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How to Measure Water Pressure & Flow in a Building InspectAPedia®  -     How do we measure water pressure and flow accurately? 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 How to diagnose loss of water pressure or loss of water in 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. Readers whose building is served by a private pump and well system should see WATER PUMP PRESSURE CONTROL.

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.

© 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Causes of Variation in Building Water Pressure

The water pressure regulator setting may not be the same as the water pressure you'll actually get in the building. Depending on the size of the building, its piping size and length of piping runs, and the number of fixtures and number of occupants (and similar variables) in a building, it is possible to draw water out of the building supply piping at such a high rate that the overall building water pressure drops considerably below the pressure setting at the water pressure regulator.

With a municipal water supply the water pressure will vary depending on how many fixtures are being run, and also the municipal water system pressure may vary in the street as well. Water flow rate in a building also depends on the diameter, length, and number of bends or valves in the piping system. Clogged water pipes reduce water flow rate, not water pressure.

With a private well and pump water supply, water pressure will also vary depending on the water pump on and off cycle and possibly on the ability of the well to deliver water.

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

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.

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.

Static well water pressure depends on the setting of the pump controls.

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 PUMP, WATER PRESSURE BOOSTER.

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 Water Flow Rate in a Building

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.

How to Fix Bad Water Pressure or Bad Water or Well Flow Rates

Our water pressure and flow diagnosis and repair guide starts at WATER PRESSURE LOSS DIAGNOSIS GUIDE.

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 REGULATOR ADJUST and see PUMP, WATER PRESSURE BOOSTER.

If your dynamic 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.

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PLUMBING TOPICS
OIL TANKS
SEPTIC SYSTEMS
WATER PRESSURE & FLOW MEASUREMENT
  Definition of Static Water Pressure
  Definition of Dynamic Water Pressure
  How to Measure Water Flow Rate in a Building
WATER PRESSURE GAUGE ACCURACY
WATER PRESSURE LOSS DIAGNOSIS GUIDE
  BAD MUNICIPAL WATER PRESSURE or FLOW
  BAD WELL WATER PRESSURE or FLOW
  Water Pressure Intermittent
  No Water Pressure
  Well Tank relation to Water Pressure
  Shaking the Water Tank?
  Water Pressure Tank Problems
  Water Pressure Tank Diagnosis
  Water pressure Later Returns "on its own"
  Water pipe clog diagnosis
  Water pipe clog repair guide
  Making the "right" repair
  Typical Water Tank & Pump Prices
  Typical Pressure Tank & Pump Prices
WATER PRESSURE REGULATOR ADJUST
  What's a Water Pressure Regulator?
  Causes of Variation in Water Pressure
  Diagnose Poor Municipal Water Pressure
  Adjust Bad Pressure Regulator on Municipal Water
WATER PRESSURE REPAIR GUIDE & COSTS
  Questions about lost water pressure
  Advice to a homeowner
  Adjusting Water Pump Pressure Control
  Right Repair to Pump & Tank?
  Well Pump & Tank Replacement Costs
WATER PUMPS & WELL TANKS
WATER PUMP CONTROLS & SWITCHES
  IDENTIFY WELL PUMP TANK COMPONENTS
  MAIN WATER SHUTOFF VALVE
  WATER PUMP ELECTRICAL SWITCHES
  WATER PUMP PRESSURE CONTROL
    WATER PUMP PRESSURE CONTROL ADJUSTMENT
    WATER PUMP PRESSURE CONTROL REPAIR
    WATER PUMP PRESSURE CONTROL REPLACE
  WATER PRESSURE REGULATOR ADJUST
  WATER PUMP RELAY SWITCH
  WATER TANK AIR VOLUME CONTROLS
  TANK AIR INLET VALVE
  TANK AIR VALVE REPAIRS
  WATER TANK PRESSURE GAUGE
  WATER TANK DRAIN VALVE
  WATER TANK RELIEF VALVE
  CHECK VALVES
  WATER FILTERS
WATER PUMP TYPES & LIFE EXPECTANCY
WATER PUMP & TANK SAFETY
WATER SUPPLY & DRAIN PIPING
WATER TANK TYPES: WATER, OIL, EXPANSION, ALL
WATER TANK LIFE EXPECTANCY
WATER TANK REPAIRS
  CONTROLS & SWITCHES on WATER TANKS
  INTERMITTENT CYCLING WATER PUMPS
WATER TANK AIR, HOW TO ADD
WATER TANK BLADDERS & CAPTIVE AIR
WATER TANK REPLACEMENT
WATER TANK BLADDER PRESSURE ADJUSTMENT
WATER TANK PRESSURE CALCULATIONS
WATER TANK SIZE & VOLUME
WATER TANKS HOW THEY WORK
WATER TESTING
WATER TREATMENT EQUIPMENT CHOICES
WELLS CISTERNS & SPRINGS
WELL LIFE EXPECTANCY
WELL PUMP TYPES & LIFE EXPECTANCY

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