CONTAMINANT LEVELS
WATER FILTERS, HOME USE
WATER HAMMER NOISE DIAGNOSE & CURE
WATER ODORS, CAUSE CURE
WATER PUMP REPAIR GUIDE
WATER PRESSURE LOSS DIAGNOSIS & REPAIR
WATER PUMP SHORT CYCLING
WATER SOFTENERS & CONDITIONERS
WATER TANK REPAIR PROCEDURES
WATER TANK: USES, TROUBLESHOOTING
WATER TESTS, CONTAMINANTS, TREATMENT
WATER TREATMENT EQUIPMENT CHOICES
WELLS CISTERNS & SPRINGS
WELL CHLORINATION & DISINFECTION
WELL FLOW RATE
WELL WATER PRESSURE DIAGNOSIS
WELL YIELD IMPROVEMENT
WINTERIZE A BUILDING
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.
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How to Measure Building Water Pressure: Definitions of Static Building Water Pressure & Dynamic Water Pressure
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.
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.
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.
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:
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.
Reader question: how much pressure is exerted by water per vertical foot of pipe or lift or rise?
3/3/2014 Rob Cunningham said: What is the water pressure per foot, I know it is 2.??? per foot but I don't remember the the formula.
Reply: basic data about water weight, volume, pressure relationships
Here are some water weight & pressure basics:
How much does a gallon of water weigh?
The actual weight of water varies by temperature as its density varies by temperature and possibly also by the effect of impurities in the water. Using our weight of a cubic foot of water data above, or 62.427 pounds, that's for water at 32 °F. If warm the water up it will weigh a little less and at 60°F a cubic foot of (pure) water weighs about 62.37 pounds.
How to convert the weight of water to pressures per foot in a well pipe or inside the well casing
At STATIC HEAD, WELL DEFINITION we show how to calculate the volume of water in a cylinder of various diameters. There we showed that
Why is this number different than our previous square foot PSI calculation? Because we're calculating the area of a 1-inch round column of water which will have less total volume than a 1-inch square column of water.
Other water conversions weights & measures converting a U.S. gallon
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
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.
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,
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,
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.
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.
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 Private Well Water Pressure & Flow Measurements
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.
Diagnosis of private water well problems divides roughly into these areas
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:
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.
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 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.
Reader Question: what is the flow rate of water out of a 6-inch diameter line at 90 psi static pressure?
Hi, we have a 6" water line with a static pressure of 90 psi. We want to put in a 6" stand pipe for truck filling at 1500gpm & 20psi. will that work?
Reply: water & water pipe hydraulics
Whew!. OK so it depends ... that "static" pressure of 90 psi is not going to give us a true answer to the flow rate in gpm you'll be seeing, since we don't know the effects of line length, friction losses, and number of bends, valves, or obstructions in your water line.
[Click to enlarge any image]
Really this is a hydraulics question. The true figures for water movement through piping are interesting and complex. For example, the velocity of water moving through piping (measured in feet per second or fps) is not uniform across the diameter of the pipe.
Water moves fastest in the middle and slows down at the edges of the cylinder or pipe walls - which makes sense if we think the walls impart friction losses on the water. OK so we'll skip that. Here are some basics:
The cool chart at left relating water flow rate in GPM to pressure in psi makes some assumptions stated in the table's note. This data is from engineering work prepared by the University of Florida, Indian River Research & Eductation Facility. - Dr. Brian Boman, "Chapter 21, Hydraulics", retrieved 3/3/2014, original source: http://irrec.ifas.ufl.edu/citrusbmp/ Water%20and%20FL%20Citrus/21%20Chap21.pdf
The second chart shown at left relates pressure loss to length for 100 feet of hose of various diameters up to greater than the 6-inch you asked about. This data is from Dultmeier Sales also cited at References. Click to enlarge for a more readable copy and you'll see that a six-inch diameter "pipe" (we don't know the material of your pipe) at a flow rate of 1500 gpm has a pressure loss of 4 psi per 100 feet.
You could ask Dultmeier or Dr. Boman for an accurate detail but I'm reluctant to bother an educator with these individual questions. Extrapolate from the table or try some basic ballpark calculations, or just give your local hydraulics engineer a call. Boman lays out the data and basic equation as follows:
As we've reported elsewhere under improving water flow in buildings or "perceived water pressure", doubling the pipe diameter increases the liquid carrying capacity of the pipe by a factor of 4.
FYI, friction losses for a 100 ft. length of 6" diameter PVC pipe are figured at 0.30 psi / 100 ft. of pipe. The friction loss data is different for different pipe materials, and is IMO always wrong for in-use systems where contaminants or usage have changed the surface properties of the piping to increase (roughing or wear or mineral deposits) or decrease (algae) friction losses.
Pressure Versus Flow is described eloquently by Boman. As I've struggled for years to explain this and to help people understand that what they call "water pressure" experienced at the kitchen sink or bath shower is better understood as flow rate, I'm quoting him here:
Continue reading at WATER PRESSURE VARIATION CAUSES or select a topic from the More Reading links shown below.
Suggested citation for this web page
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
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. But it will reduce the water flow rate. 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.
Question: how does adding plumbing fixtures reduce the water flow rate & what is the difference between upstairs & downstairs water pressure?
(July 4, 2011) mike in p.s. said:
I own a condo with the following: 2-Tub/Shower Combo's, 2-Toilets, Dishwasher, Kitchen Sink, 4-Bathroom Sinks, 1-Patio Faucet. My unit is upstairs with an identical unit below (same fixture count). Each unit HAD it's own 3/4" feed with seperate shut-off vales about 30' away at the common meter box. HERE'S the problem.
The downstairs unit had a leaf in their line undeer the slab and aftere two days of attempting to locate and repair the line the plumbers deceided to abandon the downstairs fee and tee into our 3/4" feed so now both units share one feed, escentially doubling the number of fixtures on one 3/4" line.
Additionally am I correct in thinking that because I am upstairs and if both units are using water, the downstairs unit gets more pressure? And ideas would be helpful!
If your pressure upstairs is unacceptable it may be possible to improve things with a separate pressure tank and booster pump. In the article links at page left, under
Question: how do I get more water pressure?
(Oct 19, 2012) sly said:
How do you increase the water pressure in your building building, because the water flow rate is low.
Sly: you might want to install an auxiliary water pressure tank and booster pump. See WATER PRESSURE BOOSTER PUMP
Question: pressure of water per foot of height
(Mar 3, 2014) Rob Cunningham said:
What is the water pressure per foot, I know it is 2.??? per foot but I don't remember the the formula.
Here are some water weight & pressure basics:
At 30-feet of water (say diving under the ocean) at sea level, you've equaled 1 ATM of pressure.
The Weight per US gallon of water is based on 7.48 gallons per cubic foot.
1 Gallon (U.S.) = 3.785x10-3 m3 = 3.785 dm3 (liter) = 0.13368 ft3 = 4.951x10-3 yd3 = 0.8327 Imp. gal (UK)
Details of this calculation showing how to convert the weight of water to pressures per foot in pipes of various diameters or inside the well casing are given in the article above.
Questions & answers or comments about how to accurately measure water pressure & water flow rate in buildings.
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