Water pressure gauge test (C) Daniel Friedman Causes of Varying Building Water Pressure

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Water pressure or flow rate variation:

This article describes the causes of variation in building water pressure for homes connected to municipal or community water supply.

These different causes for variation in water pressure help tell us what is needed to improve or regulate the pressure.

The process of water pressure problem diagnosis and the costs of the repair are explained.

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What are the Possible Causes of Variation in Building Water Pressure?

Water pressure regulator (C) Daniel FriedmanReaders whose building is served by a private pump and well system should

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.

The water pressure regulator setting [public water supply] 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.

The water pressure settings on your [private water supply] well pump pressure control switch may not be the same as the water pressure you'll actually get in the building. Similarly, the private well pressure control switch settings define the maximum and minimum water pressure you may see in the system, not the water pressure you'll see at any given moment when water is being run.

Even if every component of the private well, water pump, and water pressure tank and plumbing system are working normally, water pressure in such a building will fall between the time water is turned on and the time that the well pump is turned on by the pressure switch.

  1. Number of plumbing fixtures being run at once: With a municipal water supply as well as private well water supply systems, the building water pressure seen at any individual plumbing fixture will vary depending on how many fixtures are being run. We tend to see this effect more on private well water systems where the total water supply system flow rate may be more limited.
  2. Variation in municipal water delivery pressure: the municipal water system pressure may vary in the street as well as a function of level of water use in the whole community or when work is being done on the system.
  3. Variation in small community water systems and private well pump water delivery pressure: with a small community well system as with 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.

    For example, if you turn on water at a sink just after the well pump has cut out the building water pressure will be at its "cut out" pressure maximum, typically 40 psi or 50 psi. But if water has been run in the building to drop water pressure in the system to just a bit above the water pressure control's "cut in" pressure, then the water pressure observed will be lower, perhaps just above 20 psi. Details are
  4. Water pipe diameter, length, elbows and bends: Water flow rate in a building also depends on the diameter, length, and number of bends or valves in the piping system. And if water pipes become clogged with mineral deposits or debris, the water flow rate will diminish in the building, even if the static water pressure remains high.


    Technical note: The D'Arcy-Weisbach equation for pressure and head loss can be used to calculate the actual water pressure loss due to friction in a building piping system. The Engineering Toolbox provides the D'Arcy-Weisbach formula: Δp = λ (l / dh) (ρ v2 / 2)  where Δp = pressure loss (Pa, N/m2), and l = length of duct or pipe (m) and dh = hydraulic diameter (m) and finally, ρ = density (kg/m3).

    But keep in mind that even this apparently accurate calculation of the effect of piping on water pressure loss will not include the effects of obstructions in building water piping such as mineral deposits, debris, or blobs of solder from an improperly-soldered copper pipe connection.
  5. Clogged water pipes reduce water flow rate, not water pressure.
  6. Variations in building occupancy levels: Where building demand for water flow varies widely, a single pressure reducing valve may not be able to handle the maximum water demand flow rate. This condition occurs at buildings where there is a large water supply main to an apartment or office building whose water demand can vary enormously (0.5 gpm to 100 gpm) depending on the building occupants. Watts and other pressure reducing valve producers recommend a nice solution to this problem.

    As we explain
    at WATER PRESSURE REDUCER / REGULATOR, parallel pressure reducing valves are sometimes installed to correct this difficulty. One pressure regulator control is set at a higher psi flow rate than the other. For example installing a Watts Model 223 pressure reducer set at 50 psi in parallel with a second Watts 223 pressure reducer set to 60 psi will allow increased total water flow capacity during periods of peak water demand.
  7. A failed hot water pressure expansion tank or significant changes in hot water temperature may also cause surges in building water pressure: some building plumbing systems include an expansion tank on the water supply system, often on the hot water piping (details about these expansion tanks are

    If the expansion tank suffers an internal failure such as a perforated bladder, the tank will stop functioning, will not protect the system from temporary increases in hot water pressure. Also
  8. Water filter clogging: if your water system includes a cartridge-type water filter, typically installed to control or reduce sediment, debris, or odors, the filter may become clogged over time, resulting in a reduced water flow rate.

    Some folks confuse water flow rate (how fast water comes out of the tap) with water pressure (static pressure when water is not being run). And lots of people call our water flow rate "water pressure" which it is, in a practical sense - the pressure with which water comes out of the tap.
  9. Variation in performance of the pressure regulator itself: a water pressure regulator may be misbehaving.

    One water pressure regulator failiure mode causes gradual increases in building water pressure above the set point on the regulator. This may be due to an internal failure or leak in the water pressure regulator, dirt on the regulator valve seat, or a leak in an internal regulator valveo pressure-sensing diaphraghm.

    See Water Pressure Regulator Testing for a discussion of regulator tests and possible repair procedures.
  10. Variations in private water supply source, well capacity, and well pump behavior will also cause variations in water supply pressure on private well water systems.



    Problems with the private well itself are discussed beginning

How do we Diagnose Poor Municipal Water Pressure & Flow in a Building - 5 Key Checks

Key things to check when water pressure and flow are inadequate in a building served by municipal water supply are

  1. The condition of individual plumbing fixtures, faucets, valves, and controls. A clogged sink or shower strainer will cause bad water pressure at individual fixtures while others may flow freely. A plumbing shutoff valve that is partly closed or clogged with debris will cause bad water flow at that fixture.
  2. The incoming municipal supply pressure. The municipal water supply source may be delivering water at low pressure.
  3. The condition of water piping between the building and the street water main. An older water supply pipe connecting the building to the water main in the street may be small in diameter, clogged with minerals, rust, or debris, or it could be leaking underground.
  4. The condition of building water supply piping. Clogged pipes due to minerals or rust, small diameter piping, long piping

    For details about the above items,
  5. The setting of the water pressure regulator at the building water meter or water shutoff valve. This topic is discussed just below.

Photograph of a water pressure regulator on municipal water supplyBad water pressure regulator or bad pressure regulator adjustment: (poor municipal water pressure) if the building is connected to a municipal water supply there may be a water pressure regulator installed, usually close to the water meter or where the water supply pipe enters the building.

Photos of water pressure regulators and diagnosis and adjustment advice are

If the water pressure control was set too low to start with or if there is a problem with the water pressure regulator, you may see an improvement in water pressure by diagnosing and adjusting or repairing this control.

If the loss of water pressure was sudden, it's not likely to be a problem the pressure regulator except in the less common cases of a blockage or failure in the regulator itself. In this photo our flashlight lights up a (rather amateurish looking) water pressure regulator installed on a municipal water supply.

You can see that a previous connection to the water meter was cut, left shut off (see that shutoff valve at the left hand vertical pipe - what happens if someone opens that valve?), and new fittings were installed to conduct incoming water to the building water supply piping.

The use of smaller diameter flexible copper tubing might restrict the incoming water flow, but the presence of a pressure regulator at all suggests that the incoming pressure may have been excessive. (Too high water pressure leads to fixture leaks.)

How to Adjust the Water Pressure Regulator on Municipal Water Supply

Details about how to adjust the building water pressure regulator or pressure reducing valve are found

We have already mentioned checking and adjusting the pressure control for the water pump on a private well system - just above. But what if the home does not have it's own well - what if the building is connected to a municipal water supply?

Watch out: don't set the water pressure reducing valve higher than necessary. Doing so wastes water and as we discuss
at WATER PRESSURE TOO HIGH: DANGERS, setting water pressure too high can cause both plumbing problems and actual serious safety hazards at buildings.

A Guide to Building Water Pressure by Adjusting or Repairing the Water Pump Pressure Control Switch on a Private Pump and Well Water Supply

For details about diagnosing poor well water pressure and flow, start
 WELL WATER PRESSURE DIAGNOSIS or go directly to the individual items listed just below.

Question: water pressure in the system varies too much


I have a water system based on a tank plus demand pump. The problem at the moment is that the pressure in the system varies too much between full pressure and the cut in of the demand pump. i.e. while taking a shower the water temperature drops markedly. What is the problem? - Anonymous by private email.


Please take a look at the WATER PRESSURE VARIATION CAUSES article above and then also see WATER PRESSURE VARIATION FAQs.

Then watch your pressure control and pressure gauge as water is running to see if the pump is turning on when the pressure falls to the pressure switch CUT-IN pressure setting. If not the switch is clogged or damaged and needs replacement. Do let me know what questions remain after that and I'll be glad to research and respond in detail. - Editor.


Continue reading at WATER PRESSURE VARIATION FAQs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see WATER PUMP PRESSURE CONTROL ADJUSTMENT - private pump & well pressure controls

Or see WATER PRESSURE REGULATOR ADJUSTMENT - municipal water supply pressure

Suggested citation for this web page

WATER PRESSURE VARIATION CAUSES at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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